[Leica] 90mm APO Summicron M Asph Price Range

[benedenia at gmail.com (Marty Deveney)]

> Another consideration about the 90 APO Summicron Asph is that some of them 
> do not focus accurately on the M8 or, by extension, the M9.

This is not an issue with "some" of them.  The 90AA has substantial
focus shift - it's an atrefact of the design.  The lens gets
calibrated for optimum focus at a specific aperture and object
distance.  Focus shift means that even a properly calibrated lens will
focus incorrectly at some apertures and object distances.  In a
rangefinder camera, the mechanism assumes a linear focus arrangement,
but this isn't the case and there is no engineering solution to make
the cam shape change with aperture.

This problem also occurs with SLRs because they focus wide open and
then stop down before shooting without re-focusing.  In autofocus SLRs
where there is lens-camera communication I've never understood why
manufacturers didn't characterise the shift and make camera bodies
that compensate for it, or have a menu selection that enables the
camera to focus then stop down and re-focus then shoot.  The former
may not exist because autofocus is less accurate than manufacturers
like to admit, the latter might attract more complaints than praise
because it would be slower.

Marty