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View Full Version : Is a Fresnel Necessary?



alanbutler57
8-Aug-2012, 20:05
I've purchased but not really played with a Linhof Bi-Kardan 4x5. Not knowing much about it I took the ground glass out to clean hoping to help focusing.

I didn't realize that a plastic Fresnel was often used between the glass and lens and found focusing easier having removed it (by the way, this isn't the original GG, it's Acra Swiss, not Linhof). Am I likely to run into film plane problems if I don't put the Fresnel back (assuming I can find it), or at least a shim?

Thanks in advance.

Mark Stahlke
8-Aug-2012, 20:13
Am I likely to run into film plane problems if I don't put the Fresnel back (assuming I can find it), or at least a shim?It depends on the design of the camera. If the fresnel holds the GG away from its frame then you'll need the fresnel or a shim. If not then you don't need the fresnel or a shim. I'm not familiar with your camera but I can tell you that I can remove the fresnel from my Shen Hao PTB 4x5 without shimming the GG.

Leigh
8-Aug-2012, 20:15
The first question is... Was the fresnel supposed to be between the GG and the lens, or behind the GG?

Locating the fresnel behind the GG, away from the lens, is more common in my experience, but may not be correct for your camera.

You definitely need to determine which configuration is correct before you adjust the GG position.

To answer the question in the thread title...
I have a Maxwell screen (proprietary GG plus fresnel) in my Zone VI field camera, and find the
image is MUCH brighter than in my other field cameras with their stock GGs (no fresnel).

In fact I seldom use a dark cloth with the Zone VI, unless there's bright sun at my back.

- Leigh

jardel
8-Aug-2012, 20:31
Well ... I compose a lot when sun is just going away. So, a GG a little bit brighter is good.

Jardel.

mandoman7
8-Aug-2012, 22:10
As Leigh suggests, most commonly the fresnel is placed behind the GG. In some cases, as with my older Arca Swiss, the distances are not correct unless you place the fresnel underneath the GG. Measuring the distances on your film holders gives you a reference number. The same distance should be maintained with the ground glass to the appropriate surface on the camera where the holder sits. I think its beneficial to actually do the physical measurements to get an idea about what's taking place when you slide the holder in. The fresnel, is not technically necessary if the distances are correct, but they sure help you see the image better.

Rod Klukas
9-Aug-2012, 09:54
With an Arca-swiss screen the mounting flanges on the back frame were probably changed if mounted correctly. So you need to replace both pieces. Other systems are different and have other conventions for mounting. Anytime you add something into the light path between the subject and the image plane, you move the focus 1/3 the thickness of the glass/ fresnel. Some companies get around this by putting the fresnel on the back, with the Ground Glass on the lens side. However if the former owner did not have it modified it may be he quit LF and sold out because he could not get sharp focus. If you contact me I will send you a focus system to help you. It works and is free.

BTW this is also true with filters. You need to focus with a filter mounted to have sharp focus as if you decide to use one. This is especially important with wide angles. If you want to use a dense filter such as red 25 or deep yellow, you can use a filter of less density such as a UV and then substitute the filter you want to use. The UV must be the same brand as the chosen colored filter or this is moot. With a polarizer you have no choice and must focus through it as most are double thick.
Rod

Rod Klukas
9-Aug-2012, 09:57
I've purchased but not really played with a Linhof Bi-Kardan 4x5. Not knowing much about it I took the ground glass out to clean hoping to help focusing.

I didn't realize that a plastic Fresnel was often used between the glass and lens and found focusing easier having removed it (by the way, this isn't the original GG, it's Acra Swiss, not Linhof). Am I likely to run into film plane problems if I don't put the Fresnel back (assuming I can find it), or at least a shim?

One more thing. Your focus aid/loupe should not be higher than 5x and 4x is better. Higher magnification aids will began to magnify the ground surface or fresnel and get in the way of seeing the image.
Rod

Thanks in advance.

