ruthenium
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gdanmitchell wrote:
I'm sorry, but with the possible exception of some true edge cases, that simply is not true in real world photography. You are trying very, very hard adapt reality to fit your theory...
I shoot APS-C and full frame format extensively, sometimes side by side. In a situation where I do not need a higher ISO (say for low light) I typically use the camera's base ISO. On my FF system that happens to be 100 and on my APS-C system that happens to be 125.
Let's say the light is a bit lower, such that ISO400 seems warranted on my FF system. I'm almost certain to set the APS-C system to 400 as well.
In very low light and/or situations where I must keep the shutter speed shorter, I'll push both of those cameras to ISO1600 or ISO3200. (Adobe AI Denoise does an excellent job of handling the resulting noise.)
So, what about those edge cases?
If, for some reason, I actually intended to make the same extremely large print from my FF and APS-C systems (not likely) and I needed _exactly_ some particular aperture for DOF reasons, I would use about one aperture larger on the APS-C system or one smaller on the FF system. Similarly, in typical photography my default aperture (when DOF, light, or subject motion aren't issues) I'll go with one stop larger aperture and a longer shutter speed on the APS-C system — not for "more light" but to be closer to the aperture that provides the best system sharpness. (E.g. "f8 and be there" becomes "f/5.6 and be there.)
Overall, this whole "more light" business is yet another example of people going way off the track of what is actually meaningful and important in photography, in search — I suppose — of some kind of minuscule technical perfection that they imagine is what defines great photography, but which typically makes essentially no difference in their work at all.
I'm not against understanding how these things work. That is often useful. But I am these obsessions that are the equivalent of debating the thickness of the Ines of kitchen forks, the perfect flexibility of hairs on a paint brush, or whether we should tune orchestras to A-440 or A442... or the number of dancing angels that can fit on the head of a pin...
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Dan, the opening statement from Daran "taking an equivalent photo on a smaller sensor also requires using a lower ISO" is 100% correct. To be specific, to be equivalent, the ISO on the smaller sensor shall be reduced by the crop factor^2. That is, by 2.25 for APS-C vs FF. For example, for ISO 3200 on a FF camera, the equivalent ISO on an APS-C camera must be 1422 (understandably, the latter camera shall use the nearest to 1422 available ISO value in practice).
When you "push both of those cameras to ISO1600 or ISO3200" you are not capturing equivalent photos: the SS on your APS-C camera shall be faster and therefore the resulting images shall be noisier. As a rule of thumb, the ISO on your APS-C camera should be 1/2 of the ISO on your full-frame camera, e.g. 1600 and 3200, respectively.
Why is this important in practice? Because not knowing photographic equivalence makes some (many?) photographers actually believe that larger sensors have better "high ISO performance" than the smaller sensors. They may even support this argument by using Bill Claff's PDR numbers by comparing the sensors at one ISO, e.g. 6400. This belief is supported by another bogus claim that "larger sensors collect more light", which is not true at high ISO, when a sensor can collect only as much as it is allowed by the aperture and the SS - the sensor size is irrelevant.
Knowing photographic equivalence is not an obsession - this is about being literate in photography and operate a camera with full understanding of how it works and what the camera settings actually mean. The alternative is preached by some photographers who proclaim that their life-long experience of using a camera is better. I respectfully disagree and prefer combining solid knowledge with intuition instead of relying on pure intuition alone.
Edited on Sep 21, 2026 at 02:01 PM · View previous versions
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