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Photo Forum / Film Photography / Large Format / May 2004Empirical method to measure cell spacing?
Say you have two matched cells for a Dagor (Claron, whatever) that came from a barrel mount that is now lost and you want to mount them in a shutter. Is there some way to optically determine the correct spacing? Along the same lines has anyone taken a Claron (or any other lens) and unscrewed a cell one turn and then measured the resolution, two turns etc. How sensitive is it to correct spacing? Kirk > Say you have two matched cells for a Dagor (Claron, whatever) that > came from a barrel mount that is now lost and you want to mount them [quoted text clipped - 4 lines] > unscrewed a cell one turn and then measured the resolution, two turns > etc. How sensitive is it to correct spacing? ! bump ! (Just trying to provoke some response, Kirk. I'm interested in an answer, too. Where's Brian Caldwell or Richard K when we need them? :)) > How sensitive is it to correct spacing? I think I can answer this. Spacing more or less only controls edge resolution. It also controls the distance at which the lens has the best overall center to edge resolution. You can get back to the correct spacing by your hunt and peck method but you will need some resolution targets. You will also need to pick a distance where you want the best overall resolution from center to edge. Larry > > How sensitive is it to correct spacing? > [quoted text clipped - 5 lines] > will need some resolution targets. You will also need to pick a distance where > you want the best overall resolution from center to edge. Any tips on how to set up an aerial focusing device - how to fashion one? > Say you have two matched cells for a Dagor (Claron, whatever) that > came from a barrel mount that is now lost and you want to mount them [quoted text clipped - 6 lines] > > Kirk The effect of cell spacing varies with the lens type. Generally it is more sensitive for better lenses. For symmetrical lenses like the Dagor or Claron it can affect the field flatness but also affects coma and spherical aberration. In a Tessar it makes the coma increase significantly. I've also found that it increases spherical and attendant focus shift. I don't know how to translate this to MTF because it varies so much with different lenses. Also, effect varies with the image angle. One way to find the optimum spacing is to vary the spacing while examining the image of a very distant point source, like a star. Generally, some optimum point will be observed. Since spacing affects so many things at the same time it takes pretty careful examination of the image to determine the correct distance. For a high quality Tessar the tollerance for mis-spacing is very small.  Signature--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com > The effect of cell spacing varies with the lens type. > Generally it is more sensitive for better lenses. For [quoted text clipped - 12 lines] > correct distance. For a high quality Tessar the tollerance > for mis-spacing is very small. As usual, thanks very much for the information, Richard. Where does the Biogon design, in particular the Zeiss Biogon 38mm fall in this regard? > As usual, thanks very much for the information, Richard. > > Where does the Biogon design, in particular the Zeiss Biogon 38mm fall in > this regard? Biogon type designs tend to shout "astigmatism" when you mess up the cell spacing. Mind you, I'm not entirely sure I have the actual Zeiss Biogon prescription and can only guess that it corresponds one of the early Bertele patents, but most designs of this type have similar aberration behavior. Brian www.caldwellphotographic.com > "Richard Knoppow" <dickburk@ix.netcom.com> wrote in message news:YWYnc.16076$Hs1.11410@newsread2.news.pas.earthlink.net... > > The effect of cell spacing varies with the lens type. > > Generally it is more sensitive for better lenses. For [quoted text clipped - 17 lines] > Where does the Biogon design, in particular the Zeiss Biogon 38mm fall in > this regard? I see Brian Caldwell has already answered this. Brian is a very knowledgible lens designer. Signature--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com Folks, So here is what we are talking about. I figure out some way to mount my loose lens cells in an adjustable tube so they are parallel. Already I am in trouble. I then mount this on a camera somehow and focus on a distant point light source. (I have never found focusing on a star is trivial.) I then I start adjusting the spacing while watching my point source until it is sharp both at the center and at the edge of the field. Once I got my nice sharp point source I measure the spacing. So is the spacing sensitive to .001 inch or less? More? Someone must know. Again has anyone actually done the experiment by unscrewing an element and taking a few pictures after a few twists or by golly even examining a light point source. (maybe