mjh Posted October 8, 2010 Share #21 Posted October 8, 2010 Advertisement (gone after registration) o.k., it is easy enough to shoot a ruler and calculate a value for the uncertainty yourself. No, the point is that there is nothing to calculate. The value you choose for the maximum acceptable CoC captures your tolerance for unsharpness, and only you can decide what’s acceptable to you. 1/1500 of the image diagonal is just a handy rule of thumb; some like to place stricter requirements on sharpness and are free to do so. Mathematics won’t help you here, I’m afraid. Only when you have determined a CoC value that suits you can you even begin to calculate. Link to post Share on other sites More sharing options...
Advertisement Posted October 8, 2010 Posted October 8, 2010 Hi mjh, Take a look here 35 f/2 Focus Recompose (rotate) focus plane shift. I'm sure you'll find what you were looking for!
dwbell Posted October 8, 2010 Author Share #22 Posted October 8, 2010 9 deg was just a 6th of the FOV (focus in the middle the rotate 1/6FOV to put focus patch on 3rds line) and 3cm was just roughly measured on the screen when I went to the edge of the frame with the head. The CAD model is a parametric one where I can change all the variables and it adjusts the diagram (like an excel sheet in a way). For reference I used 0.03mm for the CoC - but as said I was more interested in the relative error visually than which CoC is right for me at which printing size, viewing distance etc... Link to post Share on other sites More sharing options...
Guest mc_k Posted October 8, 2010 Share #23 Posted October 8, 2010 No, the point is that there is nothing to calculate. The value you choose for the maximum acceptable CoC captures your tolerance for unsharpness, and only you can decide what’s acceptable to you. 1/1500 of the image diagonal is just a handy rule of thumb; some like to place stricter requirements on sharpness and are free to do so. Mathematics won’t help you here, I’m afraid. Only when you have determined a CoC value that suits you can you even begin to calculate. The value was originally calculated from experiment; where else would it come from? The value came from line pair resolution tests with the human eye and was adjusted upward for photography by whim or experiment. Obviously, the number you get for the depth of field varies with the value for COC. The number for the depth of field is supposed to more or less correspond to the depth of field you read off a picture of a ruler--with the usual warning there is really no abrupt limit to what's in focus, and the equation gives you one. If you read a text on depth of field theory you will probably see some pictures of rulers. You can calculate your own value by using a measured depth of field and solving for COC in the depth of field equation. It's just a number. It's no less legitimate than some "official" number. ...some like to place stricter requirements on sharpness and are free to do so... All fine, but the equation is longer predicting an assumed "depth of field," and if selective focus is your thing instead of sharpness, it may in principal be less useful. Link to post Share on other sites More sharing options...
mjh Posted October 8, 2010 Share #24 Posted October 8, 2010 It's just a number. It's no less legitimate than some "official" number. Yep, but that’s what I was saying all the time. Any CoC is as legitimate as any other. Originally you had complained that you still had decide on which CoC value to use, but that is just unavoidable. Calculating depth of field (of which the calculations concerning limits to focus and recompose are just an extension) starts with choosing a CoC value, even if one takes the default choice of 1/1500 of the image diagonal (29 µm in the case of the M9 which usually gets rounded up to 30 µm). That this CoC value is often perceived as not strict enough in this digital age isn’t specific to the issue of focus and recompose but applies to depth of field calculations in general. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 8, 2010 Share #25 Posted October 8, 2010 Yep, but that’s what I was saying all the time. Any CoC is as legitimate as any other... that's not what I meant or said. Strictly interpreted, the theory says there is only one correct value. But I understand your point. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 8, 2010 Share #26 Posted October 8, 2010 ah, a spreadsheet is excellent; I guess you could do some things with graphs, too. 9 deg was just a 6th of the FOV (focus in the middle the rotate 1/6FOV to put focus patch on 3rds line)... You want to divide the frame in thirds, not the angle of view...it's not the same thing. No worry--this is the same mistake as in some of the "formulas" for the reframing error that circulate. I think you want approx. 9.6 deg. But another issue is the angle of view is not constant for a lens that focuses as a unit. If you want to be super correct, I think you will need to use the focal length. ...3cm was just roughly measured on the screen when I went to the edge of the frame with the head... I'm pushing a point to the edge of the frame, but I think 3cm is on the low side. I think you should get approx. 8.5 cm error; this would make you stop down to f/5.6 - f/8 as you said. Link to post Share on other sites More sharing options...
Nicoleica Posted October 8, 2010 Share #27 Posted October 8, 2010 Advertisement (gone after registration) It's just a jump to the left And then a step to the right... Link to post Share on other sites More sharing options...
