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Old October 26th, 2009, 02:44 PM   #1
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Edge-to-edge sharpness

Hello everyone,
I've created a DOF adapter, and I am very pleased with the results. The brightness is very good even with 2.8 - 3.5 lenses (but mainly using it with a f1.8). The only thing I am a little bit concerned about is edge-to-edge sharpness. Is there a way to improve on that? I am using a homemade groundglass and BCX lens (cannot easily tell the diopter of it).
Usually I get a pretty sharp image in the center and loosing focus to the edge. Is this because of the f1.8?

Your help is appreciated!
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Old October 26th, 2009, 06:51 PM   #2
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Daniel.


The soft edges may be caused by the BCX condenser but if a single element dioptre is used, this will cause it too.


How close to the groundglass is the BCX?

What are you using for the close-up lens on front of the camcorder?

What power achromatic dioptre did you use?

What is diameter of dioptre?

What is diameter of the BCX?
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Old October 27th, 2009, 01:38 PM   #3
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Singlet lenses like BCX are awful for anything other than monochromatic imaging applications. You need at least a doublet like a plano-convex achromat. If you want a really nice image try an apochromat (I think Sigma produces a 72mm) but you may even be exceeding the capability of the actual camera lens at that point.
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Old October 27th, 2009, 04:59 PM   #4
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Thank you for your replies! I am currently only using a BCX lens between the 35mm lens and the ground glass. It is about 1 cm (about 0.4 inch) from the ground glass.
Will an achromat help produce better results?
EDIT: Which apochromat lens is affordable for a casual DIY-er? :)
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Old October 31st, 2009, 08:55 AM   #5
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I did some research on achromat use. Can I use a single achromatic doublet, or do I have to pair two achromatic doublet lens?
As I see, I can (have to?) keep the BCX lens, which does a wonderful job of getting rid of the vignetting. The achromat can go between the camera lens and the ground glass, as nearest as possible to the ground glass?
Thanks for your help!
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Old October 31st, 2009, 09:46 PM   #6
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You need only use a single achromatic doublet.

The achromatic dioptre should be as close to camera as you can fit it.

The BCX when used for a condensor, should be as close to the groundglass as you can fit it. The big curve side of BCX should face the camera. The nearly flat side should face the groundglass.

For a simple (non-flip) adaptor, a 4+ power achromatic dioptre should be about 8" or 600mm from the groundglass, 7+ power achromatic dioptre should be about 5" or 120mm from the groundglass.
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Old November 1st, 2009, 05:11 AM   #7
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Thanks for the reply!
My BCX lens is the same at both sides, but I am thinking about changing it to a bi-aspheric lens, but I can only get a plastic version, should I stick with a glass BCX (or PCX?) lens, or go with the plastic bi-aspheric?
How much can I "increase" the dioptre of the achromat? I would like the adapter to be compact as possible, so increasing the achromat's dioptre to like 12-14+ can I have it closer to the groundglass?
I am currently experimenting with a flip module design. How does it modify the placement of the BCX and the achromat?
Thank you!
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Old November 1st, 2009, 09:26 AM   #8
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For a flip adaptro, it seems that 4+ or 5+ is about best. The optical path is folded back on itself in the flip process so the length comes back to about 5 inches (120mm) or maybe a bit more for length from SLR lens flange to front of achromat.

If you use a stronger dioptre like more than a 7+ you may find you cannot get it close enough to the groundglass to achieve focus. You may also find with a 7+ that you may pick up an edge or corner of a mirror or prism and be forced to zoom in a long way to clear past it which will cause a crop factor.

I am not a big fan of plastic lenses but if it is not too expensive, give it a try.
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