phovsho Posted September 23, 2009 Share #1 Â Posted September 23, 2009 Advertisement (gone after registration) I'm really confused about how using the same lens on an M9 and an M8 will result in different dof (for the same aperture and distance to subject). Â The lens is the same. We aren't changing the focal length of the lens. Â The sensors are different. We are going from 1.33 crop to full frame. Â The image the lens "projects" onto the M9 and the M8 are the same, except some part of the projection falls outside the bounds of the M8's sensor. Importantly, to my thinking, the centre part of the image will be common to both sensors and will be exactly the same. There will be no difference in DoF. Â Is this correct? If not, why not. Â M Link to post Share on other sites More sharing options...
Advertisement Posted September 23, 2009 Posted September 23, 2009 Hi phovsho, Take a look here The focal length/dof mystery. I'm sure you'll find what you were looking for!
jaapv Posted September 23, 2009 Share #2 Â Posted September 23, 2009 The answer is quite simple. Focal length as such is not a factor (well, a very small factor) in DOF. The main parameters are subject distance and magnification. The reason it apparently shows up as a function of focal length is that the focal length influences the magnification and often distance. So if you use the same lens, "crop" your subject by 1.33x and enlarge your print 1.33x less the M8 will indeed show the same DOF. But in reality you won't. You'll move back, to capture the subject and you will enlarge to the same print size, thus changing the DOF. Link to post Share on other sites More sharing options...
phovsho Posted September 23, 2009 Author Share #3 Â Posted September 23, 2009 Hi So are you saying that there is or isn't a difference in DOF with the same lens on the different cameras? Magnification and distance are the same in both cases (same lens and same subject distance). The only thing which is different is field of view which is irrelevant to DOF. Best M Link to post Share on other sites More sharing options...
phovsho Posted September 23, 2009 Author Share #4 Â Posted September 23, 2009 Sorry Jaap, just re-read your post. It is late here in NZ! We agree. Â Best M Link to post Share on other sites More sharing options...
JeffreyTotaro Posted September 23, 2009 Share #5 Â Posted September 23, 2009 I like to think of it like this. If you were trying to recreate the same image on the M9 as you did on the M8 by changing the lens you would have different DOF. A hopefully clearer example might if you were shooting with a 90mm lens on 4x5 film and then your client decide they needed a higher quality image you would grab your 8x10 camera and your 180mm lens for the same view as with 4x5. The 180 has shallower DOF than does the 90mm. Link to post Share on other sites More sharing options...
yanidel Posted September 23, 2009 Share #6 Â Posted September 23, 2009 I like to think of it like this. If you were trying to recreate the same image on the M9 as you did on the M8 by changing the lens you would have different DOF. Yes, and if wish to recreate the image (FOV) with the same lens, you will have to get closer with the M9. This will also result in a different DOF. Link to post Share on other sites More sharing options...
luigi bertolotti Posted September 23, 2009 Share #7 Â Posted September 23, 2009 Advertisement (gone after registration) Hi So are you saying that there is or isn't a difference in DOF with the same lens on the different cameras? Magnification and distance are the same in both cases (same lens and same subject distance). The only thing which is different is field of view which is irrelevant to DOF. Best M Â Another "Angle Of View" () about the DOF : it is an element that one feels when LOOKS AT a picture : so, M8/M9 same lens/same f stop/same subject/same distance... ok one could say that "DOF on the sensor" is the same. But, as Jaap said, when one looks at the picture he HAS to print something different to obtain identical prints (crop & enlarge the M9 frame in respect to M8) and the visually perceived DOF changes. Jeffrey' statement is the most easy to remember : with a larger sensor or film, ceteris paribus, you'll have always less DOF, simply. Link to post Share on other sites More sharing options...