a laser pointer on a wall 30 feet away or so as stars are so faint). All yak, yak, yak, and no action here. I am actually going to have to do it. Yikes. Kirk > > "Richard Knoppow" <dickburk@ix.netcom.com> wrote in > message [quoted text clipped - 32 lines] > I see Brian Caldwell has already answered this. Brian is a > very knowledgible lens designer. > I then mount this on a camera somehow and >focus on a distant point light source. This will only work if you use two point light sources. One for the center of the cens and the other for the edge. Then all you need to do is balance the size of the two points. When both give the same size image you got it. Larry > > I then mount this on a camera somehow and > >focus on a distant point light source. [quoted text clipped - 4 lines] > Then all you need to do is balance the size of the two points. When both give > the same size image you got it. Even when the lens is wide angle, specifically a Bigon design? >Even when the lens is wide angle, specifically a Bigon design? Even more important. You must balance the center resolution with the edge resolution. Example might be Micron space Center Edge L/mm -50 285 50 -40 300 100 -30 320 300 -20 340 200 -10 360 100 0 380 50 10 350 20 You correct spacer looks to be about a -27 or -28 Larry > >Even when the lens is wide angle, specifically a Bigon design? > [quoted text clipped - 16 lines] > > Larry You will never get center and edge resolution to be the same since the effect of diffraction at the stop varies with image angle. Further, the variation in cell spacing changes a lot of things depending on the lens design. It can change spherical aberration, which appears all over the image, not just at the edges. I can affect field curvature, astigmatism, and coma, among other things. In general its useful to measure for focus shift at the center because this is a sign of spherical aberration. Focus with the lens wide open and again at a mid stop, say two or three stops down from maxium apertur. Coma appears as a tear drop shaped smear of small sources at the margins of the image. Its affected by the stop so its not easy to find the minimum. Also, symmetrical or semi-symmetrical lenses have little coma. When the cell spacing is right the coma should be minimal with the aperture at maximum. field curvature can be measured by measured the difference in focus position of a very distant object at the center, the middle, and edge of the image. Astigmatism is more difficult to measure and really requires the use of an array of proper resolution charts. Astigmatism shows up as a variation in the field curvature for light rays entering the lens along a radius and those along an annulus. In practice it results in radial and tangential lines focusing differently. That is, the spokes and circles of a spider web will not be in simultaneous focus. Proper cell spacing should minimise this effect. Properly designed Plasmat type lenses have little astigmatism so its fairly easy to tell if its bad. You need an optical bench of some sort for changing the cell spacing. This is similar to a lathe bed with clamps to hold the lens compenents. The usual viewing device is a low power microscope (30X to perhaps 100X) of very good quality. Gross problems are visible with a high quality 10x magnifier. The aerial image will make some aspects of lens performance easier to view but the magnifier or microscope has its own set of vices and can mask certain effects, especially if it acts to stop down the lens. Fine ground glass is actually better for spotting some problems, especially at the edges of the field. Brian Caldwell, a lens designer, once posted a fairly complete answer to the question of cell spacing here, I know he reads the group occasionally, perhaps he will repost. Signature--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com > [... excellent article ...] > The usual viewing device is a low > power microscope (30X to perhaps 100X) of very good quality. I have something that might be such a microscope. I don't know. It is a thick, heavy column about 2" in diameter and 18" tall on a solid, short three-legged base. On this column is mounted what looks like a small telescope that rides up and down the column on a track machined into the column. There are very fine adjustments for further vertical adjustment and for moving the telescope forward and back in exceedingly fine increments. Looking through the scope one can see vertical and horizontal crosshairs with fine subdividing lines. It focuses as close as about 4" to perhaps 16". Magnification is - well, it's been so long I can't tell you but I'd guess the field up close is about 4mm, full view. I've been debating whether I should throw it away, take it down for parts. > You will never get center and edge resolution to be the > same since the effect of diffraction at the stop varies with [quoted text clipped - 38 lines] > complete answer to the question of cell spacing here, I