Calculating Neutral Density and New Photographer’s Friend Features (Podcast 727)

Calculating Neutral Density and New Photographer’s Friend Features (Podcast 727)


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This week I’ve been busy again adding a few small new features to the Photographer’s Friend extension for Apple Watch, and this also led me to make a change to the logic behind the iOS version of our Neutral Density Filter Calculator, so I thought I’d share some details about that for the geekier listeners and readers among you. I’ve mentioned before that at school, back in England in the seventies and eighties, I was a terrible student. I was more interested in messing around with my friends than learning anything, and I never really felt like I could achieve anything academically until I went back to college here in Japan in the mid-nineties. I learned from that experience that being interested in what you are learning and being taught by enthusiastic teachers makes a world of difference.

Since then, I’ve been able to research and learn some pretty complicated math, at least to me, which is behind some of the more complicated features of my Photographer’s Friend application, such as the Pixel Peeper mode in my Depth of Field Calculator, and the Neutral Density Filter Calculator. I shared some of the theory behind the Pixel Peeper mode when we talked about Circle of Confusion, the Airy Disk, and Diffraction back in episode 594, and we talked about calculating Neutral Density back in episode 391 entitled All About Neutral Density Filters. I’ve come to a few slightly new conclusions over the last few days though, so I’m going to share that with you today, and tell you a little bit about these two new features that are available in the Photographer’s Friend version released on the App Store today.

Basic Calculations

First let’s take a look at some of the background behind the original Neutral Density filter calculations in Photographer’s Friend, which I added as a second calculator when I took over the development myself around five years ago. My calculations so far have been based purely on the multiplication factor that is marked on all neutral density filters we can buy. An ND8 for example multiplies your shutter speed by eight, so quite simply, say your base shutter speed without any ND filtration was 1/50 of a second, you can take a regular calculator and tap in 1 divided by 50 multiplied by 8 which gives us 0.16, which converts to the nearest shutter speed of 1/6 of a second.

Pretty simple stuff, and you can use this calculation for any ND filter multiplication factor, such as the ND400. 1 divided by 50 multiplied by 400 is 8, so your new exposure is literally 8 seconds. If you stack filters, you just continue to multiply, so if you add the ND8 to the ND400 we multiply our 8 seconds by the ND8 multiplication factor of 8, for the result of 64 seconds. All good fun and simple maths, and to be honest, this method of calculation is perfectly fine, and accurate, if you run with the multiplication advertised by the Neutral Density filter manufacturers.

When I developed the Neutral Density filter as part of the Photographer’s Friend Apple Watch extension, because of the limitations associated with the screen size, I decided to go with one neutral density filter dial, so that users can simply add the total density when stacking filters, and rather than using the multiplication factor, I went with exposure stops, which is also provided when you buy a neutral density filter. We know for example that an ND8 is three-stop, and an ND400 is 8.6 stops. That’s what it says on the box and can also be used to calculate our new shutter speeds, so I set about figuring out how to do that.

I came to the conclusion based on reversing some of my original math that we can find the new exposure time by multiplying the base shutter speed by 2 to the power of the number of stops provided by the neutral density filter. Here’s the calculation with the words initially.

{shutter speed} \times 2^{ND stops} = {exposure}

If we add some example numbers to that, the calculation looks like this. So, again, using the 1/50 of a second exposure and the ND400’s 8.6 stops as an example, but the result is 7.76 seconds, not 8 seconds, even though, as we saw earlier, simply multiplying by 400 gives us 8 as a whole number, not a nearby fraction.

\dfrac{1}{50}\times 2^{8.6} = 7.760468

So, I started to look into why that might be the case and found that 2 to the power of 8.6 is 388, not 400, so assuming that 8.6 is actually 8 and a third, I tried using 8.666 instead and found that this gives me 406. So, one is 12 under and the other 6 over. I’ve not been able to find any white papers on the actual variance in the marked values and actual optical performance of neutral density filters, but it’s safe to assume, anyway, that there is some variance.

{2^{8.6} = 388.023441} \hspace{50px} {but} \hspace{50px} {2^{8.6\underline{66}} = 406.186927}

I decided to use thirds rather than the .6 or .3 fraction in my calculations and moved ahead basing the new calculations on stops, rather than the multiplication factor and as the seconds are always rounded, for relatively short exposure times, the difference is not even apparent on the front end. 7.76 and 8.12 will always display as eight seconds anyway.