Frank Petronio
9-Aug-2012, 10:04
I don't like fresnels and prefer a good ground glass. Some are better than others, some of the European brands are dim but more evenly illuminated, which is usually better than the currently popular trend of having fresnels be super bright in the center and useless at the edges, with a coarse structuring that distracts from actually seeing the image.

With an old Linhof and a non-OEM glass I would start from ground zero and just put a good quality ground glass on, preferably having a good tech like Marflex doing it to get it right. You could at least save postage and just send the rear assembly (ask first though, I am only making an assumption). If you do it yourself, determine a way to make critical tests to make sure you did it right. It will likely involve shimming and repetitive real world test shots.

I think a lot of peoples' cameras are slightly off but because most large format photographers stop down to hold more depth of field, it goes unnoticed unless they do a test against a proven camera set-up.

Bob Salomon
9-Aug-2012, 10:15
On old Linhofs the Fresnel was placed between the gg and the lens. On modern Linhofs (past 40 years or so) the Fresnel goes between the ey and the gg.

To change to the modern design the gg shims need to be reset by the Linhof Service Center. Either Marflex or Nippon Photoclinic in the USA.

Modern Fresnel and Gg from Linhof are much finer and brighter then any that were contemporary to the B system or even Linhof ones from just a few years ago.

On old Linhofs the Fresnel was placed between the gg and the lens. On modern Linhofs (past 40 years or so) the Fresnel goes between the ey and the gg.

To change to the modern design the gg shims need to be reset by the Linhof Service Center. Either Marflex or Nippon Photoclinic in the USA.

Modern Fresnel and Gg from Linhof are much finer and brighter then any that were contemporary to the B system or even Linhof ones from just a few years ago.

Are they necessary? For me yes, regardless of format from half frame 35 to the largest view camera. The Fresnel spreads the light evenly over the entire gg. Eliminates dark corners and edges. The reason Linhof places modern ones between the gg and the eye is so they can be easily and quickly removed when desired. Some LF shooters complain about the Fresnel screens concentric rings but that is usually because they have not properly adjusted their loupe to be in focus on the ground side of their GG. If the loupe is properly focused there then the rings of a modern fresnel are so fine and so far away from where the loupe is focused that the rings almost, if not entirely, disappear.

A related proble can become a nuisance with an enhanced brightness gg and Fresnel. That is the blacking out of the image when the eye and the loupe is not on the optical axis. This is not a problem with most camera manufacturer's GG and Fresnel systems today.

BrianShaw
9-Aug-2012, 10:31
I've never found a real need for a fresnel, but on the cameras that already have them I find them non-offensive.

Vaughn
9-Aug-2012, 10:47
I have never cared for fresnels. I find that the same advantage that frenels give can be achieved sufficiently without them by keeping my eye on the same path that the light travels to the GG (by looking at the GG towards the center of the lens).

But then I am weird -- I like cut corners on my GG, also. (for easily checking for image cut-off by camera movements and/or lens hoods).

Vaughn

IanG
9-Aug-2012, 11:19
I agree with Bob, I find a fresnel helps enormously and I much prefer to use a camera with one fitted. With a fresnel I rarely need to use a focus cloth and it's very much easier to work handheld.

In 45 + years in photography I've never heard of or come across focus shift with filters, except with IR films and their approriate cut off filters. Thet aren't lenses :D

Ian

Vaughn
9-Aug-2012, 11:31
I agree with Bob, I find a fresnel helps enormously and I much prefer to use a camera with one fitted. With a fresnel I rarely need to use a focus cloth and it's very much easier to work handheld.

In 45 + years in photography I've never heard of or come across focus shift with filters, except with IR films and their approriate cut off filters. Thet aren't lenses :D

Ian

I have heard of focus shift using glass filters behind the lens, but not in front of the lens.

Vaughn

PS -- I like being under the darkcloth! LOL!

alanbutler57
9-Aug-2012, 15:07
Many thanks for all of the information. I knew getting into LF would be completely different vs medium and digital, just didn't realize how different. :) The re-mounting of the GG was very difficult as the retaining clips barely engage it, so, it probably would be best to get it replaced if it isn't too expensive. More to think about!