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03-03-2014, 07:31 AM   #1

Join Date: Jun 2008
Location: Flyover America
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why constant aperture zooms?

If you have enough free aperture (the actual optically effective physical size of the objective lens) to give you at, say, a FL of 50mm f/4 why maintain that same F value at 25mm?

It's been a long while since I've taken classic geometric optics (bonehead physics) but so far as I can make out f/4@50mm would require a free aperture of 12.5mm.
So, all else being equal, a free aperture of 12.5mm@fl/25mm should give you f/2.

What am I missing here?

Last edited by wildman; 03-03-2014 at 08:57 AM.
03-03-2014, 07:41 AM   #2

Join Date: Oct 2011
Location: Savannah, GA
Posts: 5,002
I'm sure you will more technical answers, but just because it might be possible to have it as fast as f/2 at wider settings doesn't mean it won't like like crap at f/2. Zooms are are balancing act of priorities and compromises, and IQ is one of them and so there must be a minimum standard of quality that you can expect at any focal length/aperture combo the lens provides, even if inevitably some are better than others...
03-03-2014, 08:33 AM   #3
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If you take the "cheaper" zooms, they do not have constant aperture ratio. You will see that they range from typically some f/3.5 at the wide end to some f/5.6 - f/6.3 at the long end. Only the more expensive zooms boast a constant aperture thoughout the zoom range.

I think, that wat you are missing is this: What you call "the actual optically effective size of the objective lens" - that is: "The Entrance Pupil" - has got very little to do with any single lens diameter. It's a function of the entire zoom design. And in all zooms the absolute size of the entrance pupil varies with focal lenght.

To maintain a large aperture (small f-number) at the wide end requires a significant amount of extra glass and complex lens groups. Just look at the faster wide-angle prime lenses: They aren't exactly small and lightweight. And then look down the large front lens element at full aperture to see, how small the entrance pupil actually is.

As another example, take my DA 18-135mm f/3.5 - f/5.6 lens. At 135mm f/5.6 the entrance pupil will be 24.1mm and at 18mm f/3.5 the entrance pupil will be a mere 5.1 mm. But the diameter of the front lens element is a whopping (relatively speaking) 62mm nontheless.

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