\dfrac{1}{50}\times 2^{8.666} = 8.123738

Getting to this point was not time wasted though. It was important for me to understand this stuff because I really needed to be able to work with stops rather than the multiplication factor, and I wanted to either maintain the accuracy that I’ve had to date, or, if possible, increase it, so I continued to dig. In my recent update to enable the synchronization of the total number of stops of neutral density applied in the Neutral Density Filter Calculator in the Apple Watch Extension for Photographer’s Friend, I had to convert the multiplication factor back to stops, which is what the Apple Watch Extension was using for its calculation, and this led me to discover an import part of the larger puzzle.

{\log_2 406.196927 = 8.666}

I found that I could reverse the calculation with the log base 2 of the multiplication factor, which equals 8.666, the stops of the original neutral density filter, and this is what I’ve used for the last few weeks as I released the first attempt at synchronization. It works well, but there were a few things left that were bugging me, and so I set further time aside this week to get it all cleaned up and to the point that I could finally walk away from this development work for a while, and get to some other important tasks that I’ve been putting off for a little too long now.

Decimals for Apple Watch ND Filter Calculator

As I mentioned, I’d originally gone with just the one dial for whole stops of neutral density filter for the Apple Watch extension, but as we’ve noted, some filters, such as the ND400 and a few others actually have decimal fractions rather than just being whole numbers. I’d figured that we could live without them, but that had been playing on my mind, and now that I was adding a synching mechanism, with the initial functionality I was going to have to throw out the decimals, and as I worked on the update, seeing the calculated times being different between the Watch extension and the iPhone based calculator really annoyed me.

The difference between the available times was a little bit to large to ignore. For example, if you stack the ND8 and ND400, you’ll get an exposure of 1 minute and 5 seconds when basing the calculation off of the stops using thirds. We need to be able to select 11.6 to get the correct exposure, but just 11 stops gives us a 40-second exposure, and 12 stops is 1 minute and 21 seconds. Both are a significant amount of time away from 1 minute 5 seconds, and could not realistically be simply ignored.

New Decimal Fractions Field
New Decimal Fractions Field

While trying to maintain the simple usability of the Extension I decided to add a Decimal Fractions field that would essentially only take up space when it was necessary. To realize this, firstly, although it’s a dial, you only see one value at a time, and that’s the selected value. I also added an Auto-hide switch to the settings, to have the Decimal Fractions field disappear automatically when it’s at zero. The result is that now if you have filters selected on the iPhone that include a decimal fraction, when you synchronize the settings to the Watch extension, the extra field will appear, and when it’s a whole number, the field will disappear.

Because the Watch extension can be used as a standalone calculator though, you can either swipe upwards on the ND Filter Calculator screen to show the Decimal Fractions field and select an option, or if you use Decimal Fractions a lot, you can simply turn off the Auto-hide option, and the field will stay on the screen even if it’s set to zero. The other thing to note is that I’ve left the dial at single-digit fractions, but behind the scenes, 0.6 is actually 0.666666 and 0.3 is 0.333333. 0.8 is also 0.833333. The result is that now, pretty much every combination of practically usable filters results in the same time on both the iPhone and the Apple Watch Extension, and because I’m using adjusted stops, the calculated times are more accurate than when using single-digit fractions. I’m really happy with how this turned out and think the usability is relatively slick as well.

Active Timer for Apple Watch ND Calculator

The other update that I wanted to quickly mention before we wrap this up for this episode, is the Active Timer, also for the Neutral Density Filter Calculator for Apple Watch. I’m putting together a video to explain these features visually which I’ll embed into this post once it’s done, but basically, there was another thing that had been annoying me about the original functionality that I’d built into version 3.5 of Photographer’s Friend’s Apple Watch extensions.

