[Leica] macro and CoF

[imx <imxputs@knoware.nl>]

Adrian wrote in part about the Curvature of Field when taking macro pictures
with the Apo 100 and using bellows or extenson tubes.
Here are the facts: The APO 100 is NOT a zoomlens design. It is a standard
Double Gauss with a two element rear group to improve macro capabilities.
The story that zoomlenses and internal focusing lenses are not as good when
using  tubes than lenses where the complete optical cell moves when focusing
is not true. 
All lenses (excepting the true micro lenses) when used with bellows or
extension tubes will have pronounced Curvature of Field, when not stopping
down a few stops. The optical argument is this:
When working very close to the object, the rays of light from the object are
angled much steeper than in normal situations. Lenses are not designed to
cope with these extreme angles and magnifacation ratios and we get a fairly
heavy amount of oblique chromatic aberrations and the steep angle introduces
new errors. The sum of all this looks like a pronounced CoF.
True Micro lenses like the Micro-Nikkor or the Elmarit 60, are corrected for
these steep angles that the rays make.

The best way to proceed here is to use as long a telephoto lens as you can
handle and experiment with extension tubes to get the right magnification.


Erwin