Why f/22 is the highest possible setting for the aperture diaphragm?

Question

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?

Answer

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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