[APML]: knife-edge focusing

[charles@jon.cjfny.org]

AS>It extremely depends on the lens used. My Minolta lenses don't have a
AS>considerable temperature shift at all. A striking example for a
AS>temperature-depending focus is Sigma's 400/5.6 APO (24x36mm). It is an
AS>excellent lens, but it is out of focus at a temperature difference of
AS>only 5 deg. C! Thats why I have to use the knife-edge method for
AS>focussing each piture throughout the night - although I gained some
AS>experience focussing with a look at the thermometer.

AS>Reinhard
AS>ilkr@ping.at



Reinhard,

How does knife-edge focusing work on a telephoto lens? Can it be used to
determine correct focus for filters?

Charles