Fresnels, ground glass, camera design...

[NewBearings]

I'm not a collector or even really into all the various camera models. To date I've used Sinar (4x5 & 8x10) Horseman 4x5, and my Chamonix 4x5. Can't recall if the first two used fresnel lenses. My images from those cameras are sharp though.

Using the Chamonix with the fresnel between the lens & GG does not allow critical focus with two of my lenses (90mm/135mm). I've jury rigged the fresnel to the back of the gg to allow light to focus straight on to the gg. Seems to focus fine now, and I do find the increased viewing light to be of great value. Especially working fast or with people.

Here's my question: Why aren't camera backs set-up like this from the start?
Why put the fresnel between the lens and gg?
I'm almost certain the gg will wear better than the plastic fresnel (loupe abrasion).
But why not sandwich the fresnel between two sheets of glass?

I've offered this to Hugo (Chamonix) and he has forwarded it to the owner.
Unless I'm missing something, I'd like to see a back designed and set up this way.

Any thoughts?

[IanG]

The position of the screen part of the glass us critical, camera backs are designed so that this is in the same plane as the film, once dark-slide is inserted. So you must not swap them around unless you know hoe to shim them.

If the fresnel was supplied fitted on the inside towards the film you'll lose critical focus as you've move the screen closer.

I fitted a fresnel to a Crown Graphic a couple of hours ago and that had to be fitted to the external side as that was how the back had been set up originally.

Ian

[David Karp]

Ian beat me to it!

Have you checked to make sure that you get sharp focus on your negs/transparencies with this new setup? It may be that swapping the location of the GG and Fresnel may actually throw off the focus, since the camera was designed for them to be placed the other way around. In other words, the camera was designed so that the point of focus and the location of the film in the holder match up with the Fresnel to the inside of the GG. Swapping them might result in problems.

[NewBearings]

Ian beat me to it!

Have you checked to make sure that you get sharp focus on your negs/transparencies with this new setup? It may be that swapping the location of the GG and Fresnel may actually throw off the focus, since the camera was designed for them to be placed the other way around. In other words, the camera was designed so that the point of focus and the location of the film in the holder match up with the Fresnel to the inside of the GG. Swapping them might result in problems.

Thanks guys. Actually with the Chamonix I found the reverse. I was having trouble obtaining critical focus -especially with larger apertures- with the manufacturer supplied/installed fresnel. After it was removed -then placed between the gg and my eye- I have been able to get the focus correct.


So my question is why don't (or do some) manufacturers place the fresnel between the gg and one's eyes?

[IanG]

I measured the brightness difference with & without the fresnel and it made a far greater than you'd imagine. Wile the centre was obviously the same, about an inch from each edge it was 2.5 stops (straight on reading with spotmeter). I also took some photo's which more than confirm this.

So if you switch the fresnel as you've done (NewBearings) then you also need to shim the ground glass by the thickness of the fresnel. It'll only really be critical with wider apertures or longer focus lenses but some people have posted threads here and other forums with constant problems of unsharp negatives/transparencies and it's been because they (or a previous owner) added or removed a fresnel fitted beween the glass & lens.

Ian

[Joanna Carter]

Thanks guys. Actually with the Chamonix I found the reverse. I was having trouble obtaining critical focus -especially with larger apertures- with the manufacturer supplied/installed fresnel. After it was removed -then placed between the gg and my eye- I have been able to get the focus correct.

If you think about it logically, a fresnel should always be behind the GG. The job of the GG is to provide a surface, equivalent to the film, upon which the image falls. It is the job of the fresnel to collect the light that has fallen on the GG and bend it towards a central point, thus yielding a brighter view of the image on the GG.

If you place the fresnel between the lens and the GG, it is going to bend the light, either in or out from the centre, before it reaches the GG. Thus you are effectively, not only altering the spacing of the GG behind the lens, you are actually adding another (flat) lens to the setup.

So my question is why don't (or do some) manufacturers place the fresnel between the gg and one's eyes?

Because they employ people who have no idea of photography to make their cameras up?

[NewBearings]

There seems to be a miscommunication here Ian.

Using the camera as supplied by Chamonix with fresnel installed - I had focus problems with a 90mm and 135mm lens.

Removing the fresnel lens the focus seems fine now.

I replaced the fresnel to the external side of the gg glass to enable a brighter viewing experience.

I don't know how to communicate this more clearly.



However the point of the thread is:

Why do manufacturers place a fresnel between the lens and the GG to begin with?
(when -apparently- it does interfere with focus on wider lenses)

[IanG]

I was writing as you posted

Some manufacturers did, but it works best the way your cameras was set up originally, as is my Wista, Cambo etc. I use the Wista & Cambo with 90mm & 65mm lenses, but some fresnel/screen combinations are better than others.

The problem is what appears sharper to you now won't be in register with the film, DOF may hide the problem with actual exposures.

Ian

[IanG]

Joanna, some screens work best with the fresnel towards the lens, that's the way Maxwell, Beattie, Wista, Cambo etc, etc design & optimise the combination.

Some of these are one piece solid objects not a piece of glass and a separate screen. The Wista screen is like taht so is the Cambo & Beattie. The Wista & Cambo have an additional protective Plexiglas or similar protective clear piece with the grid.

Ian

[David Karp]

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.