JohnyM wrote:Is every lens combo telecentric? Or it's becoming telecentric when you stop it down between lenses to certain value?
The answer to the first question is "no". To the second question it is "yes", assuming that the aperture is properly placed as nicely summarized above by Lou Jost.
Is infinity microscope telecentric? The way i see it, it looks like it's telecentric on sensor side, but not on subject side.
An infinity objective by itself is certainly not telecentric on the image side. If it were, then all the chief rays -- the central ray in the bundle of rays for each feature of the subject -- would all have to be parallel to the optical axis. But then the objective would be useless, because all the outgoing rays would be parallel for all points in the field, and there would be no way to separate rays belonging to one bundle from rays belonging to all the other bundles which would overlap it.
An infinity objective in combination with its tube lens can
be telecentric on the image side. What that requires is to place the objective far in front of the tube lens, so that the limiting aperture in the objective appears to be at infinity when viewed through the tube lens.
For applications commonly seen at photomacrography.net, telecentric on the image side would not be a great idea for at least two reasons. First, in any system that is telecentric on the image side, the rear element of the lens must be at least as large as the sensor diagonal plus the diameter of the exit cone where it leaves the lens. Even for APS-C and modest NA objectives, that means at least 38 mm, which is slightly larger than Raynox DCR-150 and much larger than any of the standard tube lenses from Mitutoyo, Nikon, or Thorlabs. As a result, this approach works only on small sensors. Second, when the system is telecentric on the image side, it becomes impossible to tweak the magnification just by changing the distance between tube lens and sensor.
However, telecentric on the image side is an excellent idea for many industrial applications. In that case, the limitation to small sensors is not a problem, and the resistance to magnification changes is a distinct advantage because it allows changing focus without recalibrating for any measurements. In fact when a lens is telecentric on both sides, then the magnification is "baked into" the optical design and will not be changed no matter how you adjust focus. It's a great simplification for inspection tasks.
On the subject side, any objective can be telecentric or not, depending on where the manufacturer happened to place the limiting aperture with respect to the lens elements in front of it. Many objectives, particularly at higher magnification, are not strictly speaking telecentric, but they are close enough to be treated as if they were because their shallow DOF leads to insignificant scale change within the in-focus slab.