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04-27-2011, 10:22 PM   #1
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Constant Aperture

Hi, I have a Sigma 70-300mm f4-5.6 lens. Now, if I use it at 70mm I can dial in f4 but when I zoom to 300mm the camera automatically adjusts the aperture to f5.6 and will not go any wider than f5.6 - not much good in poor light.

However this lens can be taken off the "A" setting and the aperture adjusted manually... Now, if I set the aperture manually to f4 and leave it there, it will then effectively be a faster f4 lens right through from 70 to 300mm won't it? Or would forcing things this way somehow affect the IQ or something else?

Just curious ...

04-27-2011, 10:34 PM   #2
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Your lens is a variable aperture lens. That is to say that it's minimum aperture will vary from f4 @ 70 mm to f5.6 @ 300mm. You can't change this in any way as it is the way the lens is built. More expensive lenses typically are designed with constant aperture at say f2.8 where throughout it's entire zoom range the minimum aperture will always be f2.8. This feature costs more, and usually tends to make the lens a bit a larger.

Again, your lens is designed to work the way it does.
04-27-2011, 10:41 PM   #3
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QuoteOriginally posted by johnske Quote
Now, if I set the aperture manually to f4 and leave it there, it will then effectively be a faster f4 lens right through from 70 to 300mm won't it?
No.

It would be F4@70mm and F5.6@300mm, same as if you left it on A.
04-27-2011, 10:48 PM   #4
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See if your lens has two aperture indicators. Most lenses with variable aperture do.

One indicator is for the wide-angle end, the other is for the tele end. If the wide-angle aperture indicator is at, say, 5.6, the other will be at 8, 1 stop slower.

04-27-2011, 11:07 PM   #5
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QuoteQuote:
See if your lens has two aperture indicators. Most lenses with variable aperture do.

One indicator is for the wide-angle end, the other is for the tele end. If the wide-angle aperture indicator is at, say, 5.6, the other will be at 8, 1 stop slower.
Ditto. Usually 5.6 on those lenses have a little line that leads to the position of 4.0, meaning that 5.6 is max aperture at the long end.
04-28-2011, 01:52 AM   #6
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Oh ok, thanks for clearing that up ...
04-28-2011, 05:54 AM   #7
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QuoteOriginally posted by blackcloudbrew Quote
Your lens is a variable aperture lens. That is to say that it's minimum aperture will vary from f4 @ 70 mm to f5.6 @ 300mm. You can't change this in any way as it is the way the lens is built. More expensive lenses typically are designed with constant aperture at say f2.8 where throughout it's entire zoom range the minimum aperture will always be f2.8. This feature costs more, and usually tends to make the lens a bit a larger.

Again, your lens is designed to work the way it does.
I believe you should be using the term maximum aperture. The f numbers are written like f/2.8 where 2.8 is in the denominator, and a larger denominator makes a smaller number (and thus a smaller aperture).
04-28-2011, 11:10 AM   #8
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QuoteOriginally posted by L33tGreg Quote
The f numbers are written like f/2.8 where 2.8 is in the denominator, and a larger denominator makes a smaller number (and thus a smaller aperture).
Quite. We must remember that an f-stop is a fraction, the ratio of the opening to the focal length. So on a 100mm lens, a 50mm opening is 50/100 or 1/2 (f/2), and a 25mm opening is 25/100 or 1/4 (f/4). Just as 1/2 is a bigger number than 1/4, so f/2 is a larger aperture opening than f/4. And apertures jump in the odd scale of 1--1.4--2--2.8--4--5.6--8--etc because those related to the square root of two, about 1.414. Iris openings are (or should be!) fairly circular; opening the iris 1.4x lets in 2x the light.

A fixed-maximum(widest)-aperture zoom adjusts the iris to keep the aperture constant; a variable-max-aperture zoom doesn't make that adjustment, because the mechanism for doing so costs money. Thus variable zooms cost less -- they're made more cheaply. If I have two otherwise-comparable zooms from the same maker even, I'll can bet that a constant f/3.5 lens will perform better than a variable f/2.8-4 zoom.

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