Binoculars: Miyauchi Saturn III - 33/39/50/71/150x100, WO 22x70ED, Fujinon 16x70 FMT-SX Pentax 10x50 PCF-V, 10x43 DCF-SP, 6.5x21 Papilio
Nikon 10x35 EII, 7x35 E, 8x30 EII, 8x23CF AS Diplomat
B&L 7x26 Custom
Scopes: C9.25, TMB130SS/FT, SV80S-LOMO 80/480
Quote:How to make the sighting when assessing edge-of-field vignetting...
Line up the edge of the objective with the edge of the internal baffle, prism aperture, etc., which intrudes into the eyepiece image the most.
It's that simple. There's nothing ambiguous about it. Even though you are outside the instrument, your sight line necessarily lies on the same path for a ray, traveling from a distant object, which passes through the objective's edge, is refracted and then just clears the most intrusive aperture before reaching the focal surface.
If this ray/sight line is at the same-side edge of the eyepiece image circle (as the Miya Saturn III nearly achieves), the circle of full illumination is as large as the field stop. Yippee!
If this ray/sight line is halfway between the same-side edge and the center of the eyepiece image circle, the circle of full illumination is half as large as the field stop. Nice!
If this ray/sight line appears to intersect the center of the circle of the eyepiece image, the system is fully illuminated on-axis [only]. That's OK!
If this ray/sight line appears to intersect the circle of the eyepiece image beyond the center, the system is not working at full aperture. Boo!
When taking a photo, try to observe these conditions:
- Set focus to infinity, as the eyepiece's image (and field stop) will be collimated by the objective and hence optically lie at infinity.
- Move the camera farther back so as to obtian better focus on the objective's edge.
- Illuminate the objective edge so that it can be readily seen. The camera's flash can help if it's not blocked by the bino's lens shade, etc.