diogenis Posted September 23, 2009 Share #8 Â Posted September 23, 2009 The image the lens "projects" onto the M9 and the M8 are the same, except some part of the projection falls outside the bounds of the M8's sensor. Importantly, to my thinking, the centre part of the image will be common to both sensors and will be exactly the same. There will be no difference in DoF. Â Is this correct? If not, why not. Â M Â If you keep everything else the same, then you get less dof on the M8 print, because eventually you will have to magnify the print to match the one of the M9. If you shrink M8's print by 33% then you get the same dof with a larger M9 print Link to post Share on other sites More sharing options...
mjh Posted September 23, 2009 Share #9 Â Posted September 23, 2009 Also see http://www.l-camera-forum.com/leica-forum/leica-m9-forum/98987-request-m9-m8-bokeh-dof-comparison.html for a recent discussion of the same topic. Link to post Share on other sites More sharing options...
markgay Posted September 23, 2009 Share #10  Posted September 23, 2009 It's the reverse of the argument I proposed when we all had the M8 .. ie, a 21mm on the M8 will basically behave as a cropped 21mm. It does not become a 28mm.  Many people really wanted to believe that the M8, truly, did give them a f/1.4 100mm lens.  As Jaap elegantly explains, the DOF should stay the same but it doesn’t because with a sensor crop, you treat the lens as if it was its longer cousin. You adjust your physical position.  I did just that. As a result, I was unsatisfied with the fact that when it came to the "normal" focal lengths, and moderate wide angles, these shorter apostates didn't feel quite right. The depth of focus I achieved was different.  That was an unpopular argument with the M8 and, not surprisingly, a suddenly popular argument once the M9 came along.  Regards, Mark Link to post Share on other sites More sharing options...
tobey bilek Posted September 23, 2009 Share #11 Â Posted September 23, 2009 DOF has nothing to do with focal length. F stop and camera to subject distance are the controlling factors. Link to post Share on other sites More sharing options...
diogenis Posted September 24, 2009 Share #12  Posted September 24, 2009 It has: Thus, for a given image format, depth of field is determined by three factors: the focal length of the lens, the f-number of the lens opening (the aperture), and the camera-to-subject distance. -- from here Depth of field - Wikipedia, the free encyclopediaImage format is also dependant on sensor sizes  But in the end you don't really care that much, unless you change lens. Then dof also changes Link to post Share on other sites More sharing options...
jaapv Posted September 24, 2009 Share #13 Â Posted September 24, 2009 Yes, but the focal length is relevant only as a function of magnification To get it completely correct one needs to expand the formula and then the focal length disappears as a parameter. Note, however, that this whole discussion is only valid for a symmetrical lens at medium distances. Retrofocus designs and tele lenses have different calculations and at close distance other formulas apply. Link to post Share on other sites More sharing options...
SJP Posted September 24, 2009 Share #14 Â Posted September 24, 2009 The focal length does not factor out completely as far a I can see from the equations on the Wikipedia site. This is also illustrated here Depth of Field When Image Size is Constant where they show DOF graphs for different focal lengths at the same image size, for 200 and 400 mm the difference is not very big, for 28 and 85 mm the difference is quite large. Â The focal length does factor out at close range (macro regime) but that is special case. Â The problem with all of this is that frequently approximate equations are used that are OK for getting a rough feeling for what is going on but do not reproduce al the salient features. Link to post Share on other sites More sharing options...
jaapv Posted September 24, 2009 Share #15 Â Posted September 24, 2009 That is right, Stephen, but it is a relatively minor factor. As you say the influence varies with the focal length. You will find that there are formulas that encompass film format as well as a separate parameter. It is far more complicated than one would suppose on first sight. My advice is just to judge the print and forget all about the mathematical hair-splitting Link to post Share on other sites More sharing options...
diogenis Posted September 24, 2009 Share #16 Â Posted September 24, 2009 Ofcourse we forget about maths and equations. All that is needed is to look at the window in the engravings on the barrel of a Leica Lens Link to post Share on other sites More sharing options...
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