PDA

View Full Version : Using Arca Swiss Ground Glass/Fresnel Combination on other Cameras?



neil poulsen
13-Nov-2016, 00:15
In my view, Arca-Swiss offers the best ground glass viewing experience that I've seen, through their Arca G.G./Fresnel combination. It's really excellent. Coming to my question, if I'm not mistaken, their system is usable only on Arca Swiss cameras? It that correct?

To elaborate, Arca places the Fresnel between the ground glass and the lens. Normally, this will throw off the plane of focus, compared to the film plane. But if I'm not mistaken, Arca corrects for this by designing the back to place the Fresnel and ground glass closer to the lens, relative to the film plane. Is this correct? If so, it seems that their ground glass/Fesnel system could not be used on other cameras, if the Fresnel, as with Arca cameras, lies between the ground glass and the lens.

On the other hand, if their correction involves placing the Fresnel/Ground Glass system further away from the lens relative to the film plane, then one could indeed use their Fresnel/Ground Glass system on other cameras by adding shims to make the needed correction. (This assumes that one would place the Fresnel between the lens and ground glass.

To my question, which is correct? Closer to the lens, or further away from the lens, relative to the film plane?

The reason I'm asking, is that, mentioned in other threads, I now have a Wista SP, and I really dislike the the Wista, build in, Fresnel/Ground glass viewing system. If I could use the Arca Fresnel/Ground galss system with shims, then all is well.

But if I can't use the Arca system, what other ground glass/Fresnel systems provide the best viewing experience?

Emmanuel BIGLER
13-Nov-2016, 07:22
Ni, Neil

My answer will not be related to a particular brand name ;)

The combination we are speaking about is the following, listing in the order of the path of light from the lens to the image plane

- flat side of the Fresnel lens;
- corrugated side of the Fresnel lens;
- frosted side of the ground glass; this is where the image is perceived by the operator;
- polished side of the ground glass.

First, let's assume that the Fresnel lens behaves like a plane and parallel piece of transparent material.
In this situation, inserting this element in the optical path in front of the GG will move the image backward by a quantity given by all textbooks in optics as

x = t.(1-1/n)

(at least, this is what you can derive from Gaussian optics, small ray angles)

where "t" is the thickness of the glass or plastic element and "n" is the refractive index.
For example, a flat piece of glass with n=1.5 moves the image backward by one third of its thickness.
In Gaussian optics, this value is totally indenpendant from the actual location of the glass part, whether it is located immediately behind the last lens vertex of just in front of the GG.

Now let's consider a Fresnel lens in front of the GG.
This is definitely not a flat piece of plastic, but certainly, the image is moved backward like for the flat piece of transparent material.

Now we want to use a Fresnel+GG combination as described above (two sides for each element and two elements stacked together make, in principle, 8 possible combinations ;) ) in a view camera initially designed for a plain GG with nothing in front.

The question is : how do I attach the Fresnel+GG combo, so that a sharp image seen of the Fresnel+GG will be sharp in the film holder?

Since the Fresnel moves the image backward, we have to push the Fresnel+GG combo forward in the direction of the lens.
Hence adding shims to the Fresnel+GG combo goes in the wrong direction.

A few years ago, several French LF-ers were tempted by a device named Bosscreen TM.
The Bosscreen TM is a composite GG made of a thin frosted layer between two layers of plane and parallel polished glass.
In order to properly install a BosscreenTM on a view camera designed for a plain GG, you have to compensate for one third of the thickness of the first glass windows in the sandwich.
Hence the edges of the BosscreennTM were machined so that when installing the BosscreennTM, the first glass surface would be closer to the lens by .. a certain quantity (see below the example of the flat glass installed in the Rolleiflex TLR)

Same will apply to a Fresnel + GG combo, the entrance side of the composite GG should seat closer to the lens than the frosted side of a plain GG.

I'm not sure that the explanations are clear, but I hope that they are helpful.

You'll find here
https://www.flickr.com/photos/43175600@N00/4573709647/
some drawings explaining the situation when installing the legendary plane glass device in the film plane of a Rolleiflex TLR, although not exactly the same problem, the principles are exactly the same.
In short : if you add a plate glass of thickness "t" and refractive index "n" in front of the GG, you should set the glass plate
2/3 "in" (lens side)
1/3 "out" (GG side)

Emmanuel BIGLER
13-Nov-2016, 14:30
157391I have attached here a drawing explaining where the image is located behind a glass plate.
Behind a glass plate, the ground glass should be moved backward by 1/3 of the glass thickness.

With respect to the image plane "in air" or on film inside a film holder, the glass plate should be located 2/3.t "in" and 1/3.t "out" and the frosted side of the GG in contact with the exit side of the glass plate.

Dan Fromm
13-Nov-2016, 17:57
Neil, why don't you put yourself out of your misery and buy an Arca-Swiss with the GG/fresnel combination you want? I know that monorails don't fold nicely.

neil poulsen
13-Nov-2016, 18:26
Ni, Neil

My answer will not be related to a particular brand name ;)


Thanks so much for your detailed response. Yeah, it's just as I thought, shimming doesn't work.

So, if one could cut the Fresnel down to fit within the 4x5 opening on the camera's back, and then perhaps glue it's rim or edge to the G.G. to hold it firm, one could then shim the G.G by 2/3rds the thickness of the Fesnel. (Seemingly.:))

Also very helpful is the following . . .

