bnelson Posted January 2, 2007 Share #21 Posted January 2, 2007 Advertisement (gone after registration) I had focus problems with myR9/DMR shot wide open or close to wide open when focussing on clos in objects. After I installed the Brightscreen magnifier most of the problems went away and my focus accuracy greatly improved. I have a 1.25 magnifier for the M8 coming today and a dipoter later (hopefully this week) and I amhoping this will help with the M8 as well. I'm also going to check my glasses perscription to see if it is up to date with my eyes. Bill Link to post Share on other sites More sharing options...
Advertisement Posted January 2, 2007 Posted January 2, 2007 Hi bnelson, Take a look here M8 backfocus. I'm sure you'll find what you were looking for!
khanosu Posted January 2, 2007 Share #22 Posted January 2, 2007 An addition to my post above (#20). Here is the 50mm Summicron wide open from a longer distance (about 6ft). Tripod and timer as before. If you use a tripod for this test then you can focus very precisely and make very fine adjustments. Furrukh 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/12510-m8-backfocus/?do=findComment&comment=131955'>More sharing options...
johnkuo Posted January 2, 2007 Author Share #23 Posted January 2, 2007 Furrukh I will try adding the vertical line and reshoot. I need to get a body plate from RRS to work with my tripod. I should also test this with my film Ms. Thanks again. Link to post Share on other sites More sharing options...
johnkuo Posted January 4, 2007 Author Share #24 Posted January 4, 2007 I did a few more test with other lenses, and it seems the 35/1.4 is spot on. I still need to try the film bodies. Now I'm not so sure if it's the body or the lenses. Could it be that the longer lenses need adjustment themselves? I would think if the bodies needs adjustment, it would affect all lenses? Link to post Share on other sites More sharing options...
jaapv Posted January 4, 2007 Share #25 Posted January 4, 2007 What aperture were the tests taken with? Most faster/longer lenses exhibit focus shift on stopping down, the Noctilux more than most. I seem to recall that it should be calibrated to be spot-on at f 2.8. Calibrating a M body is a simple thing, one used to be able to do it oneself. Calibrating a lens is quite an operation, it takes either shimming the mount or exchanging the helical, and requires an optical bench. Fortunately it is a very rare occurrence for a lens to be off. Link to post Share on other sites More sharing options...
drjon Posted January 4, 2007 Share #26 Posted January 4, 2007 I did a few more test with other lenses, and it seems the 35/1.4 is spot on. I still need to try the film bodies. Now I'm not so sure if it's the body or the lenses. Could it be that the longer lenses need adjustment themselves? I would think if the bodies needs adjustment, it would affect all lenses? I have done quite a lot of testing, as per my earlier post. My 35mm summilux defintely had a problem. I suspected the body at first but am now convinced there are several factors involved as each of my lens appears to behave differently. First, there is the tendency with digital to review at 100% which exposes any focusing errors or lens calibration issues. Second, it does seem that lenses can have cam calibration issues which are not obvious with film. Third, the M8 viewfinder, long lenses, and eyesight can consipre to make spot-on focusing a challenge - again exposed at 100% magnification. I have had two bodies each displaying the same issue with my 35mm, therefore, I doubt the M8 is the culprit. Link to post Share on other sites More sharing options...
mike prevette Posted January 4, 2007 Share #27 Posted January 4, 2007 Advertisement (gone after registration) You guys do understand that a lenses DOF is not symmetrical don't you? A rough rule of thumb is that 2/5 of the field of focus will be in front of whatever is at critical focus, and 3/5 will be behind that point. Its a fundamental rule of critical focus, and something we use in cinematography all the time. This means that focus will "fall off" faster in front of your mark, and 'fall off' less behind it. For whatever reason most stills guys never no this. What this means is it's very easy to look at a photo and assume that the focus is off, based on the fact the the DOF is lopsided. When in fact this is an optical principle critical to the creation of DOF. Looking at all the examples posted here, it looks to me that this is actually what your seeing, and not miss calibration. this is an excellent yet out of print book that covers all the nerdery of optics in a great degree: Amazon.com: Optics and Focus for Camera Assistants, Art, Science and Zen: Books: Fritz Hershey use this calculator to get an idea about what I'm talking about: Online Depth of Field Calculator _mike Link to post Share on other sites More sharing options...
