You have obtained a raw file containing the image of a slanted edge captured with good technique. How do you get the MTF curve of the camera and lens combination that took it? Download and feast your eyes on open source MTF Mapper by Frans van den Bergh. No installation required, simply store it in its own folder.
The first thing we are going to do is crop the edges and package them into a TIFF file format so that MTF Mapper has an easier time reading them. Let’s use as an example a Nikon D810+85mm:1.8G ISO 64 studio raw capture by DPReview so that you can follow along if you wish. The file, 0006-ISO64-1_25s.NEF, looks like this:
1) We are going to package the raw file as a TIFF using minimalist converter dcraw with the following command line (if you don’t have dcraw, download it from here – also no installation required, for simplicity now put it in the same folder as MTF Mapper):
dcraw -d -4 -T 0006-ISO64-1_25s.NEF
2) Next we are going to crop a section of each slanted edge about 100-400 pixels on the long side as shown in dashed lines above by opening the TIFF resulting from step 1) in a 16-bit editor of your choice. Lets call them the horizontal and vertical crops. On the short side, leave at least 30 pixels on either side of the edge. The crops should look something like this once done:
You are looking at full resolution, undemosaiced, unprocessed raw data. Note how there is no pixelation in the brighter portions of the edges because DPReview has recently been setting their cameras up with the correct white balance during testing (that’s the -w switch above). Save the two crops as h.tif and v.tif in MTF Mapper’s directory.
3) Run MTF Mapper twice, once for each tif file, renaming the resulting output files by adding a _h and _v extension to each of their names in between runs:
mtf_mapper h.tif x:\outputdir -arbef
mtf_mapper v.tif x:\outputdir -arbef
x:\outputdir is the directory in which you want MTF Mapper to store the output files it generates. Edit it to reflect your wishes.
4) Plot the resulting MTF data. You should now have several output files, two of which will be named raw_sfr_values_h.txt and raw_sfr_values_v.txt. Those are the ones that contain MTF data from the two edges (SFR and MTF are used interchangeably in photography). You can import them into Excel or any plotting program of your choice. In each file you will see four rows of data for the four edges that MTF Mapper detected, one for our slanted edge and three for the artificial edges resulting from the white frames it puts around images to help it isolate the relevant edge. Here for instance is the slanted edge in the vertical crop discovered, analyzed and annotated (MTF50 at 0.27 cycles per pixel) by MTF Mapper. It provides one dimensional spatial resolution information in the direction perpendicular to the slanted edge:
Back to the spreadsheet, the first column is the angle of the edge with respect to the horizontal or vertical axis, for the case just above the angle is equal to 8.958 degrees. The rest of that row is the MTF curve data, starting with a value of 1, which you can plot in increments of 1/64th of a cycle per pixel. The MTF of 1 corresponds to zero cycles per pixel, sometimes referred to as DC.
With a little additional information about sensor size and number of pixels the linear spatial resolution units of cy/px on the x-axis can be easily converted to a number of other common ones, such as lp/mm or lw/ph, as described in this post:
That’s what we are after, the MTF curve of the linear, one dimensional point spread function of the imaging system in question in the direction perpendicular to the edge. In this case it has been obtained by using the average spatial resolution information captured by the four raw planes (R, G1, B, G2) of a D810+85mm:1.8G @ f/5.6: full resolution, no demosaicing, no processing, pure IQ information. It relates directly and quantitatively to the performance of the lens, sensor, edge and technique used.
That’s basically it. This procedure is good enough for relative comparisons, say to compare two cameras/lenses that you own. There are better, more precise ways to prepare captures, run MTF Mapper and interpret the resulting measurements than what shown above which can also be used for absolute comparisons.
The other files generated by MTF Mapper are the ESF and LSF from which the MTF is derived and a record of the MTF50 values in cycles per pixels. If you do not rename them they will get overwritten at each new run. As far as the sharpness conscious average photographer is concerned they are there mainly for diagnostic purposes.