The whole idea behind the extensions is to enable the user to calculate the time required for a long exposure when using Neutral Density filters. Getting the iPhone out is fine, but I’ve found that I can sometimes fumble with the phone, so I developed the extensions to enable quick reference from a device that is essentially more accessible than the iPhone, because it’s already attached to our wrists. Granted, if you have a timer built into your camera, you’re likely just going to set that, but I’ve found that having the timer running on my Watch also helps me to get on with other things while a long exposure is running. Also, once the screen has gone dark on my camera, the only way to see if the exposure is still running is to go over to the camera and check for the memory card write indicator to flash red. With the timer running on my wrist I can reference it much more easily.

But, with the limitation of the Apple Watch being what they are, once I lower my wrist, allowing the watch to turn off its face and background the Photographer’s Friend extension, any kind of audible or haptic feedback that I could provide to the user as the timer counted down would also stop, until, that is, I figured out how to develop what I’ve called the Active Timer. For exposures of up to one hour, with Active Timer and what I’ve called the Audible Tap enabled, you will now get an audible ding and a haptic tap on the wrist every ten seconds during the countdown when the timer is over three minutes. Between three minutes and one minute, the taps are five seconds apart, then from 60 seconds to 10 seconds the taps are 3 seconds apart. From ten to four seconds they are every two seconds and then every second for the last three seconds before the alarm starts with double dings to let you know that it’s time to stop your exposure.

If you don’t want the audible ding as the Active Timer counts down, but you do want the heavier haptic taps, just turn on the Audible Tap option and mute the sound on your Watch. If you want a less obvious light tap, turn the Audible Tap option off, which will provide a much lighter tap on the wrist at the same frequencies that I just mentioned. I put a lot of thought into the tapping frequency and feel that it provides a good indication of where you are in the exposure. You can essentially allow the exposure to run down to the last second without looking at your Watch but know approximately how many seconds are left, and when to stop the exposure.

Although it’s highly unlikely that people will be doing exposures of over one hour, if the time is set for longer than one hour with Active Timer enabled, there will be a message to tell you that it’s running as a Passive Timer for the time being, and then when the timer reaches one our, it will automatically switch to an Active Timer. I’ve tested this with an eight hour timer when I went to bed, and seven hours into the timer I started to get the taps on my wrist as the Active Timer kicked in, so it’s pretty solid.

One other thing that I’ve added to keep this all running as solidly as possible is that you can now completely close Photographer’s Friend on the watch, and even reboot your watch, and when you reopen Photographer’s Friend and navigate back to the ND Filter Calculator, the Active Timer will continue to run working towards the originally set end time. This goes for the iOS version as well. In fact, you can have the two timers linked, and close both devices, and they’ll still be running when you restart either or both of them. I should also note that you do have to have the Extension running and the ND Filter Calculator screen displaying for the Active Timer feedback, but if you should close it and leave it closed, whether you are using the Active Timer or not, you will get a Notification, assuming you have given Photographer’s Friend permission to send you notifications that is. This is also the same for the iOS version as well.

OK, so as I say, I’ll embed the tutorial here once I’ve completed it so that I can walk you through this stuff visually as well, but if you are already using the Apple Watch Extensions for Photographer’s Friend, I hope you found this useful. And if you don’t use Photographer’s Friend, I hope you at least found the geeky talk about the calculations interesting. If you have just heard about Photographer’s Friend for the first time and would like to grab a copy, you can find more details on our product page at https://mbp.ac/app or on the Apple App Store at https://mbp.ac/pf. Note that the Apple Watch extensions cost a few more dollars as an in-app purchase. I didn’t want to include this in the base price because not everyone has an Apple Watch or necessarily want to use the extensions.


Show Notes

Get Photographer’s Friend in the Apple App Store at https://mbp.ac/pf

Music by Martin Bailey


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All About Neutral Density Filters (Podcast 391)

All About Neutral Density Filters (Podcast 391)

Having been asked many times about the Neutral Density filters that I use for my long exposure images, today I’m going to explain what Neutral Density filters are, which one’s I use, and how I use them, along with some ways to easily calculate your new shutter speeds, and some guidelines on how much neutral density to use for various effects.

Before we jump into today’s episode, I’d like to thank our sponsors Squarespace. Squarespace is the all-in-one platform that makes it fast and easy to create your own professional website, portfolio or online store.  For a free trial and 10% off, go to squarespace.com and use offer code MBP10. We’ll hear more from Squarespace later on.

