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

Started Jul 24, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 55,138
Re: Focal length 35mm equivalent, but not F-stop?

eyeswideshut wrote:

Lee Jay wrote:

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"?

Read on.

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.

Says who? Great Bustard? You see, the reason I call this exercise 'equivalism' rather than 'a paradigm of equivalence' is because it has taken on all the trappings of a religion with converts, neophites, disciples and masters. If you had read and engaged what I said in a simple four line paragraph you could have saved yourself all the rest:

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

See, once you understand (and respect) that most people use equivalent focal length as shorthand for angle of view and have done so for the longest time, you will understand that there is no need to bring in the entire equivalist model every time someone says 'this lens is the equivalent of a 50mm lens on ff'. If that person had said it is a 45° AOV lens you would no longer want to quibble, right?

Depends.  If they specify aperture then, in that case, they should do so the way we do it with telescopes - aperture diameter.  I have an 11" telescope.  Hardly anyone would call it a 2,800mm f/10 lens.

If you want to specify actual aperture, rather than f-stop, then all this goes away.  Same with angle of view though in that case you're including sensor size.

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