Large/medium format lens through focal reducer to 35mm sized Ee-s screen? at

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Old December 25th, 2007, 10:23 PM   #1
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Large/medium format lens through focal reducer to 35mm sized Ee-s screen?


I've sort of been bitten by the adapter bug. I can't do much until I get some of the stuff I've got coming in the mail, but messing around with what I have has given me some ideas. Based on what I've read around, the Ee-s focusing screen is a pretty good choice of scattering plate, given its brightness and lack of grain. I'm going to try and base my adapter around it, static, at least to start. I'm using an HV20, so sensitivity in low light is a concern given the light I expect to lose to the adapter. I'm thinking that one way to compensate for light loss might be to use a large or medium format lens in combination with a "focal reducer" to concentrate the light gathered by the lens onto the Ee-s screen. These are two popular focal reducers used by astrophotographers with their telescopes:

Meade f/3.3 CCD Focal Reducer/Field Flattener:

Meade f/6.3 Focal Reducer & Field Flattener:

Here's a comparison of two:

But I don't even know what kinds of lenses are in these things...

I've found that putting a pair of achromats directly behind the rear element of a 50mm Nikon lens shrinks the image circle of the lens and brings it closer to the lens' rear element. The result is a brighter, smaller image in the low-grade ground glass I am using. The field of view is maintained, though I believe the depth of field increases. I figure the depth of field characteristics of the 35mm format are good enough for me, so if I was able to reduce the image circle of a medium or large format lens to a size small enough to maintain a decent field of view on the Ee-s, the resultant deeper depth of field would still be shallow enough for my tastes, and the gain in brightness would be beneficial. Anyway, I'm wondering if one of these focal reducers would work better than a pair of achromats for shrinking the image circle without adding too many abberations.

Any thoughts on shrinking a larger format image circle to 35mm size to gain brightness, especially by doing it with a focal reducer?

Paul Martin is offline   Reply

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