Slight-vignetting with TE-11WZ on Kowa 883 post 38X? (1 Viewer)

[mskb]

Everyone,

I have a sharp image circle boundary until about 39X on the TE11WZ + K883. Beyond that, I observe slight vignetting that stays on until the end, 60X. Eye cup adjustments doesn't get rid of it. Is this common?

Thanks,
Kumar

 

[Tringa45]

Hi Kumar,

This has been mentioned before, but I don't think anyone would notice it in field use. With the eyepiece detached I see the onset of vignetting at about 50x.
At and above this magnification the AFoV is around 68°, so the small amount of vignetting is a very minor defect compared to many other eyepieces.
The TE-11WZ achieves a very respectable 57° AFoV at 25x, is parfocal throughout the range and has very low distortion, field curvature and astigmatism.

John

 

[mskb]

Thanks John!

 

[henry link]

For most zoom eyepieces the fieldstop is only in sharp focus for part of the zoom range. Apparent vignetting at high magnifications is probably just the field stop going out of focus. Sometimes at the very lowest magnifications there is some actual vignetting of the FOV as the fieldstop moves up close to the negative field group at the front.

 

[Tringa45]

For most zoom eyepieces the fieldstop is only in sharp focus for part of the zoom range. Apparent vignetting at high magnifications is probably just the field stop going out of focus. Sometimes at the very lowest magnifications there is some actual vignetting of the FOV as the fieldstop moves up close to the negative field group at the front.

Henry,

So the field stop on the Kowa zoom is internal? Would this be the case with other eyepieces, for instance short focal lengths with longer ER and a built-in Barlow?
Some years ago I wanted to achieve minimum magnification (and maximum exit pupil) with my ATM 65HD. The "window" which seals the body has a diameter of 27 mm, so I would have expected vignetting with a 24 mm Panoptic or 32 mm Plössl with their 27 mm field stops. The next best thing was a 28 mm Edmund RKE with 23,3 mm field stop. This gives 16,5x magnification and a 4 mm exit pupil and also works very well on the 883 for 18x and 5 mm exit pupil, although the "window" is just 25 mm diameter.

Regards,
John