Lindolfi Posted January 4, 2012 Share #1  Posted January 4, 2012 Advertisement (gone after registration) Testing of lenses and images is quite simple with the Slanted Edge Technique.  Take a picture of a high contrast slanted black/white edge and cut out a part at 100%, like this   (don't worry about the white balance, the method corrects for it)  Now run this image through sfrmat3, a software package written for Matlab (but there are also independent packages available) and you get this:   This is the Modulation Transfer Function (MTF) or Signal Frequency Response (SFR) of that part of the image.  You can see three color channels (r, g and and the Luminosity (L), so that is the black and white component of the image.  It was made with a Nokton 50/1.1 lens at f/1.1 on a Leica M9 with a sensor resolution of 3699 ppi.  You can see that the modulation is lower for the blue channel, because of the combined spherical and chromatic aberrations.  Furthermore you can see that the half-sampling resolution (Nyquist frequency) is still just represented by the lens, so the sensor is limiting at 75 linepairs/mm  So now it is quite easy to compare lenses by comparing graphs and you can see if this agrees with the data from the manufacturer.  Along the vertical axis is the modulation (1 is perfect, 0 is no information) and along the horizontal axis the detail in the image in terms of spatial frequency in line pairs/mm. The half_sampling point is the maximum the sensor can produce. The higher the curve, the better the lens.  Do this without sharpening of the RAW file to get a proper reading of the modulation (sharpening enhances modulation, but in a non-propertional way along the frequency axis).  With this method you can also see if your focussing was adequate by comparing different settings: The one with the highest curve, stretching out further to the right is the best. You just need a simple image of a slanted edge in black and white with high contrast. It is important that this edge is close to perfect: straight and super sharp.  Also you can establish the optimal aperture of a lens with this method, for instance for architecture or document photography.  Here an explanation of the method: Imatest - MTF curves and Image appearance  You can find the MatLab version here: sfrmat2, sfrmat3, SFRedge: slanted-edge analysis for digital cameras and scanners  A simple and superfluous warning on this Leica site: This is just a limited signature of the properties of a lens. It just says how contrasty a lens is and what the resolution limit is. All the other important properties are not represented. So use this method as just a partial finger print of the lens :-) Link to post Share on other sites More sharing options...
Advertisement Posted January 4, 2012 Posted January 4, 2012 Hi Lindolfi, Take a look here Slanted Edge Technique. I'm sure you'll find what you were looking for!
lct Posted January 4, 2012 Share #2 Â Posted January 4, 2012 Interesting indeed. No Mac version? Link to post Share on other sites More sharing options...
perb Posted January 4, 2012 Share #3  Posted January 4, 2012 Interesting indeed. No Mac version?  Matlab is available for Mac as shown here: MathWorks Nordic - MATLAB - Requirements  However, Matlab is very expensive for personal use. There is a student edition, and you could try and find a friend who uses Matlab (student, researcher, R&D person) to get some help at accessing and running Matlab on a temporary basis.  I use Matlab professionally, but at home I use Octave which is one of several free Matlab clones. There may be minor compatibility problems depending on the code in the sfrmat3 script, but usually they can be fixed.  Other possibilities are (not exhaustive):  Sage: Open Source Mathematics Software  PyLab -  Home - Scilab WebSite  Regards Per Link to post Share on other sites More sharing options...
Guest #12 Posted January 4, 2012 Share #4  Posted January 4, 2012 ... With this method you can also see if your focussing was adequate by comparing different settings: The one with the highest curve, stretching out further to the right is the best. You just need a simple image of a slanted edge in black and white with high contrast. It is important that this edge is close to perfect: straight and super sharp. ...  Did you compare this directly to the usual kind of focus test?  Cf. http://www.l-camera-forum.com/leica-forum/customer-forum/209621-tolerances-summicron-28mm-2.html#post1888349 where someone else used a contrast method. Link to post Share on other sites More sharing options...
Lindolfi Posted January 4, 2012 Author Share #5 Â Posted January 4, 2012 The test with a target chart at an angle with the sensor plane is quicker, but the slanted edge method is precise and if you need it for another purpose (like getting the SFR curve), it can be combined in one experiment. Â If I set out to check focussing I use the angled chart method you refer to, #12. Â By the way: it is surprising how good the human eye can see differences in contrast, so just "eye-balling" charts is not such a bad idea Link to post Share on other sites More sharing options...
Guest #12 Posted January 4, 2012 Share #6 Â Posted January 4, 2012 Thanks for the post, that's interesting. Â I was wondering if a through-focus test with this is more precise than just shooting a ruler. Did you make your own target, or buy one? Anything like this for Mathematica, not Matlab? Link to post Share on other sites More sharing options...
Lindolfi Posted January 4, 2012 Author Share #7 Â Posted January 4, 2012 Advertisement (gone after registration) Yes, I made the target by cutting a piece of black film and glue it on white glass. The source code for MatLab is open. If you have experience writing for Mathematica, perhaps you can translate the code after asking the authors permission. The code is not that long and perhaps it outweighs buying MatLab. Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.