jaapv Posted January 4, 2007 Share #28 Posted January 4, 2007 Mike is absolutely right. That, and, as I posted focus shift, tend to make wide open focussing a bit different from SLR photography. I agree there seems to be not much wrong with the tests we see here. I might add, that the central focussing patch leads to use focus-recompose which also provokes mis- (usually back-) focussing. Link to post Share on other sites More sharing options...
farnz Posted January 4, 2007 Share #29 Posted January 4, 2007 Mike, Thanks for the great link! I am aware of hyperfocal distance but I'm still uncertain what value should be used for circel of confusion (coc) for digital cameras because, as I understand it, the coc is a function of printed media; ie the smallest detail that is differentiable on a given size of print. But since digital output is typically viewed on screen the correlation is not clear to me. Would you very kindly offer some insight please? Pete. Link to post Share on other sites More sharing options...
jaapv Posted January 4, 2007 Share #30 Posted January 4, 2007 Mike, Thanks for the great link! I am aware of hyperfocal distance but I'm still uncertain what value should be used for circel of confusion (coc) for digital cameras because, as I understand it, the coc is a function of printed media; ie the smallest detail that is differentiable on a given size of print. But since digital output is typically viewed on screen the correlation is not clear to me. Would you very kindly offer some insight please? Pete. I posted this on another forum some time ago. I think it answers your question. Btw. the COC for the M8 can be put at 0.02, for cicumstances that would put it at 0.03 for film DOF: The reality of an illusion. DOF is a subject that causes heated discussion in photographic circles. It is, of course, next to light and shape, one of the main photographic symbols to express ourselves. There is a simple mathematical approach that is expressed in DOF scales on lenses and DOF tables in manuals, but, as always, that is not the whole story – by a fair margin. DOF as a phenomenon is childishly simple. The human eye is a rather imperfect instrument for judging sharpness, so with a resolution of about 5 lp at 75 cm everything that is higher resolved appears sharp. So now the compications start. It readily confuses contrast with sharpness, the only reason that sharpening algorithms in postprocessing actually work.. So a photograph at noon at the beach will appear to have a deeper DOF than one on a misty morning. Of course, a photograph is, in reality sharp only in one plane, which is theoretically infinitely thin, but at least as thin as the state of correction of the lens and the quality of the receiving medium, be it film or sensor, allows. Lens manufacturers, in their quest for simplification and standardization have decided, in the 1920-ies, that an unsharpness of 0.03 mm on 35 mm film would be judged the measure of DOF. That leads us to the first set of complications.: 1. Without knowing the end enlargement of the photo one takes and without taking the contrast into consideration, judging the amount of DOF is actually rather hit and –mostly- miss. 2. As DOF is solely dependent on field of view, the “enlargement” of the focal length of the lens, which is responsible for the apparent deep DOF of wideangle-lenses and shallow DOF of long lenses gets into play, so the subsequent crop will influence the DOF in as much that if one crops a 28 mm shot down to the FOV of a 90 mm lens, the DOF will be exactly the same as that 90 mm lens would have produced. 3. Film is not without thickness. In reality a COC of 0.03 mm will act like a torch shining into a murky plate of soup. It will produce a cone, diffractions, reflections, if the light strikes the film at an angle it will turn into an oblong, etc., the net result being a larger diffuse spot. This is complicated by the fact that the films we have now are much thinner and higher resolving than we had in the 1920íes. 4. Digital sensors react far more like the ideal thin receiving medium than film, causing the COC’s to be even less diffused. 5. The net result is that the DOF produced now, and especially with modern lenses (of which I will write later) is more pronounced than it is historically. It is safe to assume that it is about 70% of the scale indicated on your lens. Btw. let’s not forget that it is not divided equally in foreground and background. The real division is, for simple mathematical reasons, 1/3-2/3, more or less, depending on subject distance. All this caused me to call DOF in another context and another forum a RBU <rubber band unit>, which got me heavily flamed. Then we get to the real controversial point, and that is the effect of individual lenses on DOF, which relates to the elusive “boke”, which aptly translates to "chaos" or "confusion" I'm told, and to the rendering of out of focus picture elements. In general the lens is corrected optimally for the plane of sharpness only, which means that aberrations like chromatic aberration and astigmatism increase quickly as sharpness decreases. Add this to my plate of soup effect and the magnitude of possibilities gets so large that only using the lens in practice will give any firm grasp of its (lack of) qualities. The result is that, in extreme cases of not too well corrected lenses, there will be double contours, rings and general unpleasantness in the unsharp areas. That gives bad Boke. More elegantly, but still not optimally corrected lenses, and this applies to a large number of the older lenses used by Leica fans, will produce generally soft and smoothly changing unsharp areas where the forms as such are undistorted. (did I mention geometrical distortion with the aberrations? This is the three-dimensional variant;)) That are lenses with a good boke. Then there are the newest, highly corrected lenses, like the Leica ASPH’s, APO’s etc. Those define the unsharp areas so well that they will break up the contours, giving rise to harsh boke. Film will behave differently than sensors, as explained above. So a sweet lens on film may be unpredictably disappointing for digital and the other way around. Link to post Share on other sites More sharing options...
