I'm trying to run a (seemingly) simple simulation of a basic microscope using the free software WinLens 3D Basic, but the ray tracing looks wrong:
The microscope consists of a small plano-convex lens with 2 mm focal length and a larger achromatic doublet with 200 mm focal length, crudely representing an infinity-corrected 100X plan-apochromatic objective and a matched tube lens of a commercial system, respectively. The goal is to see the effect of extremely long tube lengths on the field of view (FOV) and, later, possibly also on chromatic aberrations.
Both lenses are placed in a confocal/"2f" configuration (their distance being the sum of their focal lengths) and the object distance has been adjusted until the image distance reached 200 mm (which equates to focusing onto the sample by adjusting the object distance until you get a sharp image). The object size radius was chosen to be 0.125 mm (0.250 mm diameter), corresponding to the 0.250 mm x 100X magn. = 25 mm FOV of the actual microscope system. The two ray bundles originate from one object point on the optical axis and one on the very edge of the FOV:
PROBLEM: The rays going from the tube lens to the sensor appear to be all parallel, whereas one would expect them to converge to form a real image:
What is going on here? All the parameters such as object and image distances, magnification etc. appear to be correct.
PS.: As a side note, the rays going from the objective to the tube lens appear to converge, i.e. there seems to be a "beam waist". I assume this is simply the spherical aberration? To reduce it, I tried to implement an asphere from Thorlabs using the aspheric coefficients, base sphere radius, correct glass type etc. from the datasheet (even included the small N-BK7 window of the laser diode), but the results were way worse:
Removing the laser window improved off-axis performance, but it stills seems to fall short of the simple plano-convex lens (see difference in diameter of central and peripheral ray bundle) and I'm not sure why, since the curvature/base sphere radius of the plano-convex vs. aspheric lens aren't that different (1.7 mm vs. 1.59 mm):
...in any case, this cannot explain the weird parallel rays between the tube lens and sensor plane!
This appears to be a rather basic issue. But I simply cannot find a configuration where the object points get properly focused to image points. Like so: