john_miller16 Posted February 22, 2003 Share Posted February 22, 2003 I'm considering of purchasing a fresnel in order to make focusing easier. Maybe a rather old question. However, where does the fresnel has to be placed again (on which side of the GG); towards the lens or towards the eye of the photog? If the fresnel is placed towards the lens and the GG on top of it, then of course the plane of sharp focus is totally different than what is set in the film holder. So have I already answered my question with this last phrase or does the fresnel really go towards the lens of the GG? Link to comment Share on other sites More sharing options...
janko_belaj Posted February 22, 2003 Share Posted February 22, 2003 As I understand optics picture is being "made" on the "bottom" side of Fresnel lens. On whatever camera size or type. So, you will have to place Fresnel on the "top" side of GG: "towards your eyes" (and as is on my sinar:) ) .. Link to comment Share on other sites More sharing options...
colin carron Posted February 22, 2003 Share Posted February 22, 2003 Rick, As I understand it for separate GG and fresnel : nearest the lens is the image side of the GG. The fresnel is then placed with ridged side against the outer face of the GG. In this arrangement the fresnel does not alter the distance to the GG image surface. Link to comment Share on other sites More sharing options...
tom_perkins Posted February 22, 2003 Share Posted February 22, 2003 Rick, it goes on the outside, as you have supposed. Check out the article on this subject by Ron Wisner at Wisner.com. Link to comment Share on other sites More sharing options...
sandy_king Posted February 22, 2003 Share Posted February 22, 2003 In is possible to use a fresnel lens on either side of the ground glass. I have used them in both locati ons and in practice there is really very little difference. However, there are some issues you need to address. 1. Whether used in front of the ground glass or behind the glass you should not change the position of the glass itself. This would of course result in a shift of focus. 2. The ridge side of the fresnel should face inward, either toward the ground glass surface if used in front of the glass, or toward the smooth side if used behind the glass. I have a slight preference for using the fresnel in front of the ground glass. In theory this may result in a very slight focus shift, depending on the thickness of the fresnel, but in practice I have found this difference to be so small as to be inconsequential. Sandy King Link to comment Share on other sites More sharing options...
doug_dolde Posted February 22, 2003 Share Posted February 22, 2003 My Arca Swiss has the fresnel on the lens side of the glass. Link to comment Share on other sites More sharing options...
vince_pulvirenti Posted February 22, 2003 Share Posted February 22, 2003 From my experiance a fresnel lens while making the image appear brighter it makes fine focusing more difficult. Link to comment Share on other sites More sharing options...
john_luke Posted February 22, 2003 Share Posted February 22, 2003 Sandy and Doug- Yes, the my Arca has the fresnel on the lens side , as it is designed that way. Linhoff used to be that way also, but changed to an externally placed fresnel. The exact position of the Arca GG and fresnel are designed with 'fresnel focus shift' compensation taken into account. If I decide to do away with the factory fresnel from my Arca, I will have to machine or mill the GG frame to allow me to set the GG in closer to the lens, not where the fresnel used to be, not where the GG used to be, but at a unique point based reversing out that 1/3 compensation factor. A job best left to someone like Steve Grimes. If you take an ordinary GG (no fresnel) and add a fresnel to the lens side, you will find that the focus has shifted, causing you to plant proper focus at a different point than where the film sees proper focus. When fresnels are added to the inside, the usual compensation is 1/3 of the thichness of the fresnel. This may seem like a slight amount, often disregarded, and if not done, focus error often goes undetected if using standard or long lenses, but if using super wide lenses, such as a 47mmXL, any incorrectly placed GGs will show up here in the form of out-of-focus images. Threre are many threads in the archives about this, as this topic comes up a couple of times a month. Please be careful! Link to comment Share on other sites More sharing options...
brian_ellis3 Posted February 22, 2003 Share Posted February 22, 2003 It is indeed an old question, in fact it's been discussed almost endlessly here and elsewhere. Check the archives, you'll find more information about this topic there than you ever wanted to know. Link to comment Share on other sites More sharing options...
