Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Tom_N Forum Pro • Posts: 16,610
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

f-stops are already normalized.  The letter 'f' stands for the focal length of the lens, which will vary with the sensor size for the same field of view.

Thus, for instance,

f/2 on 50mm on an APS-C body => 25mm absolute aperture

f/2 on 75mm on a full-frame body => 37.5mm absolute aperture

The (APS-C, 50mm) and (full-frame, 75mm) combinations will produce the same field of view, and the different absolute apertures will keep the light intensity per unit sensor area the same – for the same subject and lighting conditions.

Without the 'f' notation, photographers would need to keep the different absolute sizes in mind as they went from one sensor size to another.  With it, they can just assume that f/8 means f/8 means f/8 regardless of the size of the sensor or the focal length of the lens.

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