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Docter Aspectem 40x80 ED

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#51 Andresin150

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Posted 07 January 2013 - 10:44 PM

Then, at the edge, the circle has aproximately 50% of its area illuminated....

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#52 Andresin150

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Posted 07 January 2013 - 10:45 PM

I'll do this for the 40x150's too, but first I need to know if I'm on the right path...

#53 Andresin150

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Posted 07 January 2013 - 10:50 PM

FWIW, the test was exactly the same at all edges... And contrary to the image you linked, the circle stayed perfectly round in the inner/vissible part... Why in that image the circle seems clipped towards the center too?

#54 Rich V.

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Posted 08 January 2013 - 01:21 AM

Hi, Andres, it looks like your're getting the camera angle right in the second picture. I'm supposing the next aperture seen after the objective edge is the baffle at the first prism face; it's a bit hard to tell.

The linked photo of the 100mm BT shows a small chord cut off by the roof edge of a Schmidt roof prism. Perhaps the prism could have been aligned a bit more precisely to avoid the cutoff. It seems to be common with that particular prism design when used at short focal ratios.

Here's how Glenn LeDrew described this method of assessment of vignette in a thread from several years ago:

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.


Rich

#55 Andresin150

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Posted 08 January 2013 - 06:42 PM

Well, i tried some images with the flash on. It is evident that there is no cutoff, the prisms seem bigger than the eyepiece field stop.

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#56 Andresin150

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Posted 08 January 2013 - 06:44 PM

If I go farther away to take the photo, the circle begins to grow... I was like 1.5m away....

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#57 Andresin150

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Posted 08 January 2013 - 06:47 PM

In the 150's I've observed almost the same behavior, but the circle needs more distance to grow compared with the 80's, maybe because of the difference in the eyepieces Afov?...

#58 Rich V.

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Posted 08 January 2013 - 06:58 PM

It's the off axis illumination shot down the optical path from the objective edge like in your second original photo that shows vignette. I wouldn't expect any on-axis problems and don't see any! ;)

I would love to see similar off-axis shots taken through the Fuji 150s. We're looking for the "cats eye" shaped aperture at the eyepiece end of the optical train while sighting along the objective edge. That's what shows the degree of off-axis illumination of the binocular. This is what Glenn's instructions are about.

The off-axis illumination differences between the 25x150 and 40x150 would be most interesting! I'd expect a bit more vignette at 25x than 40x...

Thanks, Andres!

Rich






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