Question about the ground glass on WISTA SP 4x5 field camera

[innerlight]

I'm just learning how to use a Wista SP 4x5.

The ground glass has a central 'clear spot' which shows a different degree of focus than the rest of the glass.

For instance if i make sure the focus is crisp in the center circle then the rest of the screen is slightly off, and vice versa.

My question is this - in which part of the screen should I focus? The central spot or the rest of the screen.

[djdister]

Just making a guess here - if the subject appears to be in sharp focus in the central clear spot, but the same subject (at the same distance) outside of the clear spot does not appear in focus, then perhaps the ground glass with clear spot is installed backwards. An object at a fixed distance should appear sharp in both the clear spot and the ground glass.

[innerlight]

thanks for this

[AJ Edmondson]

As Dan noted, unless the Fresnel is installed backward the focus should be the same... you should be able to read the Wista logo from the rear of the camera - otherwise it is flipped and the Fresnel should be nearest the lens with the cover glass on top of it. If you aren't familiar with focusing the area outside the center spot may appear less sharp due to the texture.

Joel

[Bob Salomon]

Attached is an image of a brand new, just out of the factors box, Wista 45 SP back with the ground glass with spot, and the cover glass. The Fresnel does not come on it when you buy a replacement back, neither does the focusing hood.
The ground side of the gg faces the lens and, as mentioned, the WISTA name is readable from the back. The cover glass is on top of the gg so it is closest to the eye.

[Bob Salomon]

Attached is an image of a brand new, just out of the factors box, Wista 45 SP back with the ground glass with spot, and the cover glass. The Fresnel does not come on it when you buy a replacement back, neither does the focusing hood.
The ground side of the gg faces the lens and, as mentioned, the WISTA name is readable from the back. The cover glass is on top of the gg so it is closest to the eye.

Oops, that is factory box not factors

[innerlight]

Attached is an image of a brand new, just out of the factors box, Wista 45 SP back with the ground glass with spot, and the cover glass. The Fresnel does not come on it when you buy a replacement back, neither does the focusing hood.
The ground side of the gg faces the lens and, as mentioned, the WISTA name is readable from the back. The cover glass is on top of the gg so it is closest to the eye.

Thanks Bob and the other gentlemen who commented on my post. The WISTA name on the glass is right reading - it's good to know that and I think with experience and testing (which I'm doing right now) maybe this issue will become a molehill rather than the mountain I was turning it into.

[Jac@stafford.net]

For instance if i make sure the focus is crisp in the center circle then the rest of the screen is slightly off, and vice versa.

How does the focus appear on the negative? You might be experiencing something that can happen with an inappropriate loupe, or blended bi-focal glasses, uninformed technique. I'm not sure what you mean by 'clear spot', but if it is a place with no ground-glass texture (on the lens side, of course) then you might be doing improper aerial focusing.

Start with the best you can do focusing on the center, being sure the movements are parallel, and check the negative with a good, 8X loupe. Keep in mind that you are trying to render a 3D subject in 2D. Then we can review movements.
.

[Nathan Potter]

It's usually not a good idea to do aerial focusing using a loupe without having a small solid spot within the clear area. Something like a dot of india ink inside the clear area will enable you to precisely focus the loupe at the point of the image plane then bring the scene into focus simultaneously with the dot.

Using the edge of the ground area as the loupe focus is OK but the ground area has some depth to it as well as severe light scattering so the solid dot or line is a more precise approach. Of course in all cases you have to make sure that the ground area and the dot are the correct distance from what will be the film plane when the film holder is inserted.

Nate Potter, Austin TX.

[Bob Salomon]

It's usually not a good idea to do aerial focusing using a loupe without having a small solid spot within the clear area. Something like a dot of india ink inside the clear area will enable you to precisely focus the loupe at the point of the image plane then bring the scene into focus simultaneously with the dot.

Using the edge of the ground area as the loupe focus is OK but the ground area has some depth to it as well as severe light scattering so the solid dot or line is a more precise approach. Of course in all cases you have to make sure that the ground area and the dot are the correct distance from what will be the film plane when the film holder is inserted.

Nate Potter, Austin TX.

And the dot is on the ground side of the gg not the top side!