Guest mc_k Posted October 9, 2010 Share #28 Posted October 9, 2010 It's just a jump to the leftAnd then a step to the right... this is not practical for me, bobbing up and down to shoot full length, or if the camera is on a tripod, etc. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 9, 2010 Share #29 Posted October 9, 2010 ...It's not hard to get a simple formula for the error in terms of the distance and focal length (for rule-of-thirds, or any other placement)... Can anyone check this? Link to post Share on other sites More sharing options...
ho_co Posted October 9, 2010 Share #30 Posted October 9, 2010 More general, but possibly of interest: Forum member Stephen Picken has made available a very complete Excel spreadsheet that does a lot of the basic calculations, and started an interesting thread that lays bare many of the basic ramifications of depth of field: http://www.l-camera-forum.com/leica-forum/leica-m8-forum/42289-dof-resolution-perspective.html. Post 11 in the thread (http://www.l-camera-forum.com/leica-forum/leica-m8-forum/42289-dof-resolution-perspective.html#post481823) gives a later URL, linking to which generates a "page-not-found error" which, in turn, points to the current version of the file. But before you guys all go off talking about depth of field, I want to say thanks to the original poster for a very helpful addition to the discussion. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 10, 2010 Share #31 Posted October 10, 2010 thanks for the link. Thanks also to Giordano, who shared his calculations in the other thread. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 10, 2010 Share #32 Posted October 10, 2010 re the dof approach, you can draw the reframing error and the rear dof as functions of distance on the same axes...gives you the same info as the "recomposition circle" but for all distances at once, and also for cases where the error outdoes the dof. Shows you the big difference between recomposing for rule of thirds, and for recomposing to the edge. Also how the error grows compared to the dof. For reframing to the edge of the long dimension, some values are 28mm: the error at 1, 2, and 3 m is 17.9, 36.7, and 55.6 cm respectively. 50mm: the error at 1, 2, and 3 m is 5.6, 11.9, and 18.2 cm. 90mm: the error at 1, 2, and 3 m is 1.6, 3.6, and 5.6cm. (calculated from the nominal focal length, 24X36mm format). Link to post Share on other sites More sharing options...
Guest mc_k Posted October 10, 2010 Share #33 Posted October 10, 2010 ...rear dof..should be "front" dof. Also worth mentioning the current Hasselblad auto focus is based on the reframing error. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 11, 2010 Share #34 Posted October 11, 2010 This, for a 28mm lens. The dotted line is the near depth of field for those that need it (f/2, COC 0.03 mm). The top graph represents reframing to the very edge, and the bottom one is reframing for rule of thirds. You can see if you push a head to the edge of the frame you are not covered by dof until about 3m out; of course you will have to shoot a tin can to know for sure. You can read off the reframing error in cm from the scale on the left, which should be very close to the values in my post above. I used the angle of view to calculate the graphs, and used the focal length to get the more exact numbers above. 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/133176-35-f2-focus-recompose-rotate-focus-plane-shift/?do=findComment&comment=1467989'>More sharing options...
giordano Posted October 11, 2010 Share #35 Posted October 11, 2010 Can anyone check this? I got this far before my broadband connection packed up on Saturday. With a bit of luck it gives the focus error E introduced by recomposing so the focused-on object moves from the centre to a rule-of-thirds intersection. x and y are the height and width of the sensor and f and v have their usual meanings, focal length and lens-to-subject distance. 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! I'll let someone else complete the simple formula. Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! I'll let someone else complete the simple formula. ' data-webShareUrl='https://www.l-camera-forum.com/topic/133176-35-f2-focus-recompose-rotate-focus-plane-shift/?do=findComment&comment=1468837'>More sharing options...
Guest mc_k Posted October 12, 2010 Share #36 Posted October 12, 2010 thank you! My only complaint is the long dimension of the format should be x and the short dimension y Actually I had y=0 because I was just moving the subject horizontally. This is what I was calling rule of thirds placement; I know you can be fancy and place something both horizontally and vertically 1/3 from the edge. I learned the rule from painting. Anyway I appreciate your help--I haven't seen a correct formula anywhere and I have a feeling this is the correct one. Link to post Share on other sites More sharing options...
giordano Posted October 12, 2010 Share #37 Posted October 12, 2010 I worded my post badly: x is meant to be width and y height ... but it makes no difference to the equation. Link to post Share on other sites More sharing options...
Guest mc_k Posted October 12, 2010 Share #38 Posted October 12, 2010 ...I'll let someone else complete the simple formula. ...complete the square, etc., factor a v out, and use v for v - f (since f is very, very small compared to v), to get: 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/133176-35-f2-focus-recompose-rotate-focus-plane-shift/?do=findComment&comment=1469224'>More sharing options...
Guest mc_k Posted October 12, 2010 Share #39 Posted October 12, 2010 ...where E is the error for rule of thirds placement along the dimension x of the format, v is the object distance, and f is the focal length. Link to post Share on other sites More sharing options...
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