An optical bench only provides the lens MTF. At the same time the system MTF can be seen in a positive light, because system MTF, including the operator, is what eventually determines the technical quality of a photographic image. Some testers apply focus bracketing and simply use the image that yields the highest MTF value at a given position in the frame, an inelegant working method that is in glaring contrast with the calibrated optical bench and its standardized focus criterion. Autofocus, manual focus, live view, … each method may yield different results. The operator is part of the chain and influences the outcome, for instance via his ability to achieve a parallel set-up of target and sensor. There are many factors that influence the final MTF values. The measured MTF is not lens MTF, but system MTF, where the system is the entire chain of operator, lens, anti-alias filter or lack thereof, sensor response, in-camera processing, and post-processing algorithms. The use of different cameras, targets, analysis software, as well as human factors render results irreproducible and difficult to interpret.MTF measurements at infinity focus are not practically feasible.Even with live view, focusing is less accurate than with an optical bench.This is not true for MTF measurements by means of photographing test charts (ISO, USAF, Siemens star, etc.) or at least to a lesser degree. Measurements are automated, reproducible, independent of the operator, and allow a lens performance evaluation at infinity focus. ConsiderationsĪn optical bench is a calibrated measurement device designed specifically for the job. Unfortunately there are disadvantages and pitfalls, which are rarely mentioned or even recognized, and lenses may be hailed or dismissed for the wrong reasons. This method, which is popular among photo magazines and some enthusiasts on the internet, uses dedicated test charts and analysis software. However, such a system is not within reach of individuals and a new method has emerged with the advent of digital cameras. For decades, the preferred measurement device has been the scanning-slit optical bench. A more realistic performance indicator is measured MTF. The MTF can be computed theoretically for known lens designs, but there are always implementation losses and the actual performance of a production lens falls short of the theoretical ideal.
An explanation of MTF is beyond the scope of this article, but excellent texts are available elsewhere. The de facto industry standard to quantify lens performance is the modulation transfer function (MTF).
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