2/3 "in" (lens side)
1/3 "out" (GG side)

Let's take the second case, where we have the following stacking order . . .

Lens
Frost G.G.
Polished G.G.
Smooth Fresnel
Corrugated Fresnel

It would seem that the appearance of focus (or lack of focus) would occur in the plane of Frosted G.G. Then, the Fresnel would move that Frosted G.G. image out (further from lens) by 1/3rd thickness of Fresnel. But I would also think, that translation would not alter whether or not the Frosted G.G. image would appear in focus. (Translates it's position along the lens axis, but would not effect whether or not the image appears in focus.)

If this is the case, then I'm thinking that it should be fine to stack without shims, the G.G. and Fresnel in above order with the Corrugated Fresnel furthest from Lens?

Emmanuel BIGLER
15-Nov-2016, 01:54
Hi Neil

If you use this combination

Lens
Frost G.G.
Polished G.G.
Smooth Fresnel
Corrugated Fresnel

You avoid any image shift as far as the registration of the GG is concerned with respect to the film holder.
So from a mechanical point of view, this is the simplest way to do it, add a Fresnel on top of the polished side of the GG.
This was offered to Rolleiflex users in the fifties, no modification of the camera was required, only add the plastic Fresnel lens (named "Rolleigrid") on top of the GG, which was mounted polished side "up". After that, Rollei introduced a molded plastic "GG" combining both the functions of a GG and a Fresnel lens, like in any modern SLR camera. But in such a camera you do not use shift and tilts, and on a Rollei TLR, you only use a fixed focal length, hence the problem is much simpler than in a view camera with all movements and a wide range of focal lenghts.

The fact that the output image is offset by 1/3 of the total (GG + Fresnel) thickness is not an issue: if you look at the image with a loupe, you'll move the loupe until you see the image on the GG sharp, through both layers or glass and Fresnel.

Alternatively you can put the corrugated side of the Fresnel against the polished side of the GG, as follows
Lens
Frost G.G.
Polished G.G.
Corrugated Fresnel
Smooth Fresnel

The Fresnel will work very similarly (almost, see below regarding asymmetry in a Fresnel lens), and doing so you'll protect the corrugations of the Fresnel which are very fragile and prone to scratches, and can catch dust and dirt more easily than the flat side of the Fresnel.

Now the very last issue, minor IMHO, is: which is the best orientation for the Fresnel lens, from an optical point of view?
Many Fresnel lens are asymmetric and are optimized for infinity-focus focusing.
In principle, the best image quality in our GG combination with such an asymmetric Fresnel lens would be "focus side" facing the lens and "infinity side" facing the user's eye.
I confess that I have no idea on how to recognize which is which, I do not know if the flat side of a Fresnel lens is the "infinity" side or the reverse.
This is not very important, to me, the real practical issues are dust, dirt, and scratches.

This is the reason why the following combination, installed in my camera is the one I prefer
Lens
Smooth Fresnel
Corrugated Fresnel
Frost G.G.
Polished G.G.

Since in my camera, the G.G. is actually made of glass, the polished side of the GG is relatively scratch-resistant, much more than any side of the plastic Fresnel lens.
So it is better that the polished glass side of the GG is the only part of the combo exposed to scratches and dust.
The corrugated side of the Fresnel and the frosted side of the GG, both being fragile, are pressed against each other and in principle unreachable in normal use to dust, dirt, and finger prints.

But, going back to the original question in the thread, this combo requires to offset the frosted side of the G.G. by a certain amount with respect to the film plane.
If the Fresnel lens was a plain flat glass plate with a refractive index n=1.5, the offset would be 2/3 "in" and 1/3 "out", but for a plastic Fresnel lens, I have no idea, the proportions are certainly slightly different, but the idea is the same.

Emmanuel BIGLER
16-Nov-2016, 10:44
Thinking again about a simple solution to build the following stack as a DIY project

Lens
Smooth Fresnel
Corrugated Fresnel
Frost G.G.
Polished G.G.

I think I found a simple way to do it with no need of complex equipement or machining.

1/ start from a conventinal back with a plain GG, i.e.
Lens
Frost G.G.
Polished G.G.
We assume that the registration of the plain GG is good, i.e. the frosted side of the GG is located in the film plane, and the camera, properly focused, delivers sharp images.

2/ add shims under the plain GG, offset it backward by about 1/3 of the Fresnel thickness, hold the GG as usual with small clamps, assume that those clamps can accommodate about 0.5mm extra thickness for shims

3/ press the Fresnel against the frosted side of the GG, to build the following stack
Lens
Smooth Fresnel
Corrugated Fresnel
Frost G.G.
Polished G.G.

You'll have to find a way to hold the Fresnel lens, for crude experiments, gaffer tape will do the job, but eventually another set of small clamps will be required to press the Fresnel lens against the frosted side of the GG.

In this situation, the frosted side of the GG will be properly offset and no difficult machining of the Fresnel is required except that the Fresnel should be slightly smaller than the GG.

For a plastic Fresnel lens of thickess "t" and refractive index "n" the required backward offset of the GG, as mentioned above, is
t.(1-1/n)

The idea is to define this offset mechanically on the frosted side of the GG, which serves as reference plane, not on the Fresnel entrance plane.
(I'll add a drawing later)