CFI BE 4X, remove black hood for more even coverage

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rjlittlefield
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CFI BE 4X, remove black hood for more even coverage

Post by rjlittlefield »

I received a PM today asking about the effect of removing the black hood from the front of a Nikon CFI BE 4X NA 0.1 objective.

Here's the part we're talking about. In various places it's been called the "barrel cover", the "collar", the "barrel", the "barrel sleeve", the "outer barrel sleeve", the "hood", or just the "black part":

Image

It struck me as an interesting question.

The hole in the cover is only about 6.5 mm diameter while the field width is at least 5.6 mm (at 4X on an APS sensor) and the entrance pupil ignoring the cover is 10 mm (based on NA 0.1 at 4X with a 200 mm tube lens).

Given all these conditions, the hole provides unobstructed access to the center of the field, but it's not big enough to provide unobstructed access to the whole field of view. There will be some vignetting going on. This will certainly darken the corners, but does it have any other good or ill effects?

Based on some experiments, the answer is "not much other effect". The tradeoff is only between better center-to-corner uniformity with the cover removed and slightly less veiling glare with it in place. Removing the hood also increases the working distance from 25 mm to 32.3 mm.

Here is an example of the center-to-corner uniformity issue. This is with the objective pushed down to 2X on an APS-sized sensor, using as tube lens a Canon EF 100mm f/2.8 L IS USM macro lens focused at infinity, with an adapter stack of 67-to-52, 52-to-42, and 42-to-RMS. This is a screen grab from Photoshop with no levels adjustment, just the way the images came out of Zerene Stacker DMap.

Image

I'm not bothering to show detail crops of any areas because I can't see any other visible differences other than brightness (from the vignetting) and veiling glare (from loss of the lens hood effect when the cover is removed).

Most likely I'll be using this objective with its cover removed in the future. There's a slight increase in veiling glare due to stray light getting to the lens, but the veiling glare is much easier to correct with a levels adjustment or a separate hood than is darkening of the corners. Removing the cover also gives about 7 mm more working distance.

--Rik

Edit: to insert picture of the objective & hood.
Edit: to document increased working distance.
Last edited by rjlittlefield on Thu Dec 05, 2013 10:46 am, edited 4 times in total.

rjlittlefield
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Post by rjlittlefield »

Refining some numbers...

I inappropriately made the thin-lens assumption that the CFI BE's entrance pupil would be located one focal length from the subject.

By measurement, it's actually 8.4 mm diameter and located about 3 mm in front of the mounting threads, so 42 mm from subject.

Because the hole in the cover is located only 25.5 mm from subject, it gives completely unobstructed access to a circle on the subject roughly 3.6 mm diameter.

Outside that area part of the aperture will be blocked, leading to darkening of the edges and corners.

At 6.7 mm away from center of field (in the corner of an APS frame at 2X), roughly half the aperture is blocked. This leads to the severe falloff shown above.

--Rik

Craig Gerard
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Post by Craig Gerard »

Rik,

Thanks for the report. The difference is most evident.


Craig
To use a classic quote from 'Antz' - "I almost know exactly what I'm doing!"

martincito
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Post by martincito »

Is it worth doing the same thing to a 10X lens? I just discovered the black cover screws off. It certainly looks as if it will be easy to get light on the subject without the cover as the inner diameter of the lens is much less than that of the cover.

Best wishes,
Martin

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Post by rjlittlefield »

martincito wrote:Is it worth doing the same thing to a 10X lens? I just discovered the black cover screws off. It certainly looks as if it will be easy to get light on the subject without the cover as the inner diameter of the lens is much less than that of the cover.
If you're talking about the CFI BE 10X, I think you'll find that removing the black shell doesn't make much if any difference. With the 4X, the black shell sits way out in front of the glass, where it simultaneously reduces the working distance, vignettes the imaging path, and reduces the illumination angle. With the 10X, the black shell ends a bit short of the silver lens barrel. As a result, it has no effect on working distance or vignetting. I'm not sure whether it reduces the illumination angle at all. If it does, the effect will be small.

--Rik

martincito
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Post by martincito »

Thanks Rik! But I think the lens looks cooler without the cover too :-)

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