ISO Invariance in the Canon EOS 5Ds R (Podcast 520)

Thanks for taking the time to explain ISO invariance. My conclusion based upon your discoveries, when using the Canon 5DSr or 5DIII, in order to keep it simple, regardless of ISO one might as well employ an ETTR strategy and then “fix” in post, Trying to maximize exposure by taking into account ISO invariance would be too complex for myself. I suspect the degree of invariance is influenced by both the amount of light and the colour of the light. In other words, each photograph would be a unique situation.

Are you planning on testing the 80D or 1DXII to see what advantages that sensor may have regarding ISO invariance and DR?

Thanks for the interesting article Martin! One (slightly unrelated) question – I have often read that larger megapixel images will inevitably look a little soft when zoomed in to 100%. However if you viewed a 50MP image compared to, say, a 24MP image at a set size (e.g. full screen but not 100%) on a good display (e.g. a MacBook Retina), would the 50MP image look ‘sharper’ due to the fact that more pixels are being viewed in the same size space?

I hope that question makes sense!

Interesting stuff. I haven’t thought about exposure techniques for a while now. Even though I’m sporting an older camera it’s nice to hear.

Thanks Martin! “Invariance” is a new term to me, but in the past I’ve noticed I can push the shadows in my D800 files to blazes. Now I know why.

Thank Martin, another great article. This really hit home for me. I use a 7D ll and at times been confused with post processing results. I always try to ETTR, I just feel I get better results. What is a little confusing is why in post processing can sometimes I lighten a darker photo and it looks good? This has led me to think the same (why am I ETTR)? Your explaining ISO settings does influence this really helps me understand why sometimes it works and other times it’s a disaster. Pushing at lower ISO setting can work but trying to push a underexposed photo in the 3200 – 6400 range just does not get it.

Thanks for a very interesting post Martin. I had never heard of ISO variance but your explanations were very clear and I think I have grasped it now. In wildlife photography I have often heard some shooters talk about underexposing slightly to give them faster shutter speeds and avoid the risk of blowing out exposure if using ETTR. It makes sense in some regards, but I have found that depending on the amount of dark and shadow detail in the image, it would often lead to the introduction of noise when lifting that detail in post production. So, like you I have stuck with ETTR.

I have recently read quite a bit about the new 1DXII and seen some presentations by a couple of Canon sponsored photographers, who showed the results of shooting in low light and it appears that the new model is somewhat different to previous models as it deals much better with noise in the shadow and dark areas of an image. Lifting that detail in post production has shown it to be far more tolerant with cleaner results. That camera is out of my reach financially, but I am hoping that the technology will cascade into other models down the line, like the 5DIII replacement.

So maybe, there may be case for me to re consider shooting to the right in the future as the newer models role out. It’s something I hadn’t thought much about, but thanks top your article I will do in the future.

Very interesting post Martin. Thanks a lot for your time. I have recently listened to this and your ETTR posts. You always say that exposing to the right improves overall SNR in the images you get. It kept me thinking a lot on my way to work.

The noise in any analogue to digital capture system has always two sources: thermal noise and quantisation noise.

When using ETTR what you do is minimise the quantisation noise introduced by the ADC in the camera. Added to that, the logarithmic way the eye sees makes us more sensitive to this noise as we have a great latitude in our eye.

But, there is other very important noise in the system: the thermal noise. The thermal noise is the base noise the sensor, as an analogue system, has. It is always present. When we expose the sensor with light we have the sum of the desired signal (the light photons in the sensor) plus the thermal noise of every photodiode.

Using higher ISOs is said to be worse for noise, but in reality what it is is worse for the signal. Let’s say we are shooting in Av Mode. Asume that for a given scene we need a shutter speed of 1/100 for ISO 100. This means that we are going to have a value of say S photons at the sensor and a value of N noise. This gives an SNR of S/N. If we increase the ISo to 200 and change the shutter speed to 1/200 to maintain the aparent exposure level, what we get in the sensor is S/2 photons and the same amount of noise N. The SNR is then (S/N)/2.

If the value of N is low enough then the quantisation noise introduced by the analogue to signal converter is going to be the main source of noise, but if it is not, then the main source of noise is going to be the thermal noise.

IMHO this is why shotting with the highest possible quantity of light in the sensor is the best rule of thumb always. This usually means (but not always) using the lowest possible ISO, thus increasing real exposure (aperture and shutter time).

The truth is that the ISO is not changing the real exposure, only the apparent result.