DOUG66 Posted March 9, 2011 Share #21 Posted March 9, 2011 Advertisement (gone after registration) Yes, I have noticed that also and I must admit that I do not have an explaination. I would have expected as you have suggested that the curve would have just moved vertically and retained its shape, there must be other factors which are affecting the situation. What is interesting is that the general shape of the curves are similar, and they do show that leica lenses can be optimised for a particular F number. I settled on a shim size that would give an optimum compromise taking into account depth of field. I am happy that the shape moved the way it did for use on the M8 and not the other way, one needs a stroke of luck occasionaly. Doug. Link to post Share on other sites More sharing options...
Advertisement Posted March 9, 2011 Posted March 9, 2011 Hi DOUG66, Take a look here Focus shifting lenses. I'm sure you'll find what you were looking for!
!Nomad64 Posted March 9, 2011 Share #22 Posted March 9, 2011 I've come up against (what will be time honoured for you guys) the issue of lenses that focus shift.Here is my small list so far.....can anyone add to it or remove any? -Voigtlander 35 1.4 -Voigtlander 28 2.0 -Zeiss 50 1.5 -Leica Summilux 35 ASPH -Leica Noctilux 1.0 -Voigtlander Nokton 50 1.1 All help greatly appreciated. David - Leica Summicron 35/2 ASPH to a minor extent (mainly noticeable if one's into brick walls photography, much less in real world photography) - Voigtländer Heliar 50/2 Classic Cheers, Bruno Link to post Share on other sites More sharing options...
Guest #12 Posted March 11, 2011 Share #23 Posted March 11, 2011 The attached graphs may be of interest. I corrected my 50mm Summicron which was backfocussing on my M8 although focussing OK on my M6, by replacing the the original shim collar with one which was 125 microns (.005inch) thicker.It can be seen from the graphs that focus shift with F number is a charachteristic of this lens but when adjusted for the M8 is not a serious problem. What is interesting is the graph of a 1932 elmar, it is no wonder that Leica made a name for quality with those original cameras. Doug. Thank you for posting this, this is fantastic. What I want to know is, why does it shift backwards and then shift forwards??? I have seen this kind of graph before. What are the units on the "back/front focus" axis? And you measured this in the center of the field? Did you do any checks at longer distances? Thanks. Link to post Share on other sites More sharing options...
Guest #12 Posted March 11, 2011 Share #24 Posted March 11, 2011 Hi Doug, Many thanks. Very interesting. Could you please explain to me why the curve for the 50 Summicron didn't just move up, but changed its shape, when you replaced the shim. Best, K-H. You're adding a shim on one side of the lens (image space), and looking at the result on the other side. Translations (parallel lines) become beautiful cornhusks on the other side. Link to post Share on other sites More sharing options...
Guest #12 Posted March 12, 2011 Share #25 Posted March 12, 2011 o.k., I assumed those numbers were in mm and I plotted what you would expect to get theoretically if you added a 0.000125 m shim. I was mistaken; that is not a thick enough shim to distort the "graph." That is certainly about the thickness you would need. I already know you are careful about measurements. I am still wondering, why does a lens shift backwards and then forwards? I've seen that graph in a book, with no explanation. Link to post Share on other sites More sharing options...
Guest #12 Posted March 12, 2011 Share #26 Posted March 12, 2011 Hi Doug, Many thanks. Very interesting. Could you please explain to me why the curve for the 50 Summicron didn't just move up, but changed its shape, when you replaced the shim. Best, K-H. Attached graphs are Distance to focus vs. Distance to object, for a 50mm lens-- showing the effect of a shim. In the first picture it looks like a shim merely displaces the graph, but in the second you can see that's not really true. Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/145369-focus-shifting-lenses/?do=findComment&comment=1613483'>More sharing options...
lars_bergquist Posted March 12, 2011 Share #27 Posted March 12, 2011 Advertisement (gone after registration) A change of the flange-to-sensor register does not move the plane of best focus by a fixed distance. It moves it as a percentage of the subject distance (.i.e. the lens focus distance). The old man from the Age of the Slide Rule Link to post Share on other sites More sharing options...
