ISO Invariance

So this is new to me and I am not sure I fully understand all the physics behind it. All my previous cameras have not been ISO invariant, that is to say to minimise noise you needed to make sure the image was not underexposed by using an appropriately high ISO. Using a lower ISO and underexposing an image and then pushing the shadows in post would create a noiser image than using a higher ISO and “proper” exposure. Pretty standard stuff and well understood.

Now with the Sony A7Riv with its BSI sensor this is not fully the case. In essence the camera has 2 base ISOs namely 100 and 320 at other ISOs the camera is said to be ISO invariant. This can be seen from the chart below of read noise.

So what in practice does this mean ? I am still trying figure out what the real consequences of this are but in essence it means, for example, there is no need in low light to shoot at say ISO 5000 as the resulting image will be no better/worse than if shot at ISO 320 and then pushed 4 stops in post.

So how does this help ?

Well conventional wisdom is that detail in the shadows is best preserved by exposing as far to the right of the histogram as possible (ETTR) without blowing the highlights. With an ISO invariate sensor it strikes me that this is not the case as there is no need to risk blowing highlights as within reason the shadows can be lifted without a noise penalty in post, for landscape and astro photography where you are often having to deal with a scene of high dynamic range this potentially is of quite some significance and may in some circumstances even mitigate the need to bracket exposures. Furthermore and perhaps an even bigger advantage in having an ISO invariant sensor is that as ISO is increased the dynamic range available is reduced, see chart below, thus by keeping the ISO down a better dynamic range will be captured. In essence therefore it appears that the A7Riv should be used at one of its base ISOs namely ISO 100 (rather than any other ISO up to 320) or 320 rather than anything higher. It is also interesting to note from the charts that there appears to be no penalty in either noise or dynamic range terms from shooting at the expanded ISO rate of ISO 50.

To date all of this is theory and based upon what I have read rather than tested but I will do some real world tests on this as potentially it may lead to quite a different way of working.

As an aside below is a graph that shows the dynamic range of the Sony A7Riv and in this case I have compared it with the Canon R5 (a significantly more expensive camera in the UK/EU and Canon’s best mirrorless camera) and the Nikon D850 DSLR (aknowledged as one of the best landscape cameras), this graph shows one of the reasons, coupled with the extra resolution, as to why I chose the Sony A7Riv over any of Canon’s latest offerings.

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