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My first DSLR camera was Canon EOS 350D. Now I've got an upgrade to Nikon D90. Both were/are with the kit lens.

Whenever I'm taking a picture with biggest zoom (55mm on Canon, 105mm on Nikon) the diaphragm setting always stops at f/22. Also, when using aperture priority mode the biggest setting I can set is f/22. Any reason behind this number?

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Note that because the f stop number is a ratio that measures the relative size of the aperture, traditionally the smaller numbers are described as being "large" (because they represent a bigger aperture) and the high numbers are described as being small (smaller aperture). And when you "stop down" you are choosing a smaller aperture / larger aperture number. –  David Rouse Aug 4 '11 at 18:04
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Also note that f/22 isn't the smallest - many can go smaller, For example the Canon 28-135 goes to f/36 at the longer focal lengths. –  Rowland Shaw Aug 4 '11 at 18:13
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I have a view camera lens that stops down to f 128. Most view camera lenses go to f64 –  Kevin Won Aug 4 '11 at 18:37
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Meanwhile, not all even go down to f/22. I've got one that stops at f/16. –  rfusca Aug 4 '11 at 21:32
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2 Answers

up vote 10 down vote accepted

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.

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The aperture range has nothing to do with your camera. It has to do with the lens.

Each lens has its own aperture range which may be different along the zoom range for a zoom lens. The limit on the wide-end is extremely important as it dictates how much light can come in but puts more requirements on the lens design. Larger maximum apertures increase the cost of a lens significantly.

On the narrow end, the limit is somewhat arbitrary. Most modern lenses stop around F/22 which is actually too much anyway. That is beyond the diffraction limit on modern DSLRs and will cause your entire image to become softer.

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unless... your camera has a built in lens :) –  dpollitt Aug 4 '11 at 20:20
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Actually, unless you camera or lens does not have an aperture at all! Plenty of fixed lens ones use a ND filter to emulate less light entering without affecting DOF, they even report the simulated aperture in the EXIF. –  Itai Aug 4 '11 at 20:48
    
Itai, you say "plenty", and it is very surprising. Can you give an example or two? –  ysap Aug 4 '11 at 23:54
    
OK... 29 according to my DB... not sure if that counts as plenty but those include the Fuji S1800, Fuji F80 EXR, Casio EX-G1 and FC-150 to name some random examples. –  Itai Aug 5 '11 at 1:51
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