john_miller16 Posted February 22, 2003 Author Share Posted February 22, 2003 Thanks to all of you for your answers. When I read the answers in this thread, it kind of made me smile quite a bit. First one can read a couple of answers who tell that the GG is placed towards the eye of the fotog. Then come some posts who say that it is placed on the opposit side and then somebody who questions in general the usefullnes of a fresnel (makes focusing more dificult). I guess the winner is Sandy. That's the take home message for me: Both is possible! Of course with the consideration that focus shift can happen if the fresnel is placed between lens and GG. Thank you all for your contributions and patience. Link to comment Share on other sites More sharing options...
john_luke Posted February 22, 2003 Share Posted February 22, 2003 Both is possible! Of course with the consideration that focus shift can happen if the fresnel is placed between lens and GG. Rick- not "can" happen, but rather "will" happen. Again, back to my Arca with the factory fresnel on the lens side- if I take a plunge micrometer and get a measurement in thousandthes of an inch from a fixed point to a sheet of film in a holder, and take another measurement from that same fixed point to the side fresnel that faces the lens, the micrometer tells me that the fresnel is actuall closer to the lens than the sheet of film by 60 thousandths of an inch. If I remove the fresnel, leaving the GG in its original position, the micrometer tells me that the GG is further away from the lens than the film is by 30 thousandths of an inch. Neither reside at the same plane of the film. (The fresnel is 90 thousandths inch thick, so 1/3 of that is 30 thousandths, which is the compensation that I measured.) The compensated placement of the actual GG is further AWAY from the lens by exactly 30 thousandths of an inch. That is 1/32", or the thichness of 3 sheets of film. With all the pains we take to insure correct and flat placement of film, some people taping film in the middle so it doesn't bow out, some people tapping the film holder to get it to settle, etc, etc, this is a serious amount to be off. Don't neglect it. Link to comment Share on other sites More sharing options...
bruce_barlow Posted February 22, 2003 Share Posted February 22, 2003 It may make focusing SLIGHTLY more difficult, but the benefit of being able to see the whole image brightly at once vastly improved my ability to compose decent pictures. I have always had one for my 4x5 (between my nose and the GG), and recently got one for my 8x10 (ditto). WOW! Being able to see a brighter, full-screen image was a cheap high with no side effects except higher expenses in 8x10 film. And I don't recall ever making an out-of-focus negative... I think the slight problem of harder focusing is easily overcome. The deterioration of my eyesight due to advancing age poses a much more significant problem for me, solved bt reading glasses that I have to remembr to bring, not lose, not break, keep relatively free of fingerprints, etc. You get the idea. Link to comment Share on other sites More sharing options...
john_luke Posted February 23, 2003 Share Posted February 23, 2003 "And I don't recall ever making an out-of-focus negative." Bruce, as discussed very thourougly above, focus shift occurs only when placing a fresnel between the GG and the lens. Your placement will result in no shift, and hence no GG compensation measures needed. For those who do mount fresnels between the GG and lens, the focus shift often goes un-noticed unless using a super wide lens, where the depth of focus is about nil compared to a medium or long lens. Link to comment Share on other sites More sharing options...
sandy_king Posted February 23, 2003 Share Posted February 23, 2003 John, I agree with your comments on focus shift when the fresnel is placed in front of the ground glass. However, even with the parameters you cite I have some doubts that you would see any difference on the negative in all but the most extreme situations, i.e. very close-up work using maximum aperture. For example , ie. a fresnel with a thickness of 0.090", where the focus shift would be about 1/3 of the thickness, or 0.020" (= 0.5mm) would result in a focus shift of only about 1/100 of the focal length of a 47mm lens. And of course, with a lens of 200mm focal length the focus shift amounts to only about 1/400 of the focal length of the lens. In practice, assuming that you focus repeatedely on the same point, I suspect that operator error would amount to considerably more than 1/100 of focal length with a 47mm lens. With most cameras the question of front or rear placement is moot in any event since the rear is often the only place that a fresenl can be conveniently mounted. Sandy King Link to comment Share on other sites More sharing options...
