PDA

View Full Version : Fresnels, ground glass, camera design...



NewBearings
7-Jul-2009, 09:53
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
7-Jul-2009, 10:14
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
7-Jul-2009, 10:17
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
7-Jul-2009, 11:10
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
7-Jul-2009, 11:14
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
7-Jul-2009, 11:17
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
7-Jul-2009, 11:22
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
7-Jul-2009, 11:23
I was writing as you posted :D

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
7-Jul-2009, 11:32
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
7-Jul-2009, 11:37
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.

Bill_1856
7-Jul-2009, 11:39
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
7-Jul-2009, 12:01
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 :D

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
7-Jul-2009, 12:46
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
7-Jul-2009, 13:25
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
7-Jul-2009, 13:44
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
7-Jul-2009, 14:15
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
7-Jul-2009, 14:25
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
7-Jul-2009, 14:53
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
7-Jul-2009, 17:26
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
7-Jul-2009, 17:29
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

David Karp
7-Jul-2009, 20:51
Does your Walker Titan focus properly with a wide lens and fully open aperture?

The widest lens I have used with it is a 75mm Grandagon-N. It does focus properly with this lens and my 90, but, as with all short focal lengths, it is not as easy as focusing a longer lens. As mentioned above, it was easier to focus these lenses with the Bosscreen.

My favorite combination so far was with the Bosscreen and a Fresnel that I placed over the GG on the photographer side. I used the Fresnel to aid in composition when the corners were dark, and focused without it.


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!

This explains why the focus is accurate without the Fresnel in place.

NewBearings
7-Jul-2009, 23:57
Rick - Yes you got it. Sorry to hear of the hassle with the Edmund's fresnel.
Yes, a glass sandwich is what I would like too. Good luck!

Michael - Thanks for the info on different focal lengths of fresnel lenses. I'm better informed now. Yes, I was pleasantly surprised to see how well Chamonix has designed the backs. Am hoping my email through Hugo might inspire them to redesign or ad another back incorporating a sandwich: GG/Fresnel/clear glass. We'll see.

David - Thank you for the feedback. So are you using the Bosscreen with the original GG & the fresnel? Or replacing the original GG with the Bosscreen?

Peter De Smidt
8-Jul-2009, 00:14
The Bosscreen would replace the original ground glass. You have to be careful, though. I had one, and on my specific camera it did not focus at the right spot. It's two glass plates with a layer of wax between them. There are taped shims along the edges to get the proper depth. The main advantage is that the was layer provides a very fine-grained diffusion surface, and so you can use a high power loupe to focus without the grain of the ground glass obscuring image detail.

Archphoto
8-Jul-2009, 04:14
I have one for each format 4x5 and 8x10 and they work great.
On the 4x5 I even work with a fresnel lens and never had any problems in focussing or what ever, I don't have a 90mm though, just the 58XL, 72XL, 75SA, 115 Rodagon and longer.

It is a pitty the the dutch compagny that made Bosscreens no longer exists......
I would love to have a Bosscreen for my Shen Hao.

Peter

Michael Rosenberg
8-Jul-2009, 17:10
Rich,

Try looking at the Anchor Optics Commercial Fresnel lens. Particularly item 32593:
Size: 6.7” x 6.7” Effective size: 6.0” Focal Length: 3.0” Thickness: 0.06” 100 32593

http://www.anchoroptics.com/catalog/product.cfm?id=402

Mike

rdenney
9-Jul-2009, 08:04
Rich,

Try looking at the Anchor Optics Commercial Fresnel lens. Particularly item 32593:
Size: 6.7 x 6.7 Effective size: 6.0 Focal Length: 3.0 Thickness: 0.06 100 32593

http://www.anchoroptics.com/catalog/product.cfm?id=402

Mike

Those are made by Edmund Optics, and I see the one I did buy in their list, too. But they provided better dimensions than on the Edmund page. I'll experiment with the one I got to see if I think it will work before investing more. If it doesn't, then I think I need to bite the bullet and give Bill Maxwell a call.

Rick "willing to speculate at $42.50, but not at $80" Denney

IanG
9-Jul-2009, 08:55
Robert White in the UK sells a Wide angle fresnel for Ebony cameras, designed for lenses up to 150mm, actually the same price as the screen mentioned above from Edmund Optical (in the UK) 65.

Ian

Joanna Carter
9-Jul-2009, 09:33
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.
I own a Maxwell screen and I can assure you that the fresnel side of the one piece screen is definitely meant to be facing away from the lens. apart from the screen, as recommended, I use the original Ebony cover glass to protect the Maxwell whilst focusing.

