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05-13-2007, 12:40 PM   #1
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Variable vs. fixed aperture zooms: am I missing something here?

Or maybe I should rephrase this as "Why aren't all zooms at fixed apertures?"

Until a few quick calculations a minute ago, I thought that most zooms had variable apertures because it's cheaper and easier to keep the diameter of the diaphragm opening the same at all focal lengths. For a fixed-aperture zoom to work, presumably you'd have to increase the diaphragm diameter as focal length increases, adding additional complexity and production costs.

But here's what is confusing: take, for example, the kit lens, which wide open is f/3.5 at 18mm and f/5.6 at 55mm. At 18mm the diaphragm is 5.14mm in diameter (18/3.5 = 5.14), but a 5.14mm diaphragm opening corresponds to only f/10.7 at 55mm (55/5.14 = 10.7). The diaphragm would have to increase to 9.8mm at a focal length of 55mm to be at f/5.6.

So if the diaphragm actually does increase in diameter in a variable aperture zoom, what's so special about a fixed aperture zoom? Or do the normal focal length calculations not stack up when you're dealing with zooms? You can't argue with physics...

05-13-2007, 12:56 PM   #2
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ummmm, for the 55mm to stay at f/3.5 instead of f/5.6, it would have to increase MORE than it would for it to be f/5.6 (coming from f/3.5 at 18mm). If you stay at f/3.5 for 55mm, you need a 15.7mm aperture vs. a 9.8mm. Last time I checked, 15.7mm > 9.8mm. And more glass=more $$$

The increase in glass needed becomes greater when you are looking at longer telephoto lens.

Take a 70-200mm. If it is f/4-f/5.6, the aperture would have to go from 17.5mm (@70mm) to 35.7mm (@200mm). If you have a constant f/4 70-200mm, you need to have a 50mm aperture at 200mm. Now if you go to a constant f/2.8, you need a aperture that goes from 25mm to 71.4mm

I'm still not entirely sure where your confusion comes from. More glass=more $$$.
05-13-2007, 01:22 PM   #3
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Finn, your math is off. The linear relationship is about the area of the opening, not diameter. f-stop = FocalLength / LensOpeningArea. Also, there is no simple relationship from the calculated LensOpeningArea to the size of the front and rear elements (and hence cost).
05-13-2007, 01:32 PM   #4
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That's true. I was thinking about it in a different way, that a fixed aperture zoom would require the diaphragm to open (a lot) as the focal length increased, to maintain a constant aperture across the entire range of focal lengths. I suppose the increased cost is not from the continuous movement of the diaphragm per se, but rather the increase in the amount of glass required to have a large aperture at a long focal length.

I always assumed that most zooms were variable aperture because the diaphragm remained the same diameter across the entire range of focal lengths, and the difference in aperture was just a mathematical consequence of varying one part of the equation and not the other (i.e., increasing the numerator while keeping the denominator constant in the equation: aperture = focal length / diameter).

05-13-2007, 01:33 PM   #5
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QuoteOriginally posted by Ole Quote
Finn, your math is off. The linear relationship is about the area of the opening, not diameter. f-stop = FocalLength / LensOpeningArea. Also, there is no simple relationship from the calculated LensOpeningArea to the size of the front and rear elements (and hence cost).
Ah, so that's where I'm going wrong. I guess this is what happens when you try to do math on the weekend.
05-13-2007, 02:47 PM   #6
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There is another issue with the kit lens. It is actually a stretch to have it be f/3.5 at 18mm. Vigneting is so severe that the max. aperture really should have been limited to perhaps f/4.5 or f/5.6. The front or rear elements are simply too small to perform well at f/3.5 at 18mm.
05-14-2007, 06:24 AM   #7
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QuoteOriginally posted by Finn Quote
Or maybe I should rephrase this as "Why aren't all zooms at fixed apertures?"

Until a few quick calculations a minute ago, I thought that most zooms had variable apertures because it's cheaper and easier to keep the diameter of the diaphragm opening the same at all focal lengths. For a fixed-aperture zoom to work, presumably you'd have to increase the diaphragm diameter as focal length increases, adding additional complexity and production costs.