Basically, neutral density or ND filters, stop a given amount of light from passing through your lens to your sensor, making it necessary to increase the length of the shutter speed to achieve the same exposure. There are applications when you might use an ND to shoot say portraits in bright light when you still want to use a wide aperture for a shallow depth of field. There are also times when NDs are used for video, because video usually looks best when shot at 1/50 or 1/60 of a second, and I’ve used both of these techniques, but today, we’re going to concentrate on the use of ND filters used to achieve longer shutter speeds to capture movement in our subjects, such as to make waterfalls look silky smooth, or to capture movement in clouds etc.

Are They Really Neutral?

The idea behind the ND is that they are supposed to be totally neutral in color, cutting down the amount of light passing through the lens without changing the color of the resulting photographs. The filters that I use pretty much are neutral and have never been a problem, though I have heard of people getting a magenta cast on their images. My friend David duChemin for example says that he gets a magenta cast sometimes when stacking two of his square resin filters. The type that attach to the front of your lens using a screw in filter holder.

I used to use these, and I think they’re useful especially if you want to use graduated neutral density filters, which are dark on one half and clear on the other, with a graduation between the two. These are used to darken the sky for example, when you have a light sky and a much darker foreground. You’d use a graduated neutral density filter here to even out the light in the scene.

I personally stopped using graduated ND filters a few years ago, as I simply haven’t found them necessary since the dynamic range in our cameras became so much greater. I keep in mind that I might sometimes have to shoot multiple images to merge together in Photoshop later, but we can bring so much detail out from the shadow areas in Lightroom or Nik Software’s Silver Efex and Color Efex Pro now, that I haven’t merged images like this in years either. The result is that my main use of NDs now is in the form of circular screw-in solid neutral density filters. I use a mixture of Kenko, Hakuba and Hoya filters, though mainly Kenko. I have also heard good things about B+W filters, though I have never used them myself.

Neutral Density Ratings

Neutral Density filters are often rated with a number so that we can understand how much light they will stop from entering our camera. An ND2 will block out one stop of light, so if for example we use this with a base shutter speed of 1/250 of a second, with a one stop ND2 filter fitted, we’d need to adjust the shutter speed to 1/125 of a second, twice as long as the original shutter speed, because we need to let in one stop more light, and that always require us to double one of our exposure settings, such as ISO, Aperture or Shutter Speed, but as I say, because most of the time we use NDs to increase the shutter speed, that’s what we’ll concentrate on here.

The next rating you’ll see is an ND4, which blocks out two stops of light, and then an ND8, which blocks out three stops of light. I know these numbers might seem counter-intuitive, but if you think of these numbers, 2, 4 and 8 as the denominator in fractions, it gets easier to understand. An ND2 cuts the light entering the camera by 1/2, an ND4 cuts the light to 1/4 and an ND8 cuts the light down to just an 1/8 of the original amount of light that would have entered the camera without the filter fitted.

So, because an ND2 cuts the light entering our camera to 1/2, we have to double the shutter speed to compensate for this, and I’m sure you already know that if we double the shutter speed, we are increasing it by one stop. This is why an ND2 gives us one stop of darkness, and an ND4 gives us two stops, and an ND8 gives us three stops of darkness, or longer exposure.

This calculation is continued, with the not so commonly seen values ND16, ND32 and ND64 and so on. Each consecutive rating halving the amount of light is passes through compared to the previous value filter. The next value filter I own is an ND400, which of course is not a fraction. The natural progression would be ND128, ND256 then ND512, as we keep halving, but the manufacturers decided that ND400 would be a good value to add to the range too. This does still cut the light down by 1/400 though, so if we do the math, we find that this is an 8 and 2/3 stop filter.

The absolute darkest ND filter I own, which is actually made for directly photographing the sun, is an ND100000, which cuts the light down to 1/100000 of it’s original brightness. This filter can give me multiple hour exposures if I want to, or still take me down to multiple minutes in very bright conditions, like when you are shooting towards the sun over the sea, for example, when there is a lot of reflectivity going on.

