Today seems to be "kidney-beaning" day.
Indeed it was.
...and sorry for this lengthy post.
I had a chance to play a couple of hours with my grey Swaro AT and my friend's ATM (+25-50x) and I thought I found something that might hopefully shed some light on this phenomenon, which seems to be quite annoying for some users. I know exactly how maddening it can be because I have had (almost) the same issue with my Nikon 8x32SE binoculars.
Usually long ER and wide apparent FOVs are considered to be a good thing in eyepiece design, but these two features together mean that the "depth" of optimal eye placement is actually quite short. Too short or long eye distance and you will see some shadowing in the field of view (round or bean-shaped). If the ER "plane" is not the same at the center or the edges of the exit pupil, the eyepiece is said to have "spherical aberration of the exit pupil" or "SAEP", and this makes it is even more challenging to have the eye correctly positioned. Kidney bean shaped blackouts and SAEP seem to be often (always?) linked to long-ER, wide-angled, flat-field eyepieces.
I took a picture (Fig. 1) through my old Swaro 20-60x zoom with the camera slightly misaligned & a little too close to the eyepiece. This shows to my knowledge the typical form of kidney-beaning: it appears on the opposite side of the FOV compared to the "shadowing" eye/camera lens aperture. It is relatively easy to get rid of by extending the eyecup or just by taking the eye/camera farther from the eyepiece.
The second image (Fig. 2) tries to show, how the Swaro 25-50x behaves when a well centered camera (or the eye) is gradually brought closer to the eyepiece. In the upper panel (pics 2 and 3) the arrows show that vignetting starts to disappear already from the edges when there is still a heavy shadow in the FOV. When the exit and entrance pupils are lined up, the whole FOV is nicely and evenly illuminated. And further, when the camera is too close (pics 5 and 6), there is now gradually darkening vignetting towards the edges.
In the last set (Fig. 3) I tried to get a bean-shaped shadow by twisting the camera when it was somewhere at position 2 or 3 of the previous set. This time the shadow appeared on the *same* side as the shadowing aperture edge and to remove it you had to bring the camera/eye closer to the eye lens. This may be somewhat counter-intuitive if you are used to the "other kind of" SAEP, but I think one should be able to adapt to it (well, I feel like I did...). However, for those who wear glasses AND have deep set eyes, the ER may not actually be sufficient even when the eyecup is fully retracted - especially at 50x power. In this case the Leica zoom may give just enough more ER to feel "easier" for these users, but for those who don't need thick glasses or do not have deep eyes there should be no problems even with the Swaro.
To summarize my "findings":
- the SAEP of the Swaro 25-50x eyepiece seems to be curved outwards, whereas with the old Swaro 20-60x (at 20x... and the Nikon SE etc.) the outer edges are probably curved inwards.
- to minimize "kidney-beaning" with this eyepiece you should get the eye closer to the eyelens and center the pupil towards the shadow (with the SE, you have to center the eye away from the shadow).
I hope this makes sense and maybe helps some users to find a way to comfortably use this eyepiece, which IMO very successfully combines the best features of fixed wide-angles and the zooms.
Best regards,
Ilkka :t:
ps. I couldn't see a difference in the "black-out behaviour" of the 25-50x zoom whether on the AT or ATM body.