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10-20-2009, 08:53 PM   #1
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The aperture difference....or is it???

I have a question regarding the aperture reading on the lens and the aperture that you can select using the camera. Some of the lenses show aperture 3.5-5.6 and so on, but you can select an aperture smaller than 5.6 all the way upto 30+..how is it possible..?? How can the camera dictate the aperture to the lens- if the lens is not designed to handle a smaller opening..???

10-20-2009, 08:56 PM   #2
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Specs on lenses are usually the maximum aperture. Many zooms have variable maximum aperture - the largest opening at the smallest focal length - largest opening at the longest focal length.

Some list a minimum aperture, but typically above around f/15+ diffraction starts becoming an issue and the lenses are not as sharp (just as many are not as sharp until you close down to say f/8 and such from the aperture). Most lenses have smaller minimum apertures than you'll want to use.
10-20-2009, 09:09 PM   #3
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To expand on that, when your lens says 18-55/3.5-5.6, that doesn't mean 3.5 is the largest aperture, 5.6 the smallest. It means 3.5 is largest aperture available at the 18mm end of the zoom, and 5.6 is the largest aperture available at the 55mm. But at both ends, the *minimum* aperture is a lot smaller - somewhere in the 20's or 30's.
10-20-2009, 09:25 PM   #4
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QuoteOriginally posted by sagiboy Quote
How can the camera dictate the aperture to the lens- if the lens is not designed to handle a smaller opening..???
First, it is true that the camera or user cannot set an aperture that is lower than what the lens was designed with. However, it is true that aperture can be set lower than the widest opening by either the camera or user. This ability is deliberately designed into the lenses and is a good and useful thing to comtrol the amount of light going thru the lens.

I don't think it's important to discuss how this is done, as that's an electrical/mechanical issue that depends on the lens and camera.

Also, while diffraction is a physical truism and no one beats the laws of physics, in real life, when you view an image resized on a computer screen, or make small prints, many of us won't ever notice the loss in sharpness from diffraction. For good practice though, you don't want to be at small apertures if you don't need to be there. It makes the dust spots visible, for example, if your camera sensor is prone to dust.

10-20-2009, 09:55 PM   #5
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The smallest opening of the lens is usually not indicated on the lens or on paper for people to see. An exception is something like the DA 16-45, which is indicated 1:4 (22) on the lens, meaning maximum aperture is f/4 and minimum is f/22.

The kit lens is different in this regard - not only is the maximum aperture variable (according to the focal length), but so is the minimum. I think it's f/32 at 18mm and f/40 at 55mm.
10-20-2009, 11:47 PM   #6
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Of a simple lens (1 peice of glass), the aperture f number = focal length / diameter of the lens.
In case of a complex item as a modern zoom, it is not all that easy to determine from simply measuring the lens properties.

However, since with zoom lenses the diameter of the initial opening does not change, the most simple designs will have a smaller aperture (higher f number) in case you zoom in (increase the focal length).
It is all simple math. A zoom lens with f4 on 50mm will be f8 on 100mm, if the design is simple, not taking into account any internal losses etc.

- Bert
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