In our previous blog, Demystifying lens specs, we discussed the Modulation Transfer Function, also known as MTF, which gives you the performance of light through a medium. The MTF helps us understand lens characteristics, however it is extremely hard to compare using manufacturer’s data sheets. In essence, looking at a 25mm / f1.4 lens from vendor A to vendor B may look similar with basic information, but they are not! Not all lenses are created equal and in turn need extensive data for comparison.
The problem is that most lens manufacturers DO NOT supply MTF information, or do not supply complete MTFs. Lens manufacturers with high quality optics, such as Schneider Optics, are one of the few that provide a complete set of MTFs vs. transmission. (See an example on Pg 2 on this datasheet.) Many will provide basic information in terms of line pairs/mm (lp/mm) measured in the center of the lens, however this is still not enough for a true comparison. MTF data will vary with aperture (f3), light intensity and distance from the center to edges. In turn, if you are not comparing “apples to apples”, you cannot draw a conclusion on which is the better lens.
The short answer to this is: Not so easily! First off, the MTF of a computer monitor is probably around 30 lp/mm. All the lenses we are discussing in this blog are at least 2x this, if not 3 or 4x it. So the limiting factor is the monitor, and you will not be able to see any differences. If you have a resolution chart, and some software where you can get the actual pixel data and plot it vs. the test pattern, you can get a better idea. However, a fairly rigid test set up with constant lighting, constant exact FOV and other identical parameters is required. This is a very lengthy process and requires special equipment. True optical testing is the correct way to determine and compare MTF.
Our lens webpage also highlights the resolution and distortion BUT again does not tell the whole story!