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Forum: Troubleshooting and Beginner Help 05-11-2017, 10:25 AM  
f-stop crop
Posted By photoptimist
Replies: 6
Views: 836
The T-stop for a given lens should be the same whether mounted on an APS-C or FF camera.

But two different lenses of the same f-stop may have different T-stops because the T-stop depends on the number of elements in the lens and the quality of the lens coatings. Old film lenses tend to have fewer elements (good) but worse coatings (bad).
Forum: Troubleshooting and Beginner Help 05-11-2017, 09:11 AM  
f-stop crop
Posted By photoptimist
Replies: 6
Views: 836
Short answer is no, the FF lens is not slower. An FF lens has a wider field of view than it's APS-C version which means it is collecting proportionally more total light to cover that larger image circle.

Aperture and image circle are totally independent of each other although it is optically challenging to design and build lenses of given focal length (especially wide angle lenses) that have both big apertures and big image circles.

Another way to think of it is that the focal length and aperture are physical properties of the lens and the lens' properties (e.g., brightness & depth of field) don't change just because you've mounted it on a camera with a bigger or smaller sensor. The depth of field of a new APS-C lens on an APS-C camera will be identical to the depth of field of an old full-frame film lens (of the same focal length and aperture) on an APS-C camera.

Where things get messy is in comparing depth of field across formats (APS-C vs. FF) because creating the same picture on two different cameras involves using different focal lengths, different optical magnifications, and different blur circle diameters.
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