The Canon EOS and Photography Forums

I said something that might have been wrong, so I thought I would check it with you, Bob.  Does a typical zoom have an f number that increases as a function of focal length?  That is, does a manufacturer have to stop the lens down at shorter zoom lengths to make it behave with constant aperture?

If so, why?   What is so nice about constant aperture?  Or is this a relic of the manual days?

I think constant aperture was important in the good old days when flash and exposure were manually set. With a constant aperture zoom you can set the correct shutter speed then zoom to any focal length and still get the same exposure.

With modern cameras and auto exposure systems constant aperture isn't something that really makes life any easier.

Whether manufacturers deliberately stop down lenses at shorter focal lengths to make a zoomm constant aperture I don't know.

I was just thinking that a zoom should "naturally" increase the aperture since the diameter of the elements stay constant and the focal length changes.  One of these days I may try to figure out the stops per change in focal length, if I can get motivated.

ETA:  I found this discussion which seems to explain it
http://www.dgrin.com/showthread.php?t=58299

The physical size of the elements isn't a good indicator of the lens speed when you get to normal and wideangle focal lengths.

For example take the 50/1.8 and a lens like the 20/2.8.  The 20mm lens has a much larger front element than the 50mm lens, even though 50/1.8 = 28mm and 20/2.8 = 7mm.

As the above reference states, pupil magnification factor has to be taken into account as well as the size and position of any internal aperture stops.

I'm pretty sure that lens designers have to work at it to get a zoom to be constant aperture, just as they have to work to get a lens that dosn't shift focus when zoomed. There are quite a few zoom lenses that do shift focus slightly. This used to be a problem in the manual focus days, but with AF it is somewhat less of an issue.