Archived posting to the Leica Users Group, 2012/02/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Henning, Oh. The 90AA I trialled showed quite a bit of focus shift when I measured it, but it could have been due to the kind of issue you've described here. I didn't put it on an optical bench. Yes, I have a table I calculated for the 75 Summilux somewhere about how to proportionally compensate at each aperture where focus shift is an issue. I never liked the 90AA enough to buy one so I never characterised this - thanks for making me aware of that - I'll be on the lookout for it. Thanks, Marty On Fri, Feb 3, 2012 at 12:41 PM, Henning Wulff <henningw at archiphoto.com> wrote: > 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 >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information >> > > > Henning Wulff > henningw at archiphoto.com > > > > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information