Strange behavoir on my Ebony Screen

[Jeffrey Sipress]

I can't figure this out. On my relatively new 45RW, the angle of view that the ground glass priovides is so narrow that there is a huge difference in the amount of light that each eye sees. When one eye is seeing the image properly, my other eye is receiving so little light, it appears dark. Yes, this sounds strange, but even looking with just one eye and moving my head left to right indicated the narrow band of view that is not wide enough to cover both eyes with my face about 8-9 inches back from the glass. The sensation is erie, and very uncomfortable. Has anyone here experienced this?

[Ole Tjugen]

What lens are you using?

What you describe is perfectly normal with a wide-angle lens - the wider, the worse. Longer lenses will have more parallel light hitting the GG, so the differences are less.

[Donald Hutton]

This is pretty typical behaviour with a wide angle lens and the fresnel screen Ebony use. It is a very bright screen, but with wide lenses, you do have to look "through" the aperture to see what you are doing in any particular spot. The Ebony hand loupe makes focussing with this screen a bit easier. If you do not like the "hot spot" effect, you can get their wide angle fresnel which will improve the situation with wide angle lenses dramatically.

[Frank Petronio]

Change the channel to get better reception

[Harley Goldman]

Get an Arca.

[Jim Rice]

I understand that the focal length of the fresnel is optimized for a range of taking lens focal lengths. Obviously this is the symptom of a fesnel designed for a longer lens than the one being used. What is the effect of a fesnel designed for a shorter FL than the one being used?

[Frank Petronio]

Early Glaucoma - time to hit the weed

[Harley Goldman]

Frank,

I think he already tried it. If I hadn't looked through Jeffrey's Ebony myself and had the same difficulty, I would have thought the weed was the problem instead of the solution.

[Jeffrey Sipress]

The problem even exists using a Fujinon 250 W lens. It's not so bad if I move my head back, but I need to get close to focus and usually can use both eyes to do so using an Optivisor.

This situation never existed with my Toyo 45CF (with fresnel), a Toho, or Harley's Arca F, all with the same lenses. Is there something unique to the Ebony glass? I removed it, and there doesn't appear to be any fresnel type of surface on the GG, which in itself is very nice.

[Donald Hutton]

Jeffrey

There is a fresnel - but it's between two sheets of glass. It adds about two stops of brightness (rough measurement) and is a very bright screen to focus on (quite a bit brighter than on my Arca Swiss F-line, contrary to some eposts I have seen - I looked at them side by side) - but does produce a hot spot. I have no problem using my hands as a dark cloth to compose under most conditions. Personally, I have never tried using a binocular device to focus, but I can imagine that this may be an issue. Certainly, with a conventional loupe it's no problem. However, it is made a lot easier, particularly with wide lenses, if you have a hand loupe, like the Ebony (as opposed to a loupe in a permanent tube mounting) or the Silvestri tilting loupe.