Wista DX - Why the plain plastic over focus screen?

Have recently aquired a second hand Wista DX. Wasn't sure if the

focus configuration included a fresnel lens so took it apart. I have

what appears to be a focus screen (concentric ridges with clear

center) nearest the lens and what appears to be a plain piece of

plastic placed over the focus screen nearest the photographer.

What is the plain plastic for? is it simply to hold the focus screen

in place. Would I simply replace the plain plastic with a fresnel

lens if I aquire one?

It sounds like you have a plastic focusing screen and a fresnel screen for light transmission. The fresnel is closest to the lens and disperses the light so it is easier to see into the corners. If there is not a piece of ground glass, does the flat plastic have a "ground" side and a clear side?

What is important to remember, the film plane must be in the same place as the ground surface of the focusing screen. When the film holder is slid into place, the ground glass (plasitc, or whatever) is displaced and the film face now rests exactly where the ground side of the focusing screen was during focusing.

If there is some concern while setting all this up, use a stack of paper, business cards or a depth gauge to check the depth. The film holder's film plane is the same depth as the depth of the recess in the ground glass back (to the ground side of the focus screen). All you are checking is the depth from the camera to the ground side of the glass and the outside film holder's face to the film face. They should be the same.

Hi Tim,

This is exactly what I concluded when I first took the focussing configuration apart however I do not believe the plain plastic to be the focus screen.

I have taken a closer look and there is no texture on it whatsoever. There does though appear to be some small speckles under the surface, in fact it may actually be a thin piece of glass and not plastic at all. I tried focussing with just the plain glass / plastic and nothing can be focused on it, you can just see right through it. I then tried focussing on what you say could be the fresnel lens (concentric rings and plain middle) and focussing is actually quite easy using just this. Surely it is not possible to focus on a fresnel lens alone which is why I think this is in fact the focus screen. Holding the plain glass / plastic over the 'focus screen' merely darkens the image slightly.

It just seems that I have a basic focus screen (no lines inscribed apart from the faint corners of a square)covered by a plain piece of glass / plastic the use of which is to 1) hold the focus screen in place, 2) provide a surface to place the loupe against.

I did also try an approximate depth test as you suggested and the surface of the film appears to coincide with the front of the 'focus screen'.

I'll admit I'm not familiar with your camera, but I have some idea of what might be going on. You might have a screen set up for "true focusing". Using a loop deigned to focus on the focal plain and view the actual image in focused there, rather than viewing an image being scattered off the ground glass. When you use a grain finder in the dark room you're using this focusing technique. It works on cameras too, but there's no image to compose on the screen. Someone doing scientific work may work this way.

The speckles you mentioned are on the focal plain. the way to know you're in focus is to loop the screen and focus your loop to bring the speckles into focus. Then focus the camera so that both the speckles and image are in focus. This achieves an incredibly accurate focus as long as your film and holders are set up to take advantage of it.