upscan Posted February 23, 2003 Share Posted February 23, 2003 Rick: The very first rule of precision is that any error that can be eliminated should be. Errors can be additive and Murphy's laws will see to it that they always compound themselves, so hoping that errors will be kind enough to knock each other off to benefit the user is just dreaming. Placing the Fresnel on the lens side is tinkering with registration. We photographers are quite capable of making focus errors without trying and are wise to clean the system from built-in errors. What it takes to shift the focus in a 47mm wide angle lens appreciably is measured in X0.001" and putting the Fresnel on the lens side is going up stream without a paddle and poor advice no matter what its source may be. There of course is one practical reason for putting the Fresnel on the lens side: protecting it from wear. Rubbing the base of a loupe over the Fresnel in time will make it rough, after all it is just plastic. But I rather replace the Fresnel than tinker with registration. Link to comment Share on other sites More sharing options...
upscan Posted February 24, 2003 Share Posted February 24, 2003 Sandy: On a 58mm lens, a de-focus of 0.11mm (0.0045")will result in a focus shift from 5 meters to 6 meters. It will be substantially more for a 47 mm lens. For the same 58 mm lens, a de-focus of 0.34mm (0.0135") will result in a focus shift of from 5 meters to 10 meters. Notice that this latter de-focus is 1/170 of the focal length of the 58mm lens. Thus a de-focus of 1/100 of the focal length of a 58 mm or a 47 mm lens is a quite a big deal, not the small thing that 'gut feeling' would seem to indicate. Furthermore, if in addition to a deliberate 'built-in focus error', other components of the system such as the lowly film holder are off-registration as is quite likely, the result could be additive. Add to that the possibility of operator's imperfect focus and you get a photographer sending his Schneider 47 mm Super Angulon XL back to the dealer or listing it in eBay. No wonder that this business of poor registration worries optical firms who must view it as the worst enemy of their good name. Link to comment Share on other sites More sharing options...
john_luke Posted February 24, 2003 Share Posted February 24, 2003 Thank you Julio- I had so many issues trying to get my 47MMXL images sharply focused. My detents were ever so slightly out of perfect parrallelness, I was getting funny tilted planes of focus going through my images. I had the detents reset to factory specs, and I also use the Zig Align each time I mount my 47XL to insure perfect parrallelness. I also discovered the entire GG & frame assembly was defective, it was off spec, so I bought a new one. These errors "compounded" together made focusing at infinity with my 47XL show up as focus at 5' on the film. Too much for hyperfocus to "take care of". It took months of research on this forum, lots of dealing with camera technicians, and a lot of agony to trouble shoot and eventually solve the problem. I looked at everything. I tried various loupes, had my eyes checked, measured my film holders with a micrometer, checked my lens board, and was eventually ablte to isolate the variables that were causing trouble. The Arca, with its interior fresnel is an achilles heel as I found out for those who use ultra wide lenses. According to the Wisner artical from the link posted earlier, it was mentioned the when using inside fresnels with ultra wides, the focus at the corners of the GG will not be accurate as the light is so off axis it comes through with huge focus shift even when the GG and fresnel are properly placed. When using my ultra wides, I only focus in the center of the GG, or directly over the rear lens element if using a shift or rise. Unfortunately, the way the Arca is designed, if I wanted to abandon my interior fresnel and replace it with one of the many high quality outside fresnels, I would have to have my GG assemblies machined by someone like Steve Grimes to accomplish this for both my 6x9 and 4x5 backs. I'll just live with these shortcommings now that I know how to deal with them. I would advise anyone who desires to do landscapes with anything wider than a 75 to not buy a camera with an interior fresnel, period. I view these forums as invaluable and thank you to everyone for this free exchange of ideas! Link to comment Share on other sites More sharing options...
sandy_king Posted February 24, 2003 Share Posted February 24, 2003 John, Your experience with the interior fresnel on the Arca with the 47mm lens reads like a horror story. However, have you considered the possibility that some of the problem may be in the mis-match between the focal length of the fresnel and the focal length of the taking lens. As you probably know, for maximum efficiency the focal length of the fresnel should match as closely as possible the focal length of the taking lens. Have you checked to verify the actual focal length of the fresenl on your Arca? Personally I would avoid using fresnels on any super wide angle lenses, either in front of the ground glass or behind it. Unless the focal lengths of the fresnel and taking lens are closely matched the pain is just much greater than the gain, at least IMHO. Sandy King Link to comment Share on other sites More sharing options...