Optical Density

OK, so I know I’m geeking out here, but before we move on, I’d just like to cover the other neutral density number system that you might see quoted on ND filters, and that’s the Optical Density. Now, although I’ve figured out how to do these calculations in Excel using a logarithmic formula, Wikipedia says, and I quote “For an ND filter with optical density, the amount of optical power transmitted through the filter can be calculated from the logarithm of the ratio of the measurable intensity after the filter to the incident intensity.” That helped me to develop my Excel spreadsheet, but I’ve never really felt there is any benefit in trying to figure out what the hell that means, so let’s just look at a table of the values as a reference.

ND Rating
Optical Density
F-Stop Reduction
% of Transmission
No Filter 0.0 0 100%
ND2 0.3 1 50%
ND4 0.6 2 25%
ND8 0.9 3 12.5%
ND16 1.2 4 6.25%
ND32 1.5 5 3.13%
ND64 1.8 6 1.56%
ND128 2.1 7 0.78%
ND256 2.4 8 0.39%
ND400 2.6 8.64 0.25%
ND512 2.7 9 0.20%
ND1024 3.0 10 0.10%
ND2048 3.3 11 0.05%
ND4096 3.6 12 0.02%
ND8192 3.9 13 0.01%
ND100000 5.0 16.6 0.001%

You’ll also see that I included F-Stop reduction and the % of Transmission here too. The percentage of Transmission is the amount of light that gets through, starting at 100% with no filter, 50% for a one stop filter, and 25% for a two stop filter, and so on. This isn’t used to rate ND filters for photograph use though, so you don’t need to be too concerned with this. The F-Stop reduction for each filter is useful though, so do take a look at the chart though, especially if you have a filter with a known optical density, but you aren’t sure of its ND rating or how many stops it reduces the light by.

What Do I Use?

Although I own a couple of ND4 two stop filters, I actually don’t carry them with me any more. I mainly carry my ND8 and ND400 filters, in both the 77mm and 82mm filter sizes. I also have a circular polarizer in each filter size, and all six of these filters live in an envelope style filter case from a company called Marumi, that is really slim and slips nicely into my vest pocket. I basically lives there the whole time while I’m out shooting. I also often carry the ND100000 in my camera bag, just in case I need it.

Filters and Filter Case

Filters and Filter Case

Stacking Filters

I should mention too that it’s OK to stack ND fitlers, and the calculation for your new ND Value when doing this is really easy. You literally just add together the stops for each filter. For example, when I only owned an ND4 and ND8 filter, I would sometimes stack them for 5 stops of darkness. Nowadays, the ND8 and ND400 give me a hair under 12 stops of darkness, and this is enough to get me down to between 10 seconds and a couple of minutes in the middle of the day, depending on how much light I have to start with.

Stacked ND400 and ND8 Filters

Stacked ND400 and ND8 Filters

Remove the Protector

I know that there is a lot of polarity out there surrounding whether or not you should use a protector filter, and if you don’t use them, that’s fine. It’s your decision, but I do use them, and intend to continue to use them for a number of reasons that we don’t need to go into today. The reason I mention this now though, is because if you do use a protector filter, you will often need to remove them when you stack multiple neutral density filters.

I try to buy the narrow framed filters when available, so I can usually get away with one ND on top of a protector filter, but using two causes vignetting, which is where the corners of the image goes dark, on my 16-35 and 24-70mm lenses when used at the wide end, so whenever I’m stacking filters, I take my protector filter of first.

Another way to avoid vignetting of course is to use the large square filters like the LEE system, but I’m not a fan of square filters. They have their benefits, like only having to buy one system that you can easily use with many filter thread sizes, but I can use step down filters with my circular filters if I wanted to as well. I choose not to for the same reason that I choose not to use the large square filter systems, which is because there is no easy way to use a lens hood with either solution. LEE apparently do an absolutely awful lens hood for their system that very few people like, and once you start using a larger screw in filter with a step-down adapter, your lens hoods can’t be attached either.

Not a Fan of Vari-ND

And while we’re on the subject of systems that I don’t personally like, I’m not a fan of the variable neutral density filters that change in density as you rotate the front element either. These are basically two polarizing filters that create a varying amount of darkness depending on their rotational relationship to each other, but because they are using polarization to create the effect, the results can be pretty nasty when used on wide angle lenses, with some parts of the scene much darker than others. I bought a Singh-Ray Vari-ND, and although it worked well with longer focal length lenses, and I found it useful for video, it was pretty much useless on my 16-35mm lens, so I stopped taking it out with me.