know > he reads the group occasionally, perhaps he will repost. Hi Richard: Thanks for the vote of confidence. I would have answered here earlier, but recent travels have been a big distraction. Your answer is pretty complete and accurate (as usual), so there's only a little that I can add. First, to answer one of the original poster's questions, changing the cell spacing in a plasmat by 0.001" or less will essentially have no detectable change on the image. Also, optimizing the spacing by examining a point image on the ground glass will work, but it helps to make sure the point source is at a large distance. I've used distant street lights at night for this exact purpose when tweaking homemade view-camera lenses. Second, you are correct that plasmats have little astigmatism. However, a true symmetrical plasmat like the G-Claron will likely show a little astigmatism when focused near infinity. Reducing the central air space slightly should cure this, but the total cell spacing change will be on the order of 0.01" - 0.02" for a 100mm focal length lens. Even this much movement may be hard to detect unless you look very carefully at the ground glass, or unless you do a true star test on an optical bench. Plasmats designed for 1:1 magnification will also show a little coma at infinity focus, but re-spacing won't help this with a plasmat as it would with a Tessar. These are rough guides, based on both practical experience and ray tracing, and some of the details will of course vary among different plasmat designs. Brian www.caldwellphotographic.com > Say you have two matched cells for a Dagor (Claron, whatever) that > came from a barrel mount that is now lost and you want to mount them [quoted text clipped - 6 lines] > > Kirk Kirk, if you have a lens in hand that you want to put in barrel or even shutter, why don't you tell us what it is? A reader may have the very lens already mounted. If so, he/she/it might be persuaded to measure at least how thick the mounted lens is from end of barrel to end of barrel. FWIW, that's in essence how Steve Grimes did the detail design for the rings etc. he made to remount a 38/4.5 Biogon for me. I sent it to him mounted in a dead AGI F135 shutter, he put it in a Copal 0. To get the spacing right, he just matched the end of barrel-to-end of barrel distance. Cheers, Dan > FWIW, that's in essence how Steve Grimes did the detail design for the > rings etc. he made to remount a 38/4.5 Biogon for me. I sent it to > him mounted in a dead AGI F135 shutter, he put it in a Copal 0. To > get the spacing right, he just matched the end of barrel-to-end of > barrel distance. How does Grimes, Inc. determine spacing? Do they look it up or actually test the postion? I ask because they want a lot of money to assure that the spacing is correct, and I would think that with so many modern lenses such information is a given. I mean I have a Super Angulon 121mm LF lens to mount in a #0 shutter. How mysterious can that be? > > FWIW, that's in essence how Steve Grimes did the detail design for the > > rings etc. he made to remount a 38/4.5 Biogon for me. I sent it to [quoted text clipped - 9 lines] > I mean I have a Super Angulon 121mm LF lens to mount in a #0 shutter. How > mysterious can that be? Fair question. John, Steve measured front of front cell-to-rear of rear cell distance and made tubes to hold the cells in the shutter and match that spacing. Adam has done the same with at least four of the 38 Biogons I've sold. Steve couldn't just unscrew the cells from the AGI shutter and screw them into a Copal because the two shutters have different thicknesses and because the cells, as Zeiss delivered them to AGI, were not threaded to screw into a standard #00 or #0 shutter. Not every lens matches the Compur standard. Putting your 121 SA in a #0 should be pretty straightforward, as in screw the cells in and go, IF the cells were made to go in a #0. Otherwise, no dice. Another example. A couple of years ago I bought a mystery 65/8 SA clone in an OptoDynetics shutter. The cells turned out to be threaded for #00. So I got a Compur #00 and, after measuring the distance from front of front cell to rear of rear cell, screwed 'em out of the OptoDynetics and into the Compur. Measured again, the distance had grown by about 4 mm. The solution was to machine out the front threads of the shutter to allow the cells to go deeper into it. Steve called it "the shoulder problem," said it was well known. Oh, yeah, the lens seems to be an Ilex Acugon. Again, we knew what the distance should be and matched it. No looking up was required, just taking measurements from the example in hand. Hope this clarifies things a little. Cheers, Dan > Fair question. > [quoted text clipped - 7 lines] > and because the cells, as Zeiss delivered them to AGI, were not > threaded to screw into a standard #00 or #0 shutter. [...] I'm afraid I don't know what AGI means. The lens was in a Sinar board intended for their