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

Started Jul 24, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 55,159
Re: Focal length 35mm equivalent, but not F-stop?

eyeswideshut wrote:

Lee Jay wrote:

John Sheehy wrote:

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-numbers are ratios of real focal length divided by physical aperture size, and have nothing to do sensor sizes.

If manufacturers used the "35mm equivalent focal lengths" in the ratio, then they'd also have to scale the ISOs also to make exposure triangles work. Do you want that kind of a mess?

Absolutely I do.

Lee, no one is stopping you from engaging in all those calculations. Get down and dirty with the math!

Why is multiplying focal length by crop factor easy and obvious while multiplying f-stop by crop factor is "getting down and dirty with the math"?

For the great unwashed though, focal length is what it has always been: shorthand for angle of view. It's been that way for ever and ever and ever. And that is all that manufacturers are implying when they give equivalent focal lengths. Only when you ignore that simple semantic fact can you meaningfully go off on the equivalist tangent.


Equivalence includes focal length equivalence as well as f-stop equivalence.  In fact, what it's saying is that you shouldn't use one without the other.

f-stop = focal length / aperture

f-stop (equivalent) = focal length (equivalent) / aperture

Another way to put it is simple algebra - you have to do the same thing to both sides of the equation.

f-stop = focal length / aperture

multiply both sides by crop factor

f-stop * crop factor = focal length * crop factor / aperture

recognize that focal length * crop factor is focal length (equivalent) and you have:

f-stop * crop factor = focal length (equivalent) / aperture

now, rename the left side:

f-stop (equivalent) = focal length (equivalent) / aperture

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Lee Jay

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