Here are two images both shot with the Vari-ND at 20mm, but with the ND rotated slightly differently. The first image (left) was shot at 1.6 seconds, with a bit of the effect showing, but the second image (right) was a 4 second exposure, and I tried to get a longer exposure, but also in an attempt to get rid of the patchy darkness that can be clearly seen in these images. In the end I gave up, and this was pretty much the end of the Vari-ND for me. If you click on the images to view them larger in your browser, then use your mouse or keyboard arrow keys to jump back and forth between them, the effect I’m talking about is very easy to see.

Winter Scene with Vari-ND - 1.6 seconds

Winter Scene with Vari-ND – 1.6 seconds

Winter Scene with Vari-ND - 4 seconds

Winter Scene with Vari-ND – 4 seconds

Another reason that I don’t like variable ND filters, is because they don’t give you a definitive indication of the levels of density. All of the variable NDs I’ve seen, have a series of notches or marks without any numbers, so you can’t easily just set them at exactly 7 stops for example. You can take a good guess, but then you may end up basing a two minute exposure on that, only to find that it’s under or over exposed, and I don’t want to mess around with exposures that long.

The fact that they give a smooth transition between often 2 and 8 to 10 stops of density can be a benefit of course and is workable if you are using Aperture Priority or otherwise allowing the camera to control the exposure, because you just rotate the filter and let the camera do the math. I rarely use Aperture Priority though, as many of my longer exposures are more than 30 seconds anyway, which is the longest exposure my camera will do before I have to jump into Bulb mode and do my own exposure calculations anyhow.

Calculating the New Shutter Speed

So, let’s look at how I do my exposure calculations in the field. For exposures up to a few seconds, I generally just use Live View on my camera with Exposure Simulation turned on, so I can see on the live histogram exactly where my exposure is, and don’t really need to do any difficult mental calculations. If you don’t have Live View or a live histogram, you can just shoot a test frame, then check your histogram in image playback mode, and adjust as necessary.

Remember, you generally want the brightest part of your image, which is the right side of the graph, to be just touching the right shoulder of the histogram box. These days there is usually a line around the box, so it’s easy to see, but if your camera is getting a little long in the tooth, you may need to be careful with this, as many didn’t have the box. See the episode of Exposing to the Right (ETTR) if you want more information on why it’s better to expose this way.

One thing to note too is that when you use a heavy ND filter, like the ND8, or even darker, like the ND400 or ND8 and ND400, it starts to get very difficult, if not impossible to see through the viewfinder to compose your shot. What I sometimes do is set up my shot, deciding on my composition and get it focused etc. then find my ideal exposure without the neutral density filter on, and then screw the filter or filters on to the front of the lens and mentally calculate my new exposure.

For example, in this shot (below) I was photographing a waterfall in the daytime, and without an neutral density, the shutter speed was 1/20 of a second at f/16, with ISO 100. In this case, I only intended to use an ND8, so all I needed to do was put the filter on, and check the exposure in Live View, but what I generally find myself doing is counting out in clicks from my base exposure. I have my cameras set up to adjust exposure in 1/3 stop increments, as most Digital SLRs do, and this means that all I need to do is increase the shutter speed by nine clicks on the main dial. Three clicks per stop, multiplied by three.

Skógafoss (Falls)

Skógafoss (Falls)

Clicking and Counting on One Hand

If necessary, you can use this technique for heavier ND filter combinations. If I use the 8 and 2/3 stop ND400, I can literally just count in sets of three clicks, folding a finger for each three, which is one stop, until I get to 8 stops, and then add two more clicks for the last 2/3. The Japanese way of counting to ten on one hand is useful here too, because I’m using my second hand to rotate the main dial.

I start with an open hand, with all five fingers extended for zero, then fold the thumb in for one, while counting three clicks, then the index finger for two stops, counting another three clicks off on the main dial, and son on. Once I fold my little finger or pinky for five stops, I then re-extend the little finger for six stops, still counting three clicks per stop on the main dial, and work my way back, until I extend my middle finger for eight stops, then count off two more clicks for the final two thirds of a stop.

