Archived posting to the Leica Users Group, 2008/12/13
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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