f-stop and zoom

[rishi]

f-stop is defined as focal length divided by the diameter of the opening.
When you have a lens with variable focal length (zoom), does the f-stop value change when the focal length is changed? If not, what is the focal length used when numbering the f-stops on the selector?

 

[EOS_JD]

Not sure what you mean (I'm not that technical) but you see the variable aperture noted on the lens.

for example 70-300 f4.5-5.6. or 17-35f2.8-4

At the wide end the lens is faster (larger aperture) and as it zooms the aperture gets smaller.

70-300 would be f4.5 @ 70mm and f5.6@300mm

Fixed max aperture lenses are generally better

Edit
(read the word fixed as constant) - the aperture is obviously not fixed.

 

[Big Mike]

When you have a lens with variable focal length (zoom), does the f-stop value change when the focal length is changed?

Essentially, yes it does. That's why you see zoom lenses with a variable maximum aperture. For example, the Canon 18-55 F3.5-5.6; at 18mm the ratio is equal to F3.5...but when we zoom out (change the focal length to 55mm) the ratio is now equal to F5.6. This is because the aperture diameter does not change...that is it's maximum size.

Higher end zoom lenses have 'constant max apertures'...usually F2.8...that is a funny thing to call it because actually the aperture is not constant but the F-stop ratio is. For this to work, the aperture has to get bigger as the lens is zoomed out.

 

[Torus34]

Rishi;

Your question is not accurately worded. As asked, it cannot be answered.

f stop is indeed the diameter of the diaphragm divided into the focal length. In designing a zoom lens, though, it's easier to include a mechanism which indexes the diaphragm diameter to the fl and adjusts it as the lens is zoomed in or out. Otherwise, you would have a very complicated set of f stop markings to contend with when you wished to set the aperture manually.

The indexing system runs into a problem when it has to deal with the maximum available lens aperture. When the aperture is fully open [maximum diameter], it will give a different effective f stop at one end of the zoom range than at the other. For this reason, zoom lenses often have two different maximum f numbers stated.

Hope this helps.

 

[fmw]

The longer the lens physically, the less light passes and hence the loss of maximum aperture as it zooms to a longer focal length.

Constant aperture zooms are designed so that the internal length of the lens (front focal length minus back focal length) remains constant by moving elements and groups in between the front and back of the lens without moving the front and back themselves. So the lens can produce varying back focal lengths (the ones printed on the lens) and varying subject magnification without getting shorter or longer physically. In practice the length does vary a little but not enough to matter in terms of exposure. Constant aperture zooms are more difficult to design and more expensive to make because they are more complex. There are other advantages to constant aperture zooms also. They are theoretically sharper because they can have aberrations more easily corrected because the light path doesn't expand and contract. Any time you correct a moving target, you have some compromises to deal with and the more the target moves, the greater the compromise. On the other hand they also have a little less contrast than variable aperture zooms because of the additional complexity.

You should also note that focusing affects the aperture as well. Most lenses do change the length of the light path when they focus. The more closely they focus, the longer they are physically and the less light passes. This is usually trivial except in macro lenses which expand and contract quite a bit as they focus.

 

[jmmtn4aj]

Higher end zoom lenses have 'constant max apertures'...usually F2.8...that is a funny thing to call it because actually the aperture is not constant but the F-stop ratio is. For this to work, the aperture has to get bigger as the lens is zoomed out.

So why not make it NOT constant and have a larger maximum aperture at the wide angle end?

 

[Big Mike]

So why not make it NOT constant and have a larger maximum aperture at the wide angle end?

Maybe I'm mixed up on that...and it's more complicated, with internal elements moving around, as Fred points out.

I've heard that question asked before, I'm not sure if why they don't do that, if it's possible. Maybe it's because a bigger aperture than F2.8 would cause more problems and significantly decrease image quality. Typically, constant F2.8 lenses perform better at F2.8 than cheaper lenses do at their maximum (F3.5-5.6 etc.)

 

[Torus34]

Ray tracing can help to answer some of the uncertainty on this topic. If you take time to accurately draw the x-section of a lens, say a Tessar for simplicity, and then check what happens as you increase the aperature of the diaphragm, you'll find that there comes a point where you will need a wider front or rear element or else you will begin to 'vignette' the corners of the focal plane image.

 

[NEPats37]

I think I asked this question awhile ago. I was confused because my Canon FD lens was variable aperture(f stop changes as you zoom in and out). I noticed on my lens there were two marking for the desired aperture to line up with. One was pink the other was blue. They conresponded to the zoom length which is 35-70.
The best way I can explain it (on my lens atleast). If I am taking a picture of a subject and have my lens set at 35 I can go to atleast a 2.8 aperture. If I decide to zoom into that subject the widest aperture I can go to is 3.5. Also when I am at 35 I need to line up the desired aperture with the blue line. If I am at 70 I need to line up the desired aperture with the pink line(which is always 1/2 stop smaller,or towards the larger number.)
I hope this helps at all.

 

[EOS_JD]

So why not make it NOT constant and have a larger maximum aperture at the wide angle end?

Why have that when you can have a constant f2.8?
If I want it not constant I'd buy cheaper glass.

 

[Torus34]

The easiest way to get a 'constant' maximum f stop is to design the aperture/fl compensating mechanism so that it stops the WA end of the scale at the maximum of the telephoto end of the scale. In other words, equality is achieved by limiting the maximum f stop to that of the maximum fl.

Or, to quote a well-known ancient philosopher, 'There ain't no such thing as a free lunch.'