If I’m using my ND400 with my ND8 for 11 2/3 of a stop, I’d keep going until I had all my fingers extended again for ten, then I simply start again folding the thumb back in for my eleventh stop, then rotate the main dial twice more for my final two thirds. One problem with counting and clicking with this much neutral density of course, is that if you aren’t working in quite bright conditions, you will go past 30 seconds, and the camera won’t go past 30 seconds just by rotating the main dial, so you have to use Bulb mode and a timer or cable release, which we’ll get to in a moment.

For me, I know that 1/125 of a second is the slowest base shutter speed that I can be using before I will go past 30 seconds when using my ND8 and ND400 together. So, if my shutter speed is is lower than 1/125 a second, I start with two clicks on the main dial, as 2/3 of a stop is more difficult to calculate in my head later, so I like to get that out of the way.

As an example, let’s say, we just we have a base shutter speed of 1/100 of a second. From there I count my first two clicks, for 2/3 of a stop, taking me down to 1/60 of a second. Now I have 11 full stops to count to get my new exposure. That will take me through 1/30, 1/15, 1/8, 1/4, 0.5 and I get to 1 second after 6 stops. Once you get to full seconds, I stop clicking the main dial and start doubling the seconds, so 2, 4, 8, 16 to 32 seconds, which is the time I have to program into my Remote Timer.

Note too that if I want to go even longer than 32 using this same example, I’ll start looking at my aperture and ISO again. Generally, I’ll also be using ISO100, but changing that to ISO50 would take me to 64 seconds. If I was already at f/16, I might consider going to f/22, although I don’t like to because diffraction can start to make the image softer, but if that will take me from 64 seconds to 128 seconds or 2 minutes 8 seconds, then I might go for that, and deal with the diffraction later, using Canon Digital Lens Optimizer, although I rarely find I have to with my current lenses.

Anyway, this may not be a very scientific way of doing these calculations, but it works for me. Remember that a lot of the time I’m all togged up and often even have gloves on, so getting my phone out and using an ND Calculator can take me longer, plus, most of the ND Calculators available don’t give you the ability to stack filters, or they don’t have the ND400 in the list, or they use the optical density instead of the ND number, so I generally find them quite frustrating to use. When we get around to adding the functionality to my iPhone app, it will be a very different story.

Bulb + Remote Switch

Canon Timer Remote Controller TC-80N3

Canon Timer Remote Controller TC-80N3

So, as I said, you will run into problems, especially on Canon cameras, if your exposure goes past 30 seconds, because most of them only go to 30 seconds as their longest camera timed exposure. After that you have to start using the Bulb mode. In Bulb, you have to use a cable or remote release, and time the exposure yourself, or use a Remote Timer, and program the exposure time in, then press the button, and it will start and stop the exposure automatically.

If you have a basic cable release, you need to start the exposure, then time it and release the button to stop the exposure. Most cable releases have a locking mechanism so you don’t actually have to hold the button down the entire time, but I think I’d find it a pain having to sit over a stop watch or count out seconds on my watch while the exposure is made.

Another quick tip here on the use of the Remote Timer–I generally also use a 2 second timer before I start my exposure, so that the camera is totally free of any vibration that I might have caused by touching the camera, by the time the exposure starts. Because of this, I just add two seconds to the exposure time that I set with the Remote Timer. This way the Remote Timer starts the two second timer on the camera initially, and then the real exposure starts two seconds later, and I get the exposure time that I want.

Also, if it’s windy, I tie the remote timer around the tripod head so that it doesn’t dangle and bang on the tripod legs, vibrating my setup. If it’s really windy, I hold the remote timer ensuring that I don’t pull on the cable, which would again wiggle the camera, messing up my shot. I’ve often thought that it would also be a good idea to put some Velcro or magic tape on the back of the timer and another piece on one of my tripod legs, so I could just stick the timer on there while the exposure is running.

Don’t Bother with Mirror Lockup Any More

Note too that I haven’t mentioned Mirror Lockup, basically because I don’t use it any more. Some of the early DSLR cameras with Live View used to jiggle the mirror around before they started the exposure, even when shooting in Live View, so mirror lockup was necessary for a while, but now, if your shooting in Live View the mirror is already up, and doesn’t move when the exposure starts, so mirror lockup isn’t necessary when shooting in Live View, which I use pretty much exclusively when I’m shooting long exposures.

