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.

[Harley Goldman]

Don,

I looked under the darkcloth of Jeffrey's Ebony with the 250mm lens, without a loupe, and I could not see the entire scene on the ground glass. I could see parts of the scene, but I had to move my head all around to see the different scene elements. I have owned 3 different 4x5's and never experienced anything like that. There is most definitely something wrong. Figuring it out is the big mystery.

[Donald Hutton]

Yes - that sounds very strange. There is a hot spot with an Ebony, but you can certainly see the entire screen without any difficulty. As I mentioned, I use my hands instead of a darkcloth a lot of the time. With a 250, it should all be pretty even. Shoot Ian Wilson (the Ebony rep in the USA) at igbwilson AT yahoo DOT com. I'm sure he will find the answer to this.

[Jeffrey Sipress]

Don, when I removed the glass retaining clips for the first time, there were only 2 elements; the ground glass itself and a piece of plain clear glass over it, obviously to protect the GG, as I can see no other purpose for it. There was no fresnel sandwiched between. Unless you mean that the GG is actually manufactured with an integrated fresnel. I see no indication of that. Do I have a piece missing? Thanks Harley, and all you others, for the helpful replies. I am going to install the GG and fresnel out of my Toyo to see what happens. I have more experience with that screen and will know any difference immediately.

[Struan Gray]

The easiest way to tell if there is a Fresnel or not is to see if the ground glass assembly works as a lens. Try focussing the sun or a well-lit scene onto a white wall. Watch out for burns if you use the sun though.

If you do have a Fresnel, it is possible that it is mounted back-to-front. Single-sided Fresnels are essentially planar-convex lenses and can have significantly greater aberrations when used the 'wrong' way round, to the extent that they will actually vignette themselves.

[Donald Hutton]

Jeffrey

From what I understand, there is an integrated fresnel in the sandwich. However, I have never taken mine apart so actually do not know for certain exactly what should be there. Shoot an email to Ian - he will know and in my experience, is always helpful and prompt.