DOUG66 Posted March 12, 2011 Share #28 Posted March 12, 2011 Hi #12, Thanks for your posts. Sorry for not putting the units on the back/front axis, you correctly assumed they are in mm. I am making these measurements on a focus chart which slopes at 60 deg to the lens axis. I have only been making measurements in the range 0.7M to 1.2M. I am limited by my testing set up. If you would like a copy of the chart, send me a PM with your postal address and I will send you a copy, (it will not email as it loses definition). I can only speculate as to the reason for the shape of the curve, but I must admit that the error in determing the precice focus when the lens is stopped down is large. I have seen what I suspect are multiple focus points which I attribute to different colours in the spectrum (I use white light illumination). This I would expext as I have always assumed that lenses designed for colour emulsions are designed to bring each colour to a slightly different point corrisponding to the layer structure of these emulsions. Older lenses which were designed for single layer fine grain or ortho emulsions are designed to be truly achromatic and bring all wavelengths to the same focus. This is also the requirement for digital sensors. This is the reason that I included the curve for the 1932 Elmar. Doug. Link to post Share on other sites More sharing options...
01af Posted March 12, 2011 Share #29 Posted March 12, 2011 A change of the flange-to-sensor register does not move the plane of best focus by a fixed distance. It moves it as a percentage of the subject distance (.i. e. the lens focus distance). No, it's not just a constant percentage—as the second graph in "#12's" post #26 clearly shows. If it was then the dashed lines were straight. I can only speculate as to the reason for the shape of the curve ... The curve's shape can be directly and immediately derived from the thin-lens equation which says, 1/f = 1/b + 1/g where f is the focal length, b is the lens-to-image distance, and g is the subject-to-lens distance. Note that the distance d which is shown on the lens' distance scale, i. e. the subject-to-image distance, is the sum of b and g. For g = infinity, the inverse 1/g is zero and hence, b = f. Adding a shim to the lens' collar means adding the shim's thickness to b. EDIT: Oh wait, you are talking about the shape of the focus-shift curves ... umm, no idea about the reason for those funny shapes. Link to post Share on other sites More sharing options...
k-hawinkler Posted March 12, 2011 Share #30 Posted March 12, 2011 Hi Olaf, Many thanks. Are b and g distances measured to the optical center of the lens? b=f would imply something else? Best, K-H. Link to post Share on other sites More sharing options...
01af Posted March 12, 2011 Share #31 Posted March 12, 2011 Are b and g distances measured to the optical center of the lens? For a hypothetical thin lens, yes. For a real-world lens, g is measured to the lens' front principal plane, b is measured from the lens' rear principal plane, and d includes not only g and b but also the principal planes' distance ... which by the way can be negative—i. e. the rear principal plane may be located before the front principal plane (but usually isn't). Unfortunately, a lens' principal planes' distance usually is not included in the tech specs provided to regular mortals. b=f would imply something else? b = f always implies focus is at infinity. Link to post Share on other sites More sharing options...
Lindolfi Posted March 12, 2011 Share #32 Posted March 12, 2011 In addition to the explanation by 01af: you can measure focal length of a lens and the distance between the two principal planes yourself by taking two photographs at two (very different) distances of two (very different sized) objects, measure the distances of the objects to the lens mount, measure the sizes of the objects and the sizes of their images in pixel. This Excel sheet I made makes it easy to get results: click. The example I filled in are for a 90 mm Elmarit on an M9. Anyone interested I can provide with the formulas I derived and implemented in this sheet. Link to post Share on other sites More sharing options...
