Help! Ground Glass and Fresnel order

[jwaddison]

I have a Toyo 45 something rail camera. There are two pieces of glass/plastic that came with it. Number One (plastic) has a clear centre circle with one side frosted in a concentric circle pattern. The other side is smooth. This piece has "Toyaview" inscribed in a corner. Number Two (glass) is frosted on one side and clear on the other, with lines inscribed in it. I have installed number One first, with the frosted side facing the lens. Number Two is on the outside, again with the frosted side facing the lens.

The way the brackets are situated it seems number One has to go in first, as it's smaller than number Two, and Two has to hold in One.

Have I done this right? What is the purpose of having two frosted surfaces? Is one of these a "Fresnel"?

I've read Ron Wisner's article and searched the threads, but I'm still not convinced I've done it correctly. Any advice greatly appreciated.

[Dave Parker]

Well it is going to depend on if the camera was factory equipped with the fresenel or not, and telling us it is a 45 something really is of no help, I have owned several 45 models of Toyo over the years, some had the fresnel first, some had them second.

Your best bet is to contact the distributor of Toyo, which is Mamiya America and give them the serial number off of the body of the camera, and they may be able to tell you if it was a factory install or an aftermarket install.

By the way, who took it apart?

Dave

[jwaddison]

I've never been able to find a model or serial number on it. I bought it as a 45D but it looks more like a 45G.
I took it apart to clean it. I know the order they came off in, but from what I've read the frosted side should be towards the lens, which it wasn't. Is there no hint from my description of the ground glass/fresnel?

[Steve Barber]

The rough side of the ground glass is always toward the lens. Whether the ground glass goes in front of or behind the fresnel is determined by what is necessary to have the rough side of the ground glass the same distance from a common reference point, such as the plane of the front edges of the camera back, as the film plane is when the film holder is correctly inserted in the camera back.

To determine which goes closest to the lens, put a scrap sheet of film in a film holder and measure from across the front edges of the camera back to the film with the film holder inserted in the camera back. This is what should be the distance, measured in the same way, to the rough side of the ground glass when it is installed in the camera back with the rough side toward the camera lens. If the distance to the ground glass is less than the distance to the film, the difference should be the thickness of the fresnel and the fresnel will act as a spacer between the lens and the ground glass. If the distance to the ground glass, without the fresnel, is the same as the distance to the film, then the fresnel is mounted behind the ground glass away from the lens where it does not change the distance of the ground glass surface from the reference point.

[Robert A. Zeichner]

The rough side of the ground glass is always toward the lens. Whether the ground glass goes in front of or behind the fresnel is determined by what is necessary to have the rough side of the ground glass the same distance from a common reference point, such as the plane of the front edges of the camera back, as the film plane is when the film holder is correctly inserted in the camera back.

To determine which goes closest to the lens, put a scrap sheet of film in a film holder and measure from across the front edges of the camera back to the film with the film holder inserted in the camera back. This is what should be the distance, measured in the same way, to the rough side of the ground glass when it is installed in the camera back with the rough side toward the camera lens. If the distance to the ground glass is less than the distance to the film, the difference should be the thickness of the fresnel and the fresnel will act as a spacer between the lens and the ground glass. If the distance to the ground glass, without the fresnel, is the same as the distance to the film, then the fresnel is mounted behind the ground glass away from the lens where it does not change the distance of the ground glass surface from the reference point.

Well, this is almost correct. A Fresnel is actually a lens and as such, shifts the image passing through it rearward (toward your eyes) by a distance equal to roughly 1/3 the thickness of the Fresnel. That may seem like not much, in that a Fresnel is typically .060" thick, but trust me, it's enough to prevent one from ever getting a sharp negative with any but the longest lenses. If you were to use the Fresnel like a shim (as determined by the measuring technique you suggest), you are certrain to be way off the mark. The ANSI specification for film holder depth is +/- .007" for 4x5.

I think the OP needs to do an experiment as follows: Set up a flat subject, perfectly perpendicular to the lens axis and focus very carefully with a loop. Make one exposure with the gg/Fresnel in one position, making certain to shoot this at the largest aperture you have available. Then, change the gg/Fresnel the other way around and refocus, make a second exposure the same way. Use a sticky note in the scene to label which way the gg/Fresnel are stacked, so you will be able to identify the negatives after processing. The sharper result is likely the way it should be. Note: Ground side of gg always faces the lens and the grooved side of the Fresnel typically faces the lens as well. If neither result is sharp, you may be missing some additional parts like shims.

[Steve Barber]

Excuse me! I thought the question was how to determine which went closer towards the camera lens, the fresnel or the ground glass, both being original components that were removed from the same camera. I think what I wrote will accomplish that.

[Dave Parker]

Well in looking around a bit,

The Description of the fresnel kit for the 45D says the fresnel goes under the ground glass, the kit came with spacers to accomplish this and maintain the correct focal plane

This is the small description on the Calument website.

http://www.calumetphoto.com/item/TY0120/

Dave

[jwaddison]

Thanks guys. The kit on the Calumet site appears to be the one I have. I can't tell which is the ground glass and which is the fresnel though. The only way this makes sense is if the metal holders are for the ground glass, and the plastic fresnel is underneath that, held in by the ground glass.

Are fresnels typically made in a circular pattern? If the fresnel is the glass part (which does not have a circular pattern), there is no way to fit the plastic part on top of it.

I'll do the tests described in the posts, but I'd like to start thinking I've got it right.

[Dave Parker]

The fresnel will be the plate with the circles on it, the glass will be smooth on one side and ground to a frosted look on the other side.

Dave

[jwaddison]

Thanks Dave. So the way I have it set up is with the fresnel underneath the ground glass and the ground glass attached to the camera with the metal holders shown on the Calumet site. Both frosted surfaces are pointing towards the lens.

Now that this is straightened out I'll try replacing the ground glass with the one I bought from you a couple of years ago. I gave you the wrong size and finally got around to grinding 1/8th inch off one side (without breaking it). I haven't had much chance to use the camera since as I've been away working.