Archived posting to the Leica Users Group, 2010/11/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Howard, I think what you are seeing here is the effect of "focus shift". It seems many lenses have a shift in focus with aperture which is normally masked by the increasing depth of field with aperture closing, and, on film bodies, by the effectively thick sensitive layer, since film emulsion is fairly thick, and the film rarely flat anyway, the focus will fall -somewhere- within the sensitive volume (it is not that much). A digital sensor is flat and therefore less tolerant of focus shift, the 90mm f2 apo has a tiny depth of field and at some apertures the plane of focus falls outside the depth of field due to focus shift. It is also difficult to get perfectly in focus anyway, I use mine with a viewfinder magnifier. A quick search on the internet will turn up quite a lot of discussion on this and the 90mm f2 apo-asph lens is one of the most sensitive lens out there for this problem - sadly. The new 35mm f1.4 asph came out because focus shift on the existing lens was being noticed on the M8 and M9 when it was not using film bodies. cheers, Frank Dernie On 6 Nov, 2010, at 22:26, Howard Ritter wrote: > I have just discovered that the RF on my M9 does not bring the 90 ASPH to > focus in at least the middle range, specifically 7 meters, although it > does so, as far as I've been able to tell, at infinity. I have not tested > it thoroughly at closer range, but I'll do so tomorrow during daylight > when I can use a low ISO. > > I had seen comments on LUG/MUG along these lines previously but not paid > attention because I didn't yet have an M9. I recall seeing something about > lenses needing modding in order to focus properly on the M8 or M9, but > can't recall details. > > This is very counterintuitive and perplexing to me. Given that the > flange-to-sensor distance on digital Ms is the same as the flange-to-film > distance on analog Ms, how does the problem arise at all, and why does a > modification of the lens or adjustment of the RF mechanism to accomodate > one combination not affect the modified unit's performance with other > combinations? > > Any insight or advice would be highly appreciated. > > ?howard > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information