john_luke Posted February 25, 2003 Share Posted February 25, 2003 Sandy- I spoke with the Arca dealer, distributor and authorized repair technician. They told me that the Arca factory supplied fresnel is supposed to work with all lenses. I've heard what you are suggesting before, but have not had success in trying to find out specifics form anybody familiar with the Arca F Classic. I suspect you are onto something, so if you have any advice or sources you can direct me to, that would be appreciated. John Luke Link to comment Share on other sites More sharing options...
sandy_king Posted February 25, 2003 Share Posted February 25, 2003 John, I am sure that the Arca technician is correct in that the installed will work with all lenses, but my expereince is that fresnel's are most efffective in brightening the screen and giving even illumination over the whole ground glass when the focal length of the fresnel and that of the taking lens are fairly closely matched. With a fresnel of about 7" focal length that I have installed on the back of my 5X7 camera I get ok results with lenses from 120mm to 300mm, but there is no question but that results are better with my 180mm and 210mm lens than with the extreme focal lengths. And of course you are really dealing with an extreme situation with a 47mm lens on 4X5. Have you talked to the Beattie Intenscreen folks about this situation. They may be able to give you some advice and/or adapt one of their products to your needs. Their web site is http://www.intenscreen.com/ There is also someone who used to advertise in View Camera who specializes in this kind of work and came highly recommended to me, but unfortunately I can not put my hands on his address at the moment. Another thing you might do is run a web search for suppliers of fresnels and see if you can locate a soruce for a 4X5" or 5" square fresnel with a very short focal length. If such a thing exists I imagine it would not be terribly expensive and you could perhaps buy one and try it out on the back of your ground glass. Sandy Link to comment Share on other sites More sharing options...
john_luke Posted February 25, 2003 Share Posted February 25, 2003 Sandy- This could open up a can of worms here as my GG is placed to accommodate the factory fresnel which is 90 thousandths of an inch thick If I remove it or replace it with another one of different thickness, I will most likely have to machine the metal frame to allow the GG to be reset in acccordance with the 1/3 rule. Drat those bloody interior fresnels. I guess thats why Linhoff switched their design to an exterior fresnel. Link to comment Share on other sites More sharing options...
bob_salomon Posted February 25, 2003 Share Posted February 25, 2003 "This could open up a can of worms here as my GG is placed to accommodate the factory fresnel which is 90 thousandths of an inch thick If I remove it or replace it with another one of different thickness" No they just readjust the shim positions. The reason for the switch from interior to exterior positioning of the fresnel was so the user could remove the fresnel if desired. This would be done for macro as well as any application where the grooves were disturbing as when focusing with a loupe for instance. Link to comment Share on other sites More sharing options...
upscan Posted February 25, 2003 Share Posted February 25, 2003 John: Your tale is a classic problem, which thanks to your persistence and inquisitiveness got solved but should be a warning to all photographers. Additionally I like to comment that if you use an adjustable-focus loupe which is properly focused on the ground surface of the groundglass, the Fresnel lines will not interfere. This is particularly so if the Loupe is a high magnification loupe. With my Silvestry 10X and my Linhof Fresnel, the Fresnel groves totally disappear. I do not know what is the focal length of the Linhof Fresnel. Looking at B&H I do not recall seeing focal length specs for Fresnels. Has anyone? Link to comment Share on other sites More sharing options...
john_miller16 Posted February 28, 2003 Author Share Posted February 28, 2003 Thanks again for that superb information, especially upon fresnel lenses internally placed and the non-compliance with wide angle / short focal lenses (John). That's very valuable information to know for future dreams of an LF camera come true. Thank you Link to comment Share on other sites More sharing options...
bob_salomon Posted February 28, 2003 Share Posted February 28, 2003 "That's very valuable information to know for future dreams of an LF camera come true" Not necessarily. Link to comment Share on other sites More sharing options...
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