Help me understand "total light" and sensor size...

Started Mar 8, 2011 | Discussions thread
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Help me understand "total light" and sensor size...
Mar 8, 2011

One heck of a discussion arose from this concept on another thread:

I'm trying to grapple with something that could change the way I understand the exposure triangle (secular to sensor size)...

Given that f/4 on FF equates to f/2.5 on APC-C, that has to speak only to the DOF equivalence in my current paradigm since the image circles through a lens at f/4 are the same regardless of the format of the sensor.

For the sake of comparison, let's say that I'm shooting my 7D with the 24-105 at 24mm f/4. Likewise, the pro next to me is shooting his 5D2 with the 24-105 also at 24mm f/4. I realize that the field of view is going to be substantially wider on on the FF, all other things being equal. So, based on this hypothetical, I ask the following:

1) Accepting the resolution loss from cropping the 5D2 image to match the APS-C field of view at 24mm, did the 5D2 just get away with more that a stop lower ISO?

2) Let's now say that the 5D2 guy next to me zooms in to 38.4mm to match my field of view. I accept that he has a shallower DOF, but no cropping is required to compare the images. In this new situation, I again ask, was he able to get away with more than a stop lower ISO?

Always learning,

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