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04-03-2010, 06:03 PM   #16
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Needless to say, faster variable-aperture zooms do exist: the only thing about it is, who'll buy them? Constant 2.8's are pretty easily-available now to anyone who can spend enough, and beginners simply aren't sold lens speed, in an age of AF.

I'm actually sporting a 2.8-4 28-105 lens now, (First time I deliberately bought a variable aperture lens in my *life,* mind you, ) ....Quirky, but workable, considering I don't tend to expect much of zooms. But if you were a lens-maker trying to sell the things new... what would you do, try to sell something with more reach than a 28-75, or expect someone who wants the fast aperture to want it consistent? You'd sell the latter, really.

It's not really a hard and fast rule, just what's practicable and marketable.

04-04-2010, 03:32 AM   #17
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I have been invited to share my voice on this topic. So, here we go
QuoteOriginally posted by pingflood Quote
The diameter of the aperture isn't directly related to its physical size, but rather the apparent size as viewed through the front of the lens.
QuoteOriginally posted by bdery Quote
Sorry, that's not true. The aperture, as reported in f-numbers, is a ratio of the physical aperture and the focal length.
On topic, I think that "Part II - Apertures, f-stop, bellows correction factor, pupil magnification" in Lens Tutorial - sums it up pretty well. I.e., not only size but virtual position as well does matter.

I.e., anybody not understanding what p (pupil magnification) means may want to refrain from discussing it.

For all practical reasons (i.e. ignoring ultra wide angle lenses), the diameter in question is the size of the lens front element divided by p if p>1. Typically, p=1 except for retrofocus wideangle lenses where p>1. Some telephoto lenses have p<1 which is why I made the constraint above.

So, while it is technically correct to define the F-stop as focal length/entrance pupil diameter, it is IMHO good practise to simplify this rule to focal length/aperture diameter where aperture is meant to mean the lens aperture, not necessarily the iris aperture.

Because exposure and perspective depend on the entrance pupil while depth of field depends on the exit pupil (cf. Depth of Field in Depth), I for the sake of piece in this forum, strongly suggest to stick to some simplification here

Last edited by falconeye; 04-04-2010 at 04:16 AM.
04-04-2010, 08:59 AM   #18
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QuoteOriginally posted by flyer Quote
I'm pretty sure he knows a lot more about that stuff than all of us together. It's his job.
According to the usual optical standard books, Pingflood is quite right: The f-number = relative aperture is calculated by dividiing the focal length by the entrance pupil - not by the absolute dimension of the aperture opening.

Obviously the entrance pupil can be the same as the front lens diameter (extreme case), which you will find in telescopes. Scopes usually have a verysimple optical layout. Photographic lenses are more complicatet, especially if they sport sophisticated designs like IF, fixed f-number over their entire fl range or do not change thewir dimensions while zooming. In these cases the entrance pupil can be very different from the absolute dimension of the aperture opening.


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