lct Posted January 23, 2012 Share #21 Posted January 23, 2012 Advertisement (gone after registration) I'm not far except for the 135/2.8 but i may be wrong. 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/171132-rangefinder-focussing-accuracy-rule-of-thumb/?do=findComment&comment=1906949'>More sharing options...
Advertisement Posted January 23, 2012 Posted January 23, 2012 Hi lct, Take a look here Rangefinder focussing accuracy: rule of thumb. I'm sure you'll find what you were looking for!
IkarusJohn Posted January 23, 2012 Share #22 Posted January 23, 2012 Thank you for all this work (though I don't fully understand it). What interests me is that if I feed the APO Summicron 75/2 ASPH into your spreadsheet, I get a result of 76.68%, which puts that lens only marginally behind the Noctilux 50/0.95, and considerably worse than the Summilux 50/1.4 upon which it was based. I have always found this lens problematic to focus ... Thank you again. Cheers John Link to post Share on other sites More sharing options...
Lindolfi Posted January 23, 2012 Author Share #23 Posted January 23, 2012 Here's a graph of relative focussing difficulty in some Leica lenses (the 50/1.4 is the standard) Link to post Share on other sites More sharing options...
Guest #12 Posted January 23, 2012 Share #24 Posted January 23, 2012 ... nor have I found a derivation of the formula that Erwin, you and lct use. I posted one in a previous thread; there are others... "... I will give you the usual derivation. It's based on the diagram (triangle) in Osterloh. You have the distance, the physical base, and the angular uncertainty in vision (visual resolution). You can get an expression for the uncertainty in the distance in terms of those three (by the geometry, with an approximation for small angles)...this gets you a formula for the accuracy. Multiply the visual res. by the reciprocal of the magnification to account for magnification. Say you already have a formula for the d.o.f. ...neglect a small bit in the denominator and you will be able to cancel out the distance. So set double the accuracy (it was plus or minus) to be less than the d.o.f. Now you have a formula r/(mb) < cn/(f^2) where r is the visual res., m the magnification, b the physical base, c the c.o.c. constant, n the f-number, and f the focal length. You can rearrange this several ways to get all the usual beautiful charts and tables ..." Link to post Share on other sites More sharing options...
thighslapper Posted January 23, 2012 Share #25 Posted January 23, 2012 Thank you for all this work (though I don't fully understand it). What interests me is that if I feed the APO Summicron 75/2 ASPH into your spreadsheet, I get a result of 76.68%, which puts that lens only marginally behind the Noctilux 50/0.95, and considerably worse than the Summilux 50/1.4 upon which it was based. I have always found this lens problematic to focus ... Thank you again. Cheers John If you look at the DOF characteristics wide open you will see the 50/0.95 and 75/2 are identical ..... DOF at 1m of 2cm and 85m to infinity when focussed at infinity..... In fact a much simpler rule of thumb than all Lindolfi's complex calculations is that if DOF wide open at at 1m is 2cm or less (135/3.4 is 2cm at 1.5m) then the lens will be difficult to focus accurately. A quick glance at the tables shows it correlates very well... I think the moral behind all this is that if you use one of these 'problem' lenses you need to have 100% faith in your rangefinder and lens calibration plus a meticulous technique (and a magnifier) to get consistently reliable results. Link to post Share on other sites More sharing options...
Lindolfi Posted January 23, 2012 Author Share #26 Posted January 23, 2012 In fact a much simpler rule of thumb than all Lindolfi's complex calculations is that if DOF wide open at at 1m is 2cm or less (135/3.4 is 2cm at 1.5m) then the lens will be difficult to focus accurately. . That is not a simple rule: you need a dof calculator or table and it depends on focus distance, while rangefinder accuracy does not depend on focus distance. However, I agree with your moral! Thanks. Link to post Share on other sites More sharing options...
