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I once wondered, in low light would it be better to shoot underexposed pictures with a low ISO and raise the exposure in post, or to shoot a properly exposed picture with high ISO? The answer to this question answered my question.

When looking at a review on the 5D mark IV, I read something interesting about the camera's noise floor:

"[It] is able to capture a large amount of dynamic range, since it adds very little noise to the detail captured in the shadow regions of the image... it minimizes the need to amplify the sensor's signal in order to keep it above [the] noise floor."

When taking a shot at 100 ISO and increasing the exposure by 6 stops, it holds true that the photo would be better off if it was taken with 6400 ISO. But interestingly at 400 ISO, this doesn't hold true any more:

"[The camera] exhibits ISO invariance, meaning that you could... shoot at 400 ISO... raise the exposure 4 EV in post... [and have] little to no noise cost relative to shooting at ISO 6400."

This leads me to believe the answer I had read previously is only partially true. It seems that there is some point which the processed "low ISO" photo will outdo the high ISO photo in terms of noise, if it is at the perfect ISO. For the case of the 5D mark IV, it seems to be very low, around just ISO 400.

Is there any validity to my assumption and if so, how can I find this optimal ISO for my Canon t5i that will result in the minimal amount noise after post processing?

  • This depends on both quantitative (noise in sensor) and qualitative (how much you care :-) ) evaluation. Best thing to do is take a bunch of bracket exposures over a range of ISO and a range of exposure times. Then see which ones you like, which ones you can improve in post processing, etc. – Carl Witthoft Oct 5 '16 at 13:19
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You're correct that things have moved on a bit since that answer was written; Sony sensors, which are also used in Nikon SLRs, have been significantly better at this than Canon's sensors. However

there is some point which the processed "low ISO" photo will outdo the high ISO photo in terms of noise

is not true. The DPReview quote says "little to no noise cost", not "negative noise cost" - i.e. a photo taken at ISO 400 and raised to ISO 6400 in post will have the same, or maybe very slightly more, noise than one taken at ISO 6400 in camera. This is different from earlier cameras where pushing a photo taken at ISO 400 to ISO 6400 in post would have significantly more noise than one taken at ISO 6400 in camera.

how can I find this optimal ISO for my Canon t5i that will result in the minimal amount noise after post processing?

Nothing has really changed here - you should still be aiming to get it right in camera. The difference now is that the penalty for getting it wrong is much lower than it used to be.

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