Archived posting to the Leica Users Group, 2008/12/14
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]My 90 SA is such a great performer I open the refrigerator door and the light comes on and it instantly it does 20 minutes. Songs, Jokes, impressions. A little soft shoe. A little slight of hand. A SDJ joke. Takes great pictures too! mark@rabinergroup.com Mark William Rabiner > From: Leonard Taupier <len-001@verizon.net> > Reply-To: Leica Users Group <lug@leica-users.org> > Date: Sat, 13 Dec 2008 21:13:57 -0500 > To: Leica Users Group <lug@leica-users.org> > Subject: Re: [Leica] M8 upgrade - 90 Cron APO focusing problems now > Question???? > > Marty, > > Of course. The lens doesn't know what body it's on. Sorry for the > incorrect wording here. I know what focus shift is and what causes > it. The two lenses that I have that have a definite focus shift are > the Noctilux and the 75mm Summilux. I can see the plane of sharp > focus inch back as I stop down the lens. Your f1 and f5.6 performance > are exactly the way my Noctilux acted before I had it shimmed by DAG. > Now it is slightly front focused at f1 (but still acceptable) and > stays in focus albeit more to the rear from f2.0 to 4.0. As you say > at f5.6 the DOF catches up. > > The 8 lenses that don't focus shift will focus more behind the point > I focus the rangefinder to on the M8s then on the film cameras. But > these 8 lenses are still acceptably within what I call a sharp focus > area. I have 19 Leica M lenses and perhaps 10 leica LTM lenses I use > on the M8. Of these 29 lenses only 3 gave me a problem. However I'm > also convinced that Leica did not align the M8 rangefinder to account > for the difference in film and sensor thickness. For all I know they > knew about the differences but didn't change the alignment for fear > of having a bigger problem. All I know is my leica lenses focus more > behind where I focus on the M8. > > Cheers, > Len > > > On Dec 13, 2008, at 7:34 PM, Marty Deveney wrote: > >> >>> But every Leica lens I have tested has a focus shift when going >>> from film to digital. >> >> This is not focus shift. This is a difference in body-to-lens >> calibration. >> >> There are a few interrelated issues here. Focus on a rangefinder >> works by mechanical linkages. The camera focuses correctly as long >> as a certain level of mechanical precision is achieved and >> individual lenses and bodies are within spec. >> >> Focus shift is a property of lenses caused by uncontrolled >> spherical aberration - it has nothing to do with camera bodies. >> Think of depth of field as a theoretical band of acceptable >> sharpness in space that varies in width and with distance from the >> film or sensor plane as you stop down. Acceptable sharpness is >> related to the circle of confusion concept, which ultimately is >> also linked to human visual acuity, so whether something is "sharp" >> or "in focus" is ultimately also linked to your own eyesight. >> >> At close range and wide open the focus point of fast RF lenses is >> almost always ever-so-slightly in front or behind the point at >> which you focus - if the lens is properly adjusted. If the lens or >> body are poorly adjusted the result in softness, but this is 'out >> of specification collimation', not 'focus shift'. As you stop down >> the focus point shifts away or towards the film or sensor plane >> (depending on the lens' design). You run into problems when this >> shift is greater than the increase in depth-of-field. I haven't >> tested the 90/2 AA extensively, but the f1 Noctilux, for example, >> focuses close to correctly at f1, but depth-of-field doesn't catch >> up with focus shift until f5.6. This is especially apparent on the >> M8. The answer for me is simple - use the Nocti on the M8 at only >> f1 or f5.6 or larger. >> >> Why does digital make it worse? Remember that focus shift works >> backwards aswell as forwards. Even the thinnest films have an >> emulsion that is ~120 micrometres thick or so. When the virtual >> image shifts, you have some leeway for movement since the image >> will appear sharp if the image is in focus anywhere in the >> emulsion. Digital sensors have no depth and the photosites have a >> single point at which focus must be accurate, which is almost a >> theoretical plane is space although in practice it has some depth, >> though this is only a few nanometres. >> >> Some of Len's lenses do display focus shift, but having them >> calibrated to his M8 will help enormously, particularly with the >> 50/1.4 asph which has negligible focus shift courtesy of its design >> which includes an aspherical element _and_ a floating element. >> These minimise spherical aberration and therefore focus shift. Of >> course some of the others will display focus shift, but having them >> calibrated to the body with make sure that they are within spec >> wide open, which has nothing to do with focus shift. >> >> Howard's 90/AA may be displaying a lack of calibration, displaying >> some focus shift - since its design is very simple and the lens is >> not known to be a great performer at short distances anyway. I >> suggest, if you want to try to make it work, to send the body _and_ >> the lens to a competent repair person for matching and then very >> carefully testing what is happening afterwards to check for focus >> shift. If this sounds like too much trouble, you could trade your >> 90/2 AA for the 75/2 AA, which has a floating element and an >> asperical element which do a much better job of controlling >> residual aberrations than the 90/2 AA design does. Hoppy's 75/2 AA >> worked perfectly on five or six M8s that were present at the >> Melbourne LuG get-together and close-up focus was _much_ easier to >> nail than with my 75/1.4. >> >> Marty >> >> Gallery: http://gallery.leica-users.org/v/freakscene >> >> >> -- >> Be Yourself @ mail.com! >> Choose From 200+ Email Addresses >> Get a Free Account at www.mail.com >> >> >> >> _______________________________________________ >> 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