thighslapper Posted January 23, 2012 Share #27 Posted January 23, 2012 Advertisement (gone after registration) That is not a simple rule: you need a dof calculator or table and it depends on focus distance, while rangefinder accuracy does not depend on focus distance. Yes.... but it is a 'rule of thumb'...... which by definition involves some guestimation and rough and ready calculations ..... not spreadsheets .... Anyway this whole business is a minefield and shot through with assumptions and presumptions which makes real world extrapolations rather difficult.... No criticism intended and I salute your endeavours in trying to bring some rationality to a very difficult and subjective issue ! Link to post Share on other sites More sharing options...
jaapv Posted January 23, 2012 Share #28 Posted January 23, 2012 So let's get through the steps needed. [1] First of all, according to my derivations, depth of focus does not depend on focal length. That is one reason why you can have a depth of focus scale on a view camera that only shows aperture, not focal length. Here my Gandolfi view camera: click [2] The motion of the lens relative to the sensor scales linearly with the focal length at equal motion of the rangefinder cam. (for a 50 mm lens the cam moves the same amount as the lens, for longer lenses it moves more than the cam, for shorter it moves less) [3] The depth of focus scales linearly with aperture number. Checked it all again and found the error!! Assumption [2] is wrong! The lens cell moves with the square of focal length, not linearly at equal movement of the rangefinder cam. So I should use the factor F*F/2500 in stead of F/50 (F is focal length in mm). I've made a new spread sheet, and now the 135/3.4 is more difficult to focus than the 50/0.95. Spreadsheet: click Terribly sorry for the circle of confusion I created. I think we (#12, lct, Erwin and I) agree now. Only problem is that now 50/0.95, 50/1, 75/1.4, 90/2.8, 90/2, 135/3.4 and 135/2.8 all fail to meet the criterium of focussability, and that is because of the more critical CoC used of 0.014. If you relax that again to CoC=0.03 you get that only the 135/3.4 can not be focussed correctly (sorry Jaapv). The order of difficulty of focussing is (with difficulty relative to 50/1.4 in red) focal_length aperture relative_difficulty 135 3.4 2.9 135 2.8 2.4 90 2 2.2 75 1.4 2.2 90 2.8 1.6 50 0.95 1.5 50 1 1.4 50 1.4 1.0 To bring it all up to recent standards of the images possible with the M9, the rangefinder base simply (!) has to increase a factor of two in size. For that there is no space. The M house makes it possible to make the base 98 mm long, which should solve part of the deficit. There is a small conflict between the release mechanism under the button and the small range finder window however.... No problem for me, except that numbers do apparently do not say anything. Mine focusses just fine - and Leica sells magnifiers Link to post Share on other sites More sharing options...
lct Posted January 23, 2012 Share #29 Posted January 23, 2012 135/3.4 is OK with 1.25x or 1.40x magnifier on the M9. I would not expect high hit rates w/o magnifier though. Link to post Share on other sites More sharing options...
jaapv Posted January 24, 2012 Share #30 Posted January 24, 2012 I must own up that I rarely have a misfocus without magnifier. Nor have some other users as has been established in earlier threads. It seems experience can compensate for theoretical shortfall in this respect. Link to post Share on other sites More sharing options...
Lindolfi Posted January 24, 2012 Author Share #31 Posted January 24, 2012 All depends on the 1 minute of arc resolution of the human eye in the calculations. Some people have sharper vision up to about 0.6 minute of arc, while Vernier acuity (alignment of linesegments) can be as good as 0.13 minute of arc. Sufficient room for the 135/3.4 without a magnifier. And Jaapv is right that a magnifier is not always helpfull. Acuity drops when the light level drops, and the magnifier reduces the light level, which may take away part or all of its advantage from magnifying the image on the retina. Link to post Share on other sites More sharing options...
