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Old May 1, 2006, 6:28 AM   #1
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On cheaper lenses the aperture gets smaller as you get nearer the telephoto end of a zoom.

I understand that these make manual exposures tricky as you always have to be thinking about the aperture of the lens. I can see the benefits of a constant aperture zoom. One less thing to keep thinking of.

But how do they do it? Does it mean that a 70-200 f/2.8 internally could be a f/1.2 at the wide end but it is reduced to f/2.8 so that it makes the aperture constant throughout the range?
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Old May 1, 2006, 8:28 AM   #2
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No the aperture does not change! :idea:

-> Most folks do not notice this, but in the studio with constant lighting set-up or when the camera is on manual (using a handheld meter), changing the focal lenght will affect the exposure - Hence constant aperture throughout the focal lenght is quite a desirable feature... and have to be designed into the lens (and people pay for)

If they make an f/1.2 at the wide end (and advertise), I'll be all over it (especially with the 120-300 f/2.8 EX)! :lol: :-) :G
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Old May 1, 2006, 3:02 PM   #3
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Paul(UK) wrote:
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But how do they do it? Does it mean that a 70-200 f/2.8 internally could be a f/1.2 at the wide end but it is reduced to f/2.8 so that it makes the aperture constant throughout the range?
Paul - just to add to what NHL wrote:

Aperture is controlled by the diaphragm in the lens - think of it like the iris of an eye. The f-stop (f2.8) is the ratio of that opening to it's distance from the media (film or sensor). As you zoom out, in order to maintain that same ratio, the 'iris' must get larger (since the distance from the media is getting larger). Also, as you zoom you need to pull in more light becuase you're pulling from a smaller field of view to 'paint' the same size media. The ability to have a larger diaphragm opening and collect more light means you need larger lens elements and diaphragm mechanics. This obviously complicates construction which is why you don't tend to see any zooms that have a faster f-stop than 2.8 (I'm sure there are some somewhere). Most lenses capable of wider than 2.8 tend to be prime lenses so there isn't the added complexity of varying the diaphragm opening as you zoom.


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