How Lenses are tested

We use an image-analysis application called lmatest PRO (www.imatest.com)
to test the resolution of the lenses. Photos of specially designed test charts are taken in controlled and consistent conditions at range of focal lengths apertures. These are analyzed for sharpness at the center and edge. the average of these two figures is used to final resolution figure.

Resolution is quoted in 'line widths/picture height': the number of lines that can be resolved within the height of the picture. this is different to lines per millimeter figures because they only apply to a single negative size format typically 35mm. Digital cameras have different sensor sizes which mean different enlargement ratios for print hence different 'lpm' requirements for lenses.

Lenses dont just stop resolving progressively finer detail- they resolve it at lower and lower contrast. in the past photographers have disagreed about when detail becomes too soft to count. lmatest gets round this by using modulation transfer frequency MTF analysis and can define a cut-off point for resolution. this is called MTF50 or the point where the contrast falls to 50% the resolution figures are also dependent on the camera used. Different SLR have different resolution, different strength low-pass anti moire filters over the sensor and different processing and sharpening algorithms.