pgk Posted January 24, 2012 Share #32 Posted January 24, 2012 It seems experience can compensate for theoretical shortfall in this respect. Visual clues are not easily defined in purely mathematical terms. Quickly rotating the lens through the point of focus and back several times will help home in on the precise point of focus, as will a slightly angled edge cutting through the rangefinder double image and so on. Whilst such calculations illustrate where the most likely focus problem areas are likely to be they do not and cannot take into account the myriad or variables which help us to achieve precise focus. FWIW lenses like the Canon 85mm f/1.2 also have criticism levelled at them and are often regarded as difficult to achieve precise focus with - in this case I suspect (having owned and used one) because the precise required point of focus is difficult for the photographer to get the camera to determine (not because it can't, but defining it through the viewfinder can be tricky). So what the maths tells us is that fast lenses are difficult to focus and there are limitations imposed by the physical dimensions of the M rangefinder, none of which comes as a surprise to me. Link to post Share on other sites More sharing options...
jaapv Posted January 24, 2012 Share #33 Posted January 24, 2012 All depends on the 1 minute of arc resolution of the human eye in the calculations. Some people have sharper vision up to about 0.6 minute of arc, while Vernier acuity (alignment of linesegments) can be as good as 0.13 minute of arc. Sufficient room for the 135/3.4 without a magnifier. And Jaapv is right that a magnifier is not always helpfull. Acuity drops when the light level drops, and the magnifier reduces the light level, which may take away part or all of its advantage from magnifying the image on the retina. Yes,I did not mention that before in this thread; I do have the 1.25 and it lived in its little case on the strap for a while - the marginal benefit on the 135 did not compensate for the hassle of moving the diopter around and removing it for other focal lengths, where it actually compromises the rangefinder "window" experience by reducing brightness, contrast and field of view. And my shots were focussed without it anyway. Others may hold a different view (pun intended) as eyes differ. It will help, of course, with feelings of focussing insecurity, that diminish over time, as well. Link to post Share on other sites More sharing options...
IWC Doppel Posted January 24, 2012 Share #34 Posted January 24, 2012 Interesting thread, Thanks to all the guys with spreasheets and theory books. I wonder how much of the potential errors/challenges can be reduced by experience, the human is incredibly adaptable as we know. Also, given the practicality of achieving acceptable accuracy through design and the issues regarding mechinical accuracy in manufacture and set up I wonder if Leica will develop any technical aids in future models to work around some of these challenges. Manual adjustment / electronic guidance ? Link to post Share on other sites More sharing options...
lars_bergquist Posted January 24, 2012 Share #35 Posted January 24, 2012 May I chime in and say that I am focusing my Apo-Telyt with more certainty without my 1.4x Sucher-Lupe than with it? The decrease of contrast in the rangefinder patch annuls the theoretical advantage of magnification. Whichever RF focusing method we use, we do really focus on contrast. The bleary-eyed old man from the Age of the Focusing Cloth Link to post Share on other sites More sharing options...
Guest #12 Posted January 24, 2012 Share #36 Posted January 24, 2012 I must own up that I rarely have a misfocus without magnifier. Nor have some other users as has been established in earlier threads. It seems experience can compensate for theoretical shortfall in this respect. oh please...the man is a cheat and has TEN magnifiers on his camera and can't focus without a magnifier: 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/171132-rangefinder-focussing-accuracy-rule-of-thumb/?do=findComment&comment=1907720'>More sharing options...
jaapv Posted January 24, 2012 Share #37 Posted January 24, 2012 Lol - those are not magnifiers Link to post Share on other sites More sharing options...
jaapv Posted January 24, 2012 Share #38 Posted January 24, 2012 May I chime in and say that I am focusing my Apo-Telyt with more certainty without my 1.4x Sucher-Lupe than with it? The decrease of contrast in the rangefinder patch annuls the theoretical advantage of magnification. Whichever RF focusing method we use, we do really focus on contrast. The bleary-eyed old man from the Age of the Focusing Cloth Exactly! Link to post Share on other sites More sharing options...
Guest #12 Posted January 24, 2012 Share #39 Posted January 24, 2012 I can clearly see that those are super-magnifiers Link to post Share on other sites More sharing options...
jaapv Posted January 24, 2012 Share #40 Posted January 24, 2012 :D Link to post Share on other sites More sharing options...
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