As i mentioned before, if you place any fresnel towards the lens, you are going to be bending the light before you get to see it focused on the GG side of the screen; it just doesn't make sense to fit a fresnel between the lens and the GG.

Joanna Carter
9-Jul-2009, 09:36
Robert White in the UK sells a Wide angle fresnel for Ebony cameras, designed for lenses up to 150mm, actually the same price as the screen mentioned above from Edmund Optical (in the UK) 65.
Having purchased an Ebony WA fresnel before succumbing to the Maxwell, I can assure you that the difference is like night and day. The Ebony will only work with WA lenses, whereas the Maxwell will work from, at least on my camera, 72mm right up to 400mm.

Steve Hamley
9-Jul-2009, 09:39
The Maxwell screen I had is as Joanna describes, and that's the way Bill said to install it. It has a frosted side toward the lens just like the Ebony, and then a plain cover glass went over it.

I might suspect he could or would make the fresnel to accommodate different cameras though.

Cheers, Steve

Joanna Carter
9-Jul-2009, 09:48
I might suspect he could or would make the fresnel to accommodate different cameras though.
Indeed, I think you makes screens for whatever camera you care to use. He even does screens for 35mm and MF cameras.

IanG
9-Jul-2009, 11:00
There's a good logical reason why many manufacturers place the fresnel first, it's better to correct the direction of the light towards the edges and corners first, before it hits the screen and scatters, this gives a brighter overall image. This is the way the major SLR manufacturer (35mm & 120) make theirs too.

Apologies Joanne I incorrectly assumed the Maxwell screens were like the Beattie/Wista etc combination screens.

Ian

Brian Ellis
9-Jul-2009, 11:17
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?

There is no Fresnel made that works well (depending on what one considers to be "well") with any lens chosen at the moment, assuming you have a lens range from roughly 75mm to roughly 300mm. As others have noted, a Fresnel is a lens. It has to be optimized for use with a range of lens focal lengths since nobody is going to design, nor would anyone purchase, a different Fresnel for every possible focal length. Most Fresnels are optimized for moderate wide angle to long. So when they're used with a lens that's shorter than the length for which they were designed they don't work as well.

But some manufacturers - Maxwell, for example - make different Fresnels optimized for different lens focal lengths, one a "normal" Fresnel and at least one other designed specifically for use with wider angle lenses. So If you want a Fresnel that is optimized for any lens you select, I think you'll need to buy at least two Fresnels and switch them around depending on which lens you're using. Of course if you buy two Maxwells you'll be paying about as much for your Fresnels as you did for your camera.

FWIW, I've used almost every viewing screen/Fresnel that's made, everything from a plain ground glass to a Maxwell, to a Beattie, to a Linhof Super Screen, to the Fresnels that are OE on Ebony, Tachihara, and Chamonix cameras, and to BosScreens (4 of them). IMHO Maxwells are the best of the bunch, followed closely by the BosScreen. The rest aren't worth talking about in comparison to those two IMHO. A BosScreen technically isn't a "bright" screen but it gives that effect because it spreads the image evenly across the entire ground glass. And since there's no Fresnel involved you don't have the focusing problems that most Fresnels (Maxwell excepted) present.

Brian Ellis
9-Jul-2009, 18:21
I have one for each format 4x5 and 8x10 and they work great.
On the 4x5 I even work with a fresnel lens and never had any problems in focussing or what ever, I don't have a 90mm though, just the 58XL, 72XL, 75SA, 115 Rodagon and longer.

It is a pitty the the dutch compagny that made Bosscreens no longer exists......
I would love to have a Bosscreen for my Shen Hao.

Peter

Are you sure of that? If so it's a real disappointment. The only reason I ask if you're sure is that I know there was a period a few years ago when they were out of business for some reason but they came back in business shortly afterwards, possibly under another name and possibly after being acquired by another company, I forget even the sketchy details I used to know. But you're saying the reincarnated version itself has now gone under? Have you checked with Ted Bromwell of Bromwell Marketing, the U.S. distributor, to see if he possibly has any still in stock (assuming of course that he's still in business, it's getting hard to keep up with things these days).

Dave Dawson
11-Jul-2009, 12:03
Having read all the comments in this thread...Can I take it that a Boss screen is finer than many sreens (e.g. Sinar)

I have a Boss fitted in a Sinar 5"x7" back and thought it must be a 'bodge' at first sight but I think it may be correct now:)

The ground part looks undersized and the ground part faces away from the lens, this is attached to a plain piece of glass behind it....Is this correct (for starters)

Thanks Dave