I’m able to do this, even in Bulb Mode, because the Live View on Canon cameras actually still give you a pretty good idea of what you are shooting, with an ND8 and even my ND400 sometimes, depending on how light the scene is before I apply the filters.

ISO Shift Compose Technique

This brings me to another useful technique that I sometimes use when applying a lot of neutral density. Although Live View can give me a pretty good idea of what my image will look like even when I have a certain amount of neutral density applied, once the available light drops or you start to really go heavy on the NDs, even Live View goes dark, and you can’t see to compose your image or focus.

In these situations, you can remove the filters, and check the composition and focus through the lens if there is enough available light, but another trick you can try is to just crank your ISO way up. For example, if you have an ND400 on for just under nine stops of neutral density, and you are shooting at ISO 100, you can crank your ISO up as high as it goes, which will usually give you about eight or nine stops more sensitivity. The image on the LCD gets very grainy when you go this high, but you can at least see to compose your image and usually see well enough to know when your focus peaks, even if it’s not totally sharp.

This is often easier than removing the filters, and is especially useful if the front of your lens rotates when you focus, as you can shift your focus when you screw the filters back on otherwise.

When to Go Heavy?

OK, so earlier we looked at a waterfall photograph, that I shot with a 0.4 second exposure. Waterfalls will smooth over like that from around a quarter of a second, while maintaining a little bit of texture. They really start to smooth over from 1/2 a second or more. I shoot waterfalls with two or three second exposures too, if I want to really smooth the water over. It really depends on the effect you are looking for. I rarely shoot waterfalls in direct sunlight, as they just look terrible with all the contrast. It’s usually either cloudy or I visit them at a time of day that I know they’ll be in the shade due to the position of the sun. This means that you can usually get down to around half a second quite easily with a three stop ND8 filter.

Falling water is moving quite fast though. If you want to smooth over the sea, which moves slower and doesn’t smooth over as easily, or even clouds, that move slower still, you have to use more neutral density. Coincidentally, if we continue using the 1/20 of a second starting exposure that I had before adding the ND8 for the photo we viewed earlier, I would get a 20 second exposure if I used the ND400 instead.

20 seconds will smooth over a relatively rough sea, but won’t give you much cloud movement. Of course, it depends on how quickly the clouds are moving, but generally, if you want cloud movement, you’ll want to get down to at least 30 seconds, and if possible, go down to more than a minute. This photo from the Falklands a few years ago was a 2 minute 30 second exposure, so the clouds have moved significantly. You can also see that the relatively rough sea has smoothed over, but leaving some texture, which I quite like in this shot.

Wrecked Minesweeper

Wrecked Minesweeper

There’s also a seagull in the bottom left of the frame that stayed very still for me during the exposure. There were also some gulls on the boat that moved, leaving a ghostly outline, which I also generally like. Notice too that I more often than not convert my long exposure images to black and white in Silver Efex Pro. Sometimes the color works, but I generally just think long exposures look better in black and white.

Use a Good Sturdy Tripod

One last thing that I want to touch on before we finish, is that once you slow your shutter speed down with neutral density filters, you’re not only going to need a tripod, you’re going need a good sturdy tripod. Don’t think that just because you are going to have to carry it for a while that you can get away with a flimsy little thing. Also, don’t fall into the trap that all Gitzo tripods are good either. Gitzo make a whole range of tripods, and even their smaller carbon fiber tripods don’t cut it for long exposures with heavy gear, especially when the wind gets up. For long exposures you just have to bight the bullet and use a big sturdy tripod. See my episode on what tripods I use and when for more information on this.

So, I hope that was useful for you. As you know, I love long exposures, and try to make time to do them whenever possible. It does take more time, literally, but you really find yourself in a location for a while if you make a couple of exposures, but watch what the clouds are doing, try a few different shutter speeds, try to shoot a few frames, and enjoy the process. Of course, long exposures also work with people on a busy street in the middle of the day, and you maybe get one person that stands still for you. The possibilities are endless once you get into this, so I hope you give it a try if you don’t already do long exposures.


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Show Notes

Why Expose to the Right (ETTR)

What Tripods I use and When I Use Them

Music by UniqueTracks


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