Archived posting to the Leica Users Group, 2007/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hello Tom, I believe Erwins explanation of the Noctilux focus shift is under stated as it applies to the M8 or even the film series of M cameras. We should also understand where the focus shift is being measured. In Eric's case he is focusing on an object perhaps 5 feet away and than observes where the sharpest plane of focus appears on the photographed image. As the lens is stopped down this plane of sharp focus moves considerably. In Erwins case he says the focus shift is less then the thickness of 35mm film. If this shift were 5 feet away there would never be a problem. I believe he measures the focus point at the film plane where a shift of 74 or 120 microns would be considerable. Erwin did not reference his shift to target distance as Eric does. That 120 micron actually corresponds to large fractions of inches. Erwins measurements are very precise and probably taken with highly calibrated instruments. What we do is focus on an object and expect it to be sharp and in focus. When an experienced photographer, like our bar keeper says the Noctilux is unusable on an M8, that's a problem. Erwin is highly skilled and knowledgeable in the field of lens design and measurement. His Compendium is an important work. The first part of the book where he goes into lens design, distortions and measurement technique is quite technical. Only a few who buy the book will understand a large part of it. For the rest of us we head for the back and read the lens reviews in plain English. I would like to see Erwin revisit the Noctilux and explain it's performance from a users standpoint and why it is worse on the digital camera. Len On Apr 24, 2007, at 6:36 PM, Tom Schofield wrote: > Hi Eric, > > We should introduce you to the work of a former LUG member-turned- > author of the Leica Lens Compendium, Erwin Puts. You can spend > many hours on his website: www.imx.nl > > Here is an excerpt discussing the focus shift with the Noctilux > (tables did not paste well) In a nutshell, as you stop down, while > the focus shift continues, depth of field compensates for it, so > that it is most problematic at f 2.0, where the focus shift is > greater than, the depth of field: > > "MTF measurements of the Noctilux and the Summicron can be compared > to give this discussion a more quantifiable dimension. > > The Noctilux however has some very special problems in this area. > Consider the results in the tables below. > > % Contrast in center Noctilux-M > lp/mm 1.0 2.0 5.6 > 10 84 79 90 > 20 59 41 72 > 40 27 1 33 > The 10lp/mm value at f/1,0 is very good, the 20lp/mm is still quite > good and the 40lp/mm result is acceptable: the Summicron has the > same value for the 40lp/mm in the extreme corners. (at f/2,0 mind > you!). At f/2,0 we notice an overall drop in performance , > especially at 40lp/mm, while at f/5,6 the performance is slightly > better than at full aperture. Modern Photography tested the > Noctilux a number of years ago and noticed the same behaviour. They > just accepted the figures. We know that the Noctilux cannot be > fully corrected for spherical aberration. One important result of > this defect is a slight focusshift. (?Blendendifferenz? the Germans > call it). When refocused the Noctilux showed these results: > > > % Contrast in center Noctilux-M (refocused) > lp/mm 2.0 5.6 > 10 94 96 > 20 80 91 > 40 51 79 > > The focusshift of the Noctilux is 74 micron when stopping down from > 1,0 to 2,0 and 120 micron when going to 5.6. The reason that the f/ > 5,6 values do not drop as much as the f/2,0 values (in the first > series of measurements) is the depth of field. At f/5,6 this depth > of field is greater than the focusshift, so the results stay within > tolerances. By the way: 120 microns is the total thickness of a > typical fine grain 35mm film! These results show the pitfalls when > testing an extreme aperture lens. As most objects in the real world > are three dimensional and have depth, the effects of the focusshift > are hardly noticable. What you could notice is a slight softening > of the image at f/5,6, because the focusshift will produce a > somewhat larger diameter of the blur circle." > > > > > > Enjoy Erwin's site, but keep in mind some of the technical issues > he discusses are so minute that most people will never realize them > in ordinary shooting. > > Tom > > > > > > > > On Apr 24, 2007, at 1:15 PM, Eric Korenman wrote: > >> So I spent an evening with my M8, a sturdy tripod and a focus chart. >> I know this is obsessive naval gazing and not art but here is what >> I found: >> >> The 35mm asph summicron and 4th generation 50mm summicron are dead >> on at >> near focus and wide open. >> Stepping down aperture shows the focus stays near center. >> >> However- the Noctilux, while dead on at f/1.0 shows severe back >> focus the >> higher the f stop. >> At f/5.6, the plane of focus is nearly 6 cm behind the target, at >> target is >> blurred. >> >> I plan to rerun the test with a borrowed M6 and film. >> >> Eric >> >> left calls with Leica NJ - everyone techincal seems to be out of >> the office! >> >> _______________________________________________ >> 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