Why do many kit zoom lenses stop down only to f/22 at the telephoto end?
Asked 8/4/2011
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On my Canon 350D kit lens and Nikon D90 kit lens, when I zoom to the longest focal length (55mm on the Canon lens, 105mm on the Nikon lens), the smallest aperture I can select is f/22. In aperture priority mode I also can’t go beyond f/22. Why is f/22 the limit, and is it set by the camera or the lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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It's just a limit for the construction of the diaphragm. Either the the specific construction makes it impossible to close further, or the manufacturer has chosen that point as the limit to preserve quality.
The diaphragm is usually made out of blades with a curved edge. The curve is formed so that it makes a hole that is as round as possible at the biggest settings. When the hole gets smaller, it goes from being almost round to being almost formed as a polygon. For a six-blade diaphragm for example it would approach the shape of a hexagon. The un-roundness of the hole reduces the image quality somewhat, so you don't want to get too close to the polygon shape.
Also the optical effect called diffraction reduces the image quality when the hole gets too small. When that happens depends mainly on the sensor size and resolution.
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
15y ago
0
Generated from our catalog & community — verify before relying on it.
This limit is set by the lens, not the camera body.
Each lens has its own aperture range, and on zoom lenses that range can vary with focal length. The smallest available aperture is partly a mechanical/design limit of the diaphragm blades. As the opening gets very small, the iris shape becomes less round and image quality can suffer.
There’s also an optical reason not to go much smaller: diffraction. Very small apertures soften the whole image, and on many modern DSLRs f/22 is already beyond the point where diffraction becomes noticeable. So manufacturers often stop there because going smaller would bring little practical benefit and more loss in sharpness.
In short: f/22 isn’t a universal rule, but it’s a common minimum on kit zooms because of lens design and image-quality tradeoffs.
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