proprietary rear shutter. Are you suggesting that Zeiss made different lens threads for these Sinar affairs? > > > Fair question. [quoted text clipped - 12 lines] > intended for their proprietary rear shutter. Are you suggesting that Zeiss > made different lens threads for these Sinar affairs? Sorry for the late reply, John, I've been away. AGI is an acronym for Aeronautical & General Instruments, formerly of Croydon, Surrey. They made a variety of small cameras of their own design, e.g., Agilux, Agifold, as well the Agiflex, a knockoff of a prewar german 120 film SLR. Korelle, I think. When Williamson, a maker of cine and aircraft cameras, folded AGI took over production and support of Williamson aircraft cameras, e.g., the F139 that was later rebadged Agiflite and the F135. The F135 had two 38/4.5 Biogons in bizarre solenoid actuated shutters that were, I believe, Williamson designs. These are the shutters I referred to. Zeiss would and still will do nearly anything if paid enough. This includes putting lens cells in barrels specified by their customers. We here tend to think that what we're familiar with, in the context of this discussion lenses in leaf shutters, is all there is. Not so, especially in other applications even odder and more specialized than ours. Cheers, Dan > > > > > Fair question. [quoted text clipped - 33 lines] > especially in other applications even odder and more specialized than > ours. Great information, Dan. Thank you. A question remains concerning the Schneider 121mm Super-Angulon found in the Sinar board intended for their proprietary shutter: do you know if the lens threads into the Copal directly, or is it a proprietary thread? > > > Fair question. [quoted text clipped - 12 lines] > intended for their proprietary rear shutter. Are you suggesting that Zeiss > made different lens threads for these Sinar affairs? Sorry for the late reply, John, I've been away. AGI is an acronym for Aeronautical & General Instruments, formerly of Croydon, Surrey. They made a variety of small cameras of their own design, e.g., Agilux, Agifold, as well the Agiflex, a knockoff of a prewar german 120 film SLR. Korelle, I think. When Williamson, a maker of cine and aircraft cameras, folded AGI took over production and support of Williamson aircraft cameras, e.g., the F139 that was later rebadged Agiflite and the F135. The F135 had two 38/4.5 Biogons in bizarre solenoid actuated shutters that were, I believe, Williamson designs. These are the shutters I referred to. Zeiss would and still will do nearly anything if paid enough. This includes putting lens cells in barrels specified by their customers. We here tend to think that what we're familiar with, in the context of this discussion lenses in spring powered leaf shutters, is all there is. Not so, especially in other applications even odder and more specialized than ours. Cheers, Dan > [...] > We here tend to think that what we're familiar with, in the context of > this discussion lenses in spring powered leaf shutters, is all there > is. [...] Have you ever worked with the Pacific Optical 3" F4.5? > > [...] > > We here tend to think that what we're familiar with, in the context of > > this discussion lenses in spring powered leaf shutters, is all there > > is. [...] > > Have you ever worked with the Pacific Optical 3" F4.5? Never. I've seen 'em, also Zeiss-badged 75/4.5s, and marvelled, though. Cheers, Dan > > Have you ever worked with the Pacific Optical 3" F4.5? > > Never. I've seen 'em, also Zeiss-badged 75/4.5s, and marvelled, though. Same thing. Well here's one of the four I am using for a new camera: http://course1.winona.edu/jstafford/newlens1 (That is definitely not the final version. The final will not have a conventional bellows or frame. It's not going well without a machinist and I sure can't afford SK Grimes, Inc. :)) Dan, This was kind of a theoretical question. The lens in question is a 150mm G Claron for which I have the barrel. The solution is obvious and I have done the experiment. It came out to within 0.001 inch the same. It is hard to measure actually as the micrometer has to be held parallel. However, I do have several old dagors for which one never really knows if they are in the original shutter. If not in the original shutter one can never be sure they are mounted correctly. Obviously some lens designs will be more sensitive to spacing than others. I was hoping there was a simple, easy way to do this. It's not looking good. I might have a go at it with the point source and start unscrewing elements. This only tests what happens when elements are too far apart. The part I am having the most trouble with is how to make an adjustable tube to accurately mount the elements in. Kirk > > Say you have two matched cells for a Dagor (Claron, whatever) that > > came from a barrel mount that is now lost and you want to mount them [quoted text clipped - 22 lines] > > Dan |
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