# [rollei_list] Re: 3.5E and 2.8F questions

• From: Emmanuel BIGLER <bigler@xxxxxxxx>
• To: rollei_list@xxxxxxxxxxxxx
• Date: Sat, 18 Jul 2015 15:41:11 +0200

----- Message from John Wild ---------

I do not see why thickness, within certain tolerances, should make much
difference unless the screen is located using its the top surface.

"The recipient has indicated that they prefer replies in these languages: English. "

Hence, let's respect the recipient's preferences: too bad, today I would have preferred answering in modern Greek, to better fit our current EU's concerns ;-)

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Hi John, hello from across the Channel.

I hope that in Bosham (UK), you are enjoying fresh and cool air as usual in the British Isles; here on the Continent, located quite far from the Ocean, we are suffering from those infamous 100F that I had only experienced in the past in the Wild American West ;-)

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Precise registration of a Fresnel+groud glass combination.

This is a delicate question.

In the good old Rollei days, when the ground glass was actually a ground glass, there was no mystery.
The frosted side of the glass defined a reference plane that should be located as the conjugate of the film plane seen through the reflex mirror. I.e. in geometrical, mathematical temrs: symmetric from the film plane with respect to the metallised surface of the reflex mirror.

As of year 1958, things became less clear with Rollei introducing a single piece of molded plastic, combining a Fresnel lens and a frosted plane.

Where is the reference plane ?

I must confess that for the original 1958-style Rollei ground ... ooops, not glasss but plastic, I do not know.

The question is well-known to view camera owners.

When you want to replace an original plain ground glass with a Fresnel (F) + ground glass (GG) combination, you immediately face the problem of proper registration of the new F + GG combination.

Since F has two sides and GG has two sides, a stubborn Rolleiphile would enumerate nothing less than 8 possibilities for the F + GG combination! Actually only a few combinations among the 8 theroretically possible are actualy in use ;-)

Well, to put it shortly, when F is in front of GG, you have to take into account some optical thickness for F.
Before 1958 Rolleiphiles could add an overlay named as "Rolleigrid" over their GG.
This combination (1/8) (lens - GG - F) did not change anything for proper focusing.

But when you want to fit a (F+GG) combination of manufacturer A to a reflex camera made by manufacturer B, you probably come into some difficulties evluating where is the proper optical or mechanical reference plane can be.

All the best !!

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