Fresnels, ground glass, camera design...

[Bill_1856]

IanG, all Pacemaker Graphics were factory installed with the Fresnel BETWEEN the lens and the GG (except the first batch, which had no Fresnel). They allowed about 1/3 thickness of the plastic additional to compensate for the different optical path.
This was done for two reasons: 1) it protected the soft plastic by having the GG behind it, and 2) it actually distributes the light better into the corners.
My Technika IV also came with the Fresnel that way.
All-in-one plastic screens such as the Linhof Super Screen and Maxwell are designed to have the Fresnel pattern away from the lens, requiring a protective clear glass cover, somewhat as suggested by the OP.

[IanG]

IanG, all Pacemaker Graphics were factory installed with the Fresnel BETWEEN the lens and the GG (except the first batch, which had no Fresnel).

The first batch was rather large and ran into the 60's then

None of my Graphics had a fresnel and the 2 Pacemakers had original screens. They were sold without as well. But I think the Graflex fresnel is very much thinner than many modern fresnels so has neglible differance on plane of focus.

Ian

[NewBearings]

What Ian and I are saying, is that even though it may be easier to focus the image on your GG after you made your modification, that does not mean that the image will be in focus on the film. The camera was designed for a Fresnel on the film side of the GG. If you disrupt the design, it is likely that the sharp image you focus on your GG will be in a different plane than the film. That is why Ian is recommending that you shim the GG. Whether it will be the thickness of the Fresnel, I don't know. You may have to experiment. Or perhaps Hugo can find out for you from the factory.

Why some makes put the Fresnel on the film side of the GG is a question for the camera manufacturers.

My Cambo cameras had the Fresnel on the photographer side of the GG. My Walker Titan SF has the Fresnel on the film side. My favorite screen of all, however, was the Bosscreen that I used on my Cambo cameras in place of the GG/Fresnel combination.

David, it's no easier to focus with fresnel on one side verses the other (except for corners). The quality (sharpness) of focus I am getting now without the fresnel between lens and GG is reflected in the media exposed.

This is exactly the question I'm asking. To camera makers or others.

Does your Walker Titan focus properly with a wide lens and fully open aperture?

[IanG]

All my cameras focus perfectly.

With 65mm, 75mm, 90mm Super Angulon's or Grandagon's all my LF cameras focus peferctly.

The question you've avoided is whether that fresnel is the best for WA lenses. Most aren't, if you're only shooting WA's then you need a re-think.

Most fresnel's aren't perfect for Wid-angle lenses, so you have a choice none, or keep it there, and laern to livr with it.

Ian

[NewBearings]

Ian,

It's pointless to "learn to live with it" if I want sharp pictures.

I do use wides and have come to realize from reading posts here that a fresnel doesn't focus well with wide angle lenses if the fresnel is between the lens and gg.
My adaptation (hence my question - the point of this thread) is to use the fresnel externally on the gg.

I'd like a viewing screen that works well with any lens chosen at that moment.


When you say, most fresnels aren't best for WA lenses. What exactly do you mean by "best"? Focus? Or other issues?

[timparkin]

Because a fresnel is a lens, the focus will depend on the bellows length. The longer the bellows length, the further away from the screen the critical focus is. Most fresnels are 'tuned' to work between about 100mm to 360mm (at least that is what it appears on the fresnels I have used). When using a 360mm lens with a fresnel, you have to position your head furrther away from the gg. When using a 90mm lens with a fresnel you have to get really close to the screen. When using a 65mm lens with a 'normal' fresnel, you would have to be tens of millimeters away from the gg and so would not be able to see the whole gg. If you move your head back, the light from the corners will miss your eye (you get dark corners).

So - a wide angle fresnel will focus the light at a 'reasonable' distance from the fresnel for wide angle lenses. However, when used with long lenses, the focal point is a very long way away from the gg and you get strange effects like one eye can see the picture and the other can't ...

Tim

[NewBearings]

Interesting, Tim. Thanks. But if one is using a loupe for critical focus with a short or long bellows how would this affect you summation?

[rdenney]

My adaptation (hence my question - the point of this thread) is to use the fresnel externally on the gg.

I do feel for you. I have a feeling that a couple of your readers got on the wrong path and can't see the right one.

So, to sum up, your Chamonix had a problem in that it did not focus accurately. When you removed the fresnel from the sandwich, the repositioned ground glass did focus accurately.

And you are asking why manufacturers do what they do.

I've seen in both ways. My Cambo doesn't have a fresnel, but all my medium-format cameras do, and they all put the fresnel under the ground glass. But in the case of those cameras, the fresnel has less work to do, and the whole package is calibrated to show correct focus with the fresnel in place.

My Sinar uses a fresnel in a removable frame that mounts behind the ground glass. At first, I didn't like it--both my fresnels are somewhat scratched up from using a loupe. But the scratches don't seem to cause much problem in practice, and I often like removing the fresnel altogether.

One problem with the Sinar fresnel is that its focal length is too long for use with wide-angle lenses. That's a point in favor of a removable fresnel in the way Sinar does it--I can have a fresnel with a shorter focal length made for viewing shorter lenses. I have experimented with that concept by buying a fresnel with a 2.5" focal length from Edmund Optics, and it arrive yesterday. But it's a bit of a disappointment--it was listed in the catalog as being 5" square, and the plastic is, but the fresnel pattern itself is a 4" circle. That will leave the corners high and dry, but I can still experiment with it with a 47mm Super Angulon and 6x9 film.

Using the removable Sinar frame might also make it possible to have made (by Maxwell, possibly) a special fresnel optimized for my more extreme wide-angle lenses. The factory fresnel works fine for lenses of 120mm and longer, but I can't see the corners at all with it for lenses shorter than 90.

I don't think it would be too difficult to sandwich thin glass behind the fresnel in the Sinar frame to protect against scratches, but my experience with it so far suggests the scratches are no that annoying, especially since I can remove it at any moment.

Rick "who also needs a loupe with a narrower nose" Denney

[Michael Rosenberg]

What NewBearing did not point out was that on the Chamonix the fresnel is trimmed to sit in a recessed position under the gg. So if you remove the fresnel the position of the gg is not altered in the least. This is truly Intelligent Design!

Mike

[Michael Rosenberg]

One other comment - there are indeed fresnel of differing focal lengths, and Maxwell offers one for most lenses used, and one for short focal lengths. Fresnels also differ in the number of lines/inch and depth of etched ridges, and the Maxwell screens are the highest I have seen.

Mike