by Henry Koren, inspired by Paul Romanczyk, edited by Norman Koren
Not all MTF measurement systems will necessarily provide the same results. The quality of the test target can impact the measurements you obtain. Long distance tests are ideally performed at the hyperfocal distance, where there is enough depth of field to have acceptable focus at infinity. Long-range tests which exceed the available space within your lab or factory are the most challenging. A collimator or relay lens system can be used to produce virtual targets at a simulated distance. The measurements obtained through a collimator can diverge from measurements obtained from free-space test targets. This article will discuss how to cope with that.
Reflective collimators, such as ones which include off-axis parabolic mirrors, can offer no MTF loss but are not compact. Refractive collimators offer compact form factors, but like all lenses, are not as sharp in the outer portions of the lens as they are in the center. Since this is less of a problem with narrow field of view collimators, one approach is to have an array of narrow collimators aligned in a “star field” in order to cover a sufficient portion of your lens. Or you can use a single collimator, and tilt and rotate the camera relative to the collimator in order to measure the necessary points across your image. These approaches are complex, more difficult to align and could be prohibitively costly or time-consuming.