Guest #12 Posted March 12, 2011 Share #33 Posted March 12, 2011 ...The curve's shape can be directly and immediately derived from the thin-lens equation which says, 1/f = 1/b + 1/g where f is the focal length, b is the lens-to-image distance, and g is the subject-to-lens distance. Note that the distance d which is shown on the lens' distance scale, i. e. the subject-to-image distance, is the sum of b and g. For g = infinity, the inverse 1/g is zero and hence, b = f. Adding a shim to the lens' collar means adding the shim's thickness to b. EDIT: Oh wait, you are talking about the shape of the focus-shift curves ... umm, no idea about the reason for those funny shapes. thank you for the nice summary; that is how I drew my graph. My point was that the Before and After-shimming curves should look just about (but not exactly) the same. Link to post Share on other sites More sharing options...
k-hawinkler Posted March 12, 2011 Share #34 Posted March 12, 2011 In addition to the explanation by 01af: you can measure focal length of a lens and the distance between the two principal planes yourself by taking two photographs at two (very different) distances of two (very different sized) objects, measure the distances of the objects to the lens mount, measure the sizes of the objects and the sizes of their images in pixel. This Excel sheet I made makes it easy to get results: click. The example I filled in are for a 90 mm Elmarit on an M9. Anyone interested I can provide with the formulas I derived and implemented in this sheet. Hi Lindolfi, Many thanks. I would be interested in your formulas. Could you also please state the assumptions used in your derivation? Best, K-H. Link to post Share on other sites More sharing options...
Lindolfi Posted March 12, 2011 Share #35 Posted March 12, 2011 Hello K-H, Here the derivation. The translation from dutch to english: "bekend" = "known" "onbekend" = "unknown" "6 vergelijkingen met 6 onbekenden" = "6 equations with 6 unknowns" f = focal length v = object distance b = image distance M = magnification d = distance object to image plane h = distance between principal planes The subscripts 1 and 2 refer to two conditions in which the two images were taken. Assumption: h does not change with change of focus (so that is a only a false assumption with floating elements in a design, but most Leica designs do not have a floating element) Link to post Share on other sites More sharing options...
k-hawinkler Posted March 12, 2011 Share #36 Posted March 12, 2011 Hi Lindolfi, Many thanks. Great stuff. I would like to brush up on my optics knowledge. Can you please recommend a book in English or German? I don't mind if it is rather technical and mathematical. Indeed, I would welcome that. Best, K-H. Link to post Share on other sites More sharing options...
Lindolfi Posted March 12, 2011 Share #37 Posted March 12, 2011 Hey K-H Here are the two books I use Born and Wolf (click) and Hecht (click) Link to post Share on other sites More sharing options...
Guest #12 Posted March 12, 2011 Share #38 Posted March 12, 2011 Hello K-H, Here the derivation. The translation from dutch to english: "bekend" = "known" "onbekend" = "unknown" "6 vergelijkingen met 6 onbekenden" = "6 equations with 6 unknowns" f = focal length v = object distance b = image distance M = magnification d = distance object to image plane h = distance between principal planes The subscripts 1 and 2 refer to two conditions in which the two images were taken. Assumption: h does not change with change of focus (so that is a only a false assumption with floating elements in a design, but most Leica designs do not have a floating element) I don't have Excel, so I didn't look at your spreadsheet. Maybe that is my problem. I was expecting a formula for the focal length and the distance h between principal pts. in terms of the two magnifications and object-to-image distances you measured: M1, M2, d1, d2. Link to post Share on other sites More sharing options...
k-hawinkler Posted March 12, 2011 Share #39 Posted March 12, 2011 Hi Lindolfi, Excellent recommendations, especially the classic by Max Born. Many thanks, K-H. Link to post Share on other sites More sharing options...
k-hawinkler Posted March 12, 2011 Share #40 Posted March 12, 2011 Hi #12, You could use "Google Docs" for free. It's an online system with Excel functionality as well. Best, K-H. Link to post Share on other sites More sharing options...
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