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#1
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swing lens cameras and focussing distance
I would have posted this in the original thread but it has got long
now and fragmented as to topic. I posted some detailed calculations in the thread with the subject line "The opposite of a close-up lens?" on April 6th 2004 two times and April 7th 2004 once and in addition on April 14th 2004 showing an important link for people interested in swing lens cameras, namely http://www.funsci.com/fun3_en/panoram2/pan2_en.htm I have posted a lot of technical stuff and maths backing it up. I use swing lens cameras for maybe 10% of all my photography so maybe that is a high proportion. I have a college degree (BSc) in Physics for what it is worth and of course I studied Optics as part of that. I am not claiming to be an expert on Optics, especially since I have earned my living developing software for many years, but at least I can claim to have some sort of awareness of the subject and a rudimentary grounding. These cameras are important to me so I thought that anybody who followed my calculations might be interested in what I have to say next. Here goes... Now this is theorising and speculation but I personally do not believe that anybody can make a lens with an accurate focal length. I assume a 0.5% error at least. So taking the Noblex 150, assumed fixed-focussed at 10.4m, then this has a 50.93 millimeter drum radius according to the film width and angle specs and there won't be much variation in the examples they make. Fair enough! They say their lens is a 50.75mm focal lens lens. You what?!? It could be anything from 51mm to 50.5mm. There is nothing they can do to stop that unless they adjust the gap between the front converging lens and the diverging lens behind it to correct it. Do they do that? Do they do any adjustment at all? I suspect that the manufacturers of these cameras do make some adjustment but the most important adjustment, if you have followed my calculations, is to make sure the secondary principal point of the lens is exactly on the axis so that the magnification is correct and so that as the lens swings round then parts of an image will always stay on the same place on the film surface. Now I challenge anybody here to find out from manufacturer-published specifications where the lens is fixed-focussed to on any swing lens camera except the Noblex range. I doubt you wil find it. And why? It could be that the only adjustment they are making to the camera is to make sure the secondary principal point is on the axis so that the image doesn't move on the film as the lens swings round. But then the focussing distance will be thrown out when they adjust this. So they will not be able to publish any figures. So you don't know what distance your camera is fixed-focussed to and you have got to take pot luck in getting one that focusses at the distance you have a preference for. In my case I will not pick bones and will say outright that I want my swing lens camera to be focussed at infinity because I use it most at that distance. But secondary principal point on the axis is the most important thing otherwise you lose horizontal resolution in a big way. The two are opposites. For me, at least, anybody claiming a distance focussed to is setting the lens to focus at that distance rather than doing the far more important thing of getting the secondary principal point sited exactly on the axis. If Noblex are claiming figures for focussing distance then they will be adjusting the lens to achieve that figure and not adjusting the lens to make sure the secondary principal point lies exactly on the axis which I feel they should be doing instead. It is possible to have the best of both worlds but it takes a lot of adjustment. If you want a lens in a swing lens camera fixed-focussed at infinity then you have got to make sure that the secondary principal point stays on the axis at all time but you can adjust the gap between the converging lens and the diverging lens behind it (for as Tessar design) to focus it. This will shift the secondary principal point again off the axis but by a process of adjusting them both you will finally arrive at the optimum setting. But it seems to me that the emphasis for the Noblex focussing models has shifted entirely away from the position of the secondary principal point being on the axis which to me (with calculations to back it up) is the most important thing. |
#2
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swing lens cameras and focussing distance
Hi Roland, Sorry, but the Horizon 202 cameras are not focused at infinity, as I have shown in a related posting, citing the Noblex 135 series (same format) models, but more like focused at a mid-distance of 7m or so. The noblex 135 models are focused at an even farther mid-distance point (7.5m), as shown by the comparison of focusing tables for the two cameras in my posting. For both the horizon 202 and Noblex 135 (and 150 series) cameras, infinity is on one edge of the hyperfocal setting, rather than at the precise focusing point. So any calculations assuming an infinity focus are simply in error, yes? Second, the critical value is not focal length variations of the lens etc. but whether the (curved) plane of focus is placed on the (curved) emulsion. See critical focusing at http://medfmt.8k.com/mf/critical.html regards bobm -- ************************************************** ********************* * Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 * ********************Standard Disclaimers Apply************************* |
#4
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swing lens cameras and focussing distance
the evidence that the horizon 202 is focused at 7 meters is very simple; it is virtually identical to the noblex 135 series in format/coverage, which we know per mfger's specs is focused at 7.5 meters. If you compare the noblex 135 DOF for various f/stops against matching horizon 202 DOF, you get nearly identical values - if anything, the horizon is slightly closer, meaning its focusing point is less than 7.5m at same stops etc. so if the noblex 135 is focused at 7.5m, the horizon 202 has to be focused at about 7.5m as well, given similar DOF values, lenses, and film formats. Again, you can't use the fixed lens DOF calculator with the fixed 28mm lens and apply it to the swing lens case with a different film format and much wider coverage on the larger film format etc., even if you do modify CoC. That's why your answer for fixed 28mm lens doesn't match case of noblex 135, which has similar specs, swinging lens, and format and coverage. It is just a happy coincidence (or more russian copying?) that the horizon 202 matches the Noblex 135 so closely... in short, the horizon 202 is nearly identical to the Noblex 135 in DOF vs. f/stop charts, format, swing lens type, etc., so it must be focused at the same point, viz., 7.5 meters or so, not infinity. bobm -- ************************************************** ********************* * Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 * ********************Standard Disclaimers Apply************************* |
#5
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swing lens cameras and focussing distance
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#6
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swing lens cameras and focussing distance
Your calculations say the horizon is focused at 280 meters, while the specs of the similar lens/format noblex 135 swing lens camera are 7.5m, yet the DOF between the two are remarkably similar ;-), viz.: horizon 202 noblex 135 5.6 2.9m to inf 2.3m to inf 8 2 m to inf 1.8m to inf DOF DOF deals with image size and image magnification to allowable printing sizes for a given quality level (viz., CoC). There is no underlying physics, it is purely a darkroom print quality range based on assumptions of film format and enlargement factors. why swing lens is not same as fixed lens The swing lens camera produces an image that is equivalent in coverage and magnification to one which would be produced by a much wