[Leica] Image circle

[lrzeitlin at gmail.com (Lawrence Zeitlin)]

Richard,

The size of a lens image circle is not absolute but depends on your
criteria. Most lenses are sharp and relatively distortionless in the
center of the image. Quality and illumination fall off as you move
away from the axis. You have reached the edge of the lens image circle
when the image quality no longer meets your photographic requirements.
That being said, the easiest way to determine the approximate size of
the image circle is to hold the lens in front of a ground glass. Adjust
it so that it focuses a distant scene and simply measure the
illuminated circle on the ground glass. Most normal lenses have a
reasonable quality of image in a circle with a diameter approximating
their focal
length. Symmetrical lens designs tend to have a bigger image circle and
are frequently used for wide angle lenses.
Larry Z