[Leica] 90mm APO Summicron M Asph Price Range

[henningw at archiphoto.com (Henning Wulff)]

Marty,

This is not a focus shift issue as in - it shifts plane of focus as it is 
stopped down. This is an issue as in - it focusses accurately at one 
distance, but not at another, all at the same aperture. The 90AA exhibits 
very little focus shift, as stopping it down doesn't change the plane of 
focus much at all, at any distance.

The problem with mine, and I believe Howard's, was that the cam did not 
follow the focus properly. If the lens focussed properly at infinity, it was 
off at 10m and then on again at 3m. Not very user friendly. I too now have 
some other 90 options which work better.

In a related vein, the 75 Summilux exhibits focus shift in the traditional 
manner, but at f/2.8 it is off the intended focus plane by the same 
proportional amount at every distance, and I have learned to compensate. I 
can use it very well, and have a high degree of confidence in it. 

Henning



On 2012-02-02, at 4:56 PM, Marty Deveney wrote:

>> 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
> 
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Henning Wulff
henningw at archiphoto.com