Removing Fresnal

[jonreid]

I'd really like to remove the fresnal on my deardorff's 4x5 back but if I do so the GG doesn't sit in the correct position. Has anyone figured a workaround for this? Some sort of spacer that will hold the GG accurately toward the back of the graflok assembly?

J

[RichardRitter]

Best to send the back to some one that can measure the back for the correct "t" and shin or correct as needed.

[Neal Chaves]

When I removed the Fresnel from my Crown Graphic, I found that the correct ground glass shim thickness is not the same as the thickness of the Fresnel, but quite a bit thinner, because the Fresnel is an optical element not just a spacer.

To determine the thickness of the shim required, measure the depth of a film holder to the film septum and subtract five thousands or so for film thickness.

Now measure the depth of your ground glass frame with the glass in place. The thickness of the shim required is the difference between these two measurements.

[Ari]

Send it to a pro, otherwise the missed focus will drive you mad for months to come.

[jonreid]

thanks everyone, looks more involved than I though. Glad I asked.

Jon

[Neal Chaves]

Before you spend money or tie up you camera for repairs that might not even be correct, try this.

Once I had my measurement, and I can't tell you now what it was, I took my micrometer to the hobby store to look for shim stock. I found some brass that was the right thickness, but brass and aluminum tend to corrode when placed in contact with one another. There was no stainless steel available. Then, by chance in the model rail road department I found some gray plastic that was just the right thickness. It comes in strips about 1" wide by 10" long. It's about 30 thousands, the thickness of a credit card. I still have some around but can't find it tonight.

With a utility knife and a metal ruler, I cut thin strips to go in place under the ground glass and glued them to the ground glass frame with a thin coat of epoxy just prior to fitting the glass. My focus accuracy is as close as it can get. I too dislike the Fresnel lens in a camera to be used with movements or wide lenses like the 65 and 90 I frequently use on the Crown, so I wanted plain ground glass focusing.

[TheDeardorffGuy]

Good For you!! Send me some images so I can see what back you have, Shoot the back side and the black side.
Ken


I'd really like to remove the fresnal on my deardorff's 4x5 back but if I do so the GG doesn't sit in the correct position. Has anyone figured a workaround for this? Some sort of spacer that will hold the GG accurately toward the back of the graflok assembly?

J

[jonreid]

Hi Ken,
Will do as you requested over the next day or so.

Jon

[Steve Smith]

Send it to a pro, otherwise the missed focus will drive you mad for months to come.

It's easy enough to do yourself:

Fit a lens board without a lens and put a filmholder in place with a scrap piece of film fitted and the darkslide removed.

Measure the distance from the film to the front of the lens board.

Now remove the film holder. The distance from the ground glass to the front of the lens board should be the same as the film to board distance just measured.


Steve.

[IanG]

As Steve & Neal say it's extremely easy to measure and then shim the screen accurately yourself. I prefer Neal's method and there is an ISO standard that covers the International backs and the spacings.

Ian

[Roger Cole]

Why do you want to remove it? I'd like to add one to my Linhof. It's maddeningly dim with wide lenses.