But here's what is confusing: take, for example, the kit lens, which wide open is f/3.5 at 18mm and f/5.6 at 55mm. At 18mm the diaphragm is 5.14mm in diameter (18/3.5 = 5.14), but a 5.14mm diaphragm opening corresponds to only f/10.7 at 55mm (55/5.14 = 10.7). The diaphragm would have to increase to 9.8mm at a focal length of 55mm to be at f/5.6.

So if the diaphragm actually does increase in diameter in a variable aperture zoom, what's so special about a fixed aperture zoom? Or do the normal focal length calculations not stack up when you're dealing with zooms? You can't argue with physics...
I have seen others comment on this and the real issue is that it is cheaper and produces usually a smaller lighter zooms.

In the normal to telephoto range, this is generally due to the front element diameter (or filter size) , but on wide angle lenses, the front diameter is a function of field of view so the filter diameter is not really defined directly by front element.
05-14-2007, 09:50 PM   #8
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QuoteQuote:
Until a few quick calculations a minute ago, I thought that most zooms had variable apertures because it's cheaper and easier to keep the diameter of the diaphragm opening the same at all focal lengths. For a fixed-aperture zoom to work, presumably you'd have to increase the diaphragm diameter as focal length increases, adding additional complexity and production costs.


So if the diaphragm actually does increase in diameter in a variable aperture zoom, what's so special about a fixed aperture zoom? Or do the normal focal length calculations not stack up when you're dealing with zooms? You can't argue with physics...
I think you have your terminology backwards. Aperture is the "hole". Diaphragm is also the "hole". You can not simultaneously have a fixed yet variable aperture/diaphragm.

If the hole size stays the same as the focal length increases, that causes the variable f-stop.

If the hole increases as the FL increases, that makes the a fixed f-stop lens.

05-15-2007, 03:20 AM   #9
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Okay, to continue the theme here, if the camera says I need 1/60s at f4, will I require 1/60s on ALL lenses set to f4? Or will "faster" lenses require a faster shutter? (All other things being equal of course).
05-15-2007, 04:22 AM   #10
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QuoteOriginally posted by Arpe Quote
Okay, to continue the theme here, if the camera says I need 1/60s at f4, will I require 1/60s on ALL lenses set to f4? Or will "faster" lenses require a faster shutter? (All other things being equal of course).
The exposure is the same -- the faster lens is only potentially faster. However, lenses generally perform best stopped down a little bit -- that is, not at their maximum aperture. So the faster lens stopped down will probably have some degree better image quality. That advantage won't necessarily persist if you stop down even further, though.
05-15-2007, 05:00 AM   #11
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Nope!

.... The "hole" for 18mm f/3.5 is ~5mm. It increases to 9.8mm for 55mm f/5.6. But the kit lens is definetly not a fixed/constant aperture lens.

This was what confused the OP, the aperture can increase, but NOT enough to maintain the same aperture as the shorter focal lengths. To maintain f/3.5 at 55mm, the hole would have to increase to 15.7mm.

Folks, this isn't rocket science! Size of hole=focal length/f-stop.

QuoteOriginally posted by SpecialK Quote
If the hole size stays the same as the focal length increases, that causes the variable f-stop.

If the hole increases as the FL increases, that makes the a fixed f-stop lens.
05-15-2007, 05:05 AM   #12
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Not really sure what you are asking. Say the meter gives you f/4, 1/60s. What do you mean by a "faster" lens? A lens that is capable of being faster than f/4? Well, if you stay at f/4, then you need to keep the shutter speed at 1/60s. If you open up to f/2.8, you go to 1/120s. f/2->1/240 (~1/250s), f/1.4->1/500s. (or you could decrease the ISO, which might be a good idea if the f/4, 1/60s has you at ISO3200!)

When you change one exposure parameter, you need to change one of the other two in the "opposite" direction to maintain the same EV (exposure value)

All other things being equal of course!

QuoteOriginally posted by Arpe Quote
Okay, to continue the theme here, if the camera says I need 1/60s at f4, will I require 1/60s on ALL lenses set to f4? Or will "faster" lenses require a faster shutter? (All other things being equal of course).
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