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pjviitas · Joined: 27 Jun 2006 Posts: 13 Location: Baffin Island

I have taken the time to read all the posts about fresnel lenses and have yet to find a consistent procedure for replacing a fresnel lense.

Some things I have concluded from reading the posts are:
-Among other things, the fresnel lense can be though of as a "shim" used to place the ground glass into its proper position in relation to the film plane.
-Removal of the fresnel lense would require the shimming of the ground glass into its proper position in relation to the film plane. The thickness of the shim would not necessarily be the thickness of the fresnel lense.

So, my question is this:
What size shim would be required for the ground glass/fresnel assembly if the fresnel was replaced with a thinner model.

With a shortage in original fresnel lenses for the Graflok back and the apparent availability of fresnel lenses of varying thickness, discussion of this question may shed light on what seems a common predicament.

Best Regards, Hedgehog

45PSS · Posted: Sat Aug 05, 2006 4:31 am Post subject:

The optical shift of the "origional" ektalite screen of the Graphics is 1/2 the thickness of the ektalite screen resulting in the ground glass being off set from the film plane focal point 1/2 the thickness of the ektalite screen.
If the ektalite screen is removed then the ground glass needs to be shimed so that it is at the film focal plane. A fresnel can be added to the rear or behind the ground glass without affecting the focal plane of the ground glass.
If replacing the ektalite screen with a newer fresnel then one must know the focal plane shift for that fresnel and shim from the mount bosses to put the ground glass in the corrected focal plane.
If the focal shift of the new fresnel is NOT known then I suggest shimming so that the ground glass sits at the non fresnel focal point and test using a fence or wall with measurements markers, shoot down it at an angle, so that you can tell how much shift there is if any and in which direction. Polaroid back and film would be good for this.

Shutterfinger

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Graflex Corp.was a (silent) supporter of Rain Forest Devastation.

[ This Message was edited by: 45PSS on 2006-08-04 21:32 ]

pjviitas · Joined: 27 Jun 2006 Posts: 13 Location: Baffin Island

Thats too bad since I unknowingly ruined part of my fresnel while cleaning it with rubbing alchohol.

If my camera came with a ground glass/fresnel assembly and I needed to change the fresnel, there seems to be no easy way to do this.

In any event, thanks for all your help.

45PSS · Posted: Thu Aug 10, 2006 4:27 am Post subject:

If you need to establish the film focal plane on your camera:
1. Remove the back from the camera noting the position of the screws as they are not all the same size.
2. Place a scrap piece of film in a standard film holder. Mark this film into six quardants (3rds verticle and horizional).
3. Insert film holder into the back with the film facing the camera mount surface.
4. Lay a straight edge across the center of the camera back mount side.
5. Clamp a small ruller or piece of wood perpindicular to the straight edge so that it touches the film and makes it flat against the film holder.
6. Check that the center of the other quardants are within .001 inch of the center measurement, if not try a different film holder.
7. Without moving the perpindicular piece, position the straight edge so that the perpindicular piece is next to a mount boss.
8. The distance fron the mount boss face to the tip of the perpindiculat piece is the shim thickness needed to place the ground glass at the film plane.

Make sure that all measurements are made with the straight edge on the same level across the back frame, avoid bumps and dips in the frame edge.
Charles.

_________________
The best camera ever made is the one that YOU enjoy using and produces the image quality that satifies YOU.

john wilton · Joined: 26 Aug 2008 Posts: 26

john wilton · Joined: 26 Aug 2008 Posts: 26

By using the rod that emerges from the far end of my calipers as an improvised depth micrometer to compare the position of the ground glass without ektalite with the surface of a piece of film in a Lisco holder I figured the shim should be 1.0mm. I happened to have some toothpicks that were cut from a 1.0mm thick sheet and proved ideal to shim in the very narrow shoulder. Shooting an inclined test target showed the shim is correct (enough).

I actually find it easier and more precise focusing without the Ektalite, at least with my possibly degraded old Ektalite.