Archived posting to the Leica Users Group, 2010/11/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The 90 APO has focus shift that is less that the depth of field. What Howard is seeing is not focus shift; something in his camera, or more likely his lens, is out of spec. Marty On Sun, Nov 7, 2010 at 9:32 PM, Frank Dernie <Frank.Dernie at btinternet.com> wrote: > 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 > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >