I had a person talking yesterday about long exposure, high ISO, etc. Really for long exposure photos during the day. They mentioned shooting at higher ISO, mentioning ISO 6400 as one example. Another person stated the noise at that level would be deafening. And for many that is quite likel so.
However, some cameras are very ISO invariant, but not all. What is ISO invariance? Really it is the ability to shoot in as underexposed and then increase the ISO dramatically, maybe by half a dozen stops, and get the same photo essentially as one shot at that higher ISO level.
But before we go into that there are even some crop bodies, that while not ISO invariant can shoot at amazingly high ISO levels. Here is one I did a test on with Nikon D500 – shutter speed of 1/50th second – f/5.6 – using a low cost kit lens. It should be noted that the D7500 should be able to do very close or even equally as well. This photo was taken in low light – where you could not read a book – and has zero post processing applied. Yes there is noise in the photo – but ISO was – are you ready for this – ISO 51,200.
But lets get back to the topic of invariance. having tested several cameras the ones which appear to be invariant to at least some degree were the Pentax K5, Sony A7RIII, Nikon D810, D750, D7100 (tested to ISO 800 only), D500. Of these the D750 was the best with the D810 being quite close. The D750 was tested to ISO 6400 and someday I may push further. To test invariance – set camera up at ISO 6400 and adjust exposure and take photograph (in RAW). Leave shutter speed and aperture the same and drop ISO to 100 – and take second photo (again in RAW). Put photos into Lightroom and increase exposure on the ISO 100 photo (which will be greatly underexposed) by six full stops. Then compare the two photographs. Don’t be afraid to zoom in and be critical. For a truly invariant camera there will be very little difference in the two photographs. I have yet to see any Canon cameras that are even close to being invariant. I am going to test some Fuji cameras as I was told they have some invariant models. Is invariance important? Certainly for daylight photography it make zero difference or benefit. For nighttime photography and astrophotography it can make a difference. This is one reason the D750 does so well at astrophotography (although the D810 or D810A are great as well with their higher pixel count).
ISO Invariance? Should you care? Not likely one little bit!! It is something that will impact few photographers and your camera will take great photos in normal daylight regardless of the invariance capabilities.
But if you want to play here is a guide on how to do it. Take you camera and set at a specific shutter speed and aperture as mentioned above. Let’s say f/2.8 and 20 seconds with ISO 3200. This is a fairly reasonable level for night photos and you will likely get a fairly decent photograph. Now keep shutter speed and aperture the same and push ISO down to 100. Take another photograph and as mentioned above it is now WAY underexposed. But take that photo into Lightroom and bring the exposure up, in this case five full stops. Now increasing exposure in Lightroom will add noise to the photo but with an invariant sensor the noise added by Lightroom should be very close to the original photograph. If you are able to do this then you have a camera with an invariant sensor! With the tests I have done, I have yet to find a camera from Canon which is anywhere near close to being invariant, however there are several Nikons and likely several Sony cameras as well.