farnz Posted January 4, 2007 Share #31 Posted January 4, 2007 Jaap, Thanks for the response and I love your 'murky soup' analogy! It provides some of the answer but I don't want to hi-jack this thread so I'll be content with 0.02 mm coc for (most) digital sensors. Pete. Link to post Share on other sites More sharing options...
jaapv Posted January 4, 2007 Share #32 Posted January 4, 2007 I was rather pleased with that myself- "Is film dead?- No- it has just turned into yesterdays soup":D Link to post Share on other sites More sharing options...
rvaubel Posted January 4, 2007 Share #33 Posted January 4, 2007 An addition to my post above (#20). Here is the 50mm Summicron wide open from a longer distance (about 6ft). Tripod and timer as before. If you use a tripod for this test then you can focus very precisely and make very fine adjustments. Furrukh Furrukh Where did you get that cool focus chart. I'm getting tired of reading newsprint Rex Link to post Share on other sites More sharing options...
farnz Posted January 4, 2007 Share #34 Posted January 4, 2007 Rex, There's a link to the focus chart in post #11 in this thread. Pete. Link to post Share on other sites More sharing options...
khanosu Posted January 4, 2007 Share #35 Posted January 4, 2007 Hi Rex, Here it is: http://www.focustestchart.com/focus21.pdf Material relevant to back focusing starts on page 13. The chart is on Page 18 Furrukh Link to post Share on other sites More sharing options...
khanosu Posted January 4, 2007 Share #36 Posted January 4, 2007 Pete, we must have hit the submit reply button at the same time Did not read your post. Furrukh Link to post Share on other sites More sharing options...
carstenw Posted January 4, 2007 Share #37 Posted January 4, 2007 My M8, which I have just received from Solms after sending it there in early November to fix focusing problems, appears to back-focus about 3cm at 80cm. Practically unusable. I feel soooo tired about now. Link to post Share on other sites More sharing options...
khanosu Posted January 4, 2007 Share #38 Posted January 4, 2007 Sorry to hear this, Carsten. Is it doing it with all your lenses? Is the intended focus point completely out of the depth of field range? Could you post an example? Thanks! Furrukh Link to post Share on other sites More sharing options...
carstenw Posted January 4, 2007 Share #39 Posted January 4, 2007 This is with the 50 Lux Asph, which was sent back with the camera. I do seem able to focus on infinity now, although I need to test this from the office tomorrow, where I have an unobstructed view of several kilometres. From my apartment I can only see about 200m, but still have some slack on the long end of the focusing. The only other two lenses I have at the moment are the CV15, with no focusing feedback at all, and an older 135/2.8, whose focusing accuracy I have no faith in. I will test it tomorrow too. I took the shot vertically, since this makes it easier to focus on the line. It is slightly slanted due to my using a too-large tripod with a too-small tripod head which doesn't allow me to turn it any more. In spite of the inaccuracy this introduces, there is clearly more than enough focus inaccuracy to be sure in any case. I also tried focusing on a bookcase at around 2m, and took perhaps 15 shots at slightly different distances. The sharpest one was visibly off in the viewfinder. . 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/12510-m8-backfocus/?do=findComment&comment=133802'>More sharing options...
khanosu Posted January 4, 2007 Share #40 Posted January 4, 2007 Given the asymmetry in the DOF looks like about a 1.5cm - 2cm back focus. How is the focus at say 6ft? Thanks, Furrukh Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.