FStop Question! (I can teach some things, but I'm still learning most things)

[TCimages]

maybe this will help some folks:

 

[Dubious Drewski]

Oh yeah TC, primes are easy enough to understand, it was really just the zooms that had me(us?) stumped.

You guys are awesome, by the way.

But there's still this one thing:

Even with zoom lenses that don't have constant aperture the diameter of the entrance pupil changes. If it didn't the change in f-number would be in the same proportion as the change in focal length.

in lenses like yours, the aperture is at it's maximum physical size at any point in the zoom...

These two quotes seem to contradict each other. One says the physical diameter of the aperture changes during zoom while the other says it doesn't. I'm not sure what to think about that!

 

[alabama1980]

Gotta wonder if some of you people get headaches from having all that knowledge crammed in there....lol. This post explained a lot to me as well, so thanks!!

A

 

[Helen B]

But there's still this one thing:


These two quotes seem to contradict each other. One says the physical diameter of the aperture changes during zoom while the other says it doesn't. I'm not sure what to think about that!

I am referring to the entrance pupil, not the physical diameter of the aperture. They are two different things, and should not be confused.

I have a number of constant-aperture zooms. None of them vary their physical aperture while zooming, but they all vary the entrance pupil diameter. The entrance pupil is not a physical object, it is an image of a physical object. It's quite important to understand that the f-number is not the focal length divided by the diameter of the physical aperture, but it is the focal length divided by the diameter of the entrance pupil.

As an aside, the fastest lenses tend to be 'normal' focal length lenses. As the focal length gets longer, the entrance pupil has to get larger to maintain the same f-number - that's quite easy to understand. The saving factor is that the angle of view gets smaller, and the ratio of entrance pupil size to front element size can increase. For many telephotos the entrance pupil is roughly the same size as the front element.

As the focal length gets shorter the entrance pupil can get smaller. That would suggest that it gets easier to make fast wide angle lenses, but it isn't. The wide angle of view usually means that the front element has to be much larger than the entrance pupil.

This relationship between entrance pupil and front element diameter is easy to see with primes. Here are some examples of my 35 mm full-frame SLR primes:

A 24 mm f/1.4 has an entrance pupil diameter of 17 mm, but a front element diameter of 51 mm, a ratio of 1:3.

A 50 mm f/1.4 has an entrance pupil of 35 mm diameter and a front element of 37 mm diameter.

That's all highly simplified, but I hope that it gives the general idea.

Best,
Helen

 

[Monz]

I was going to ask why I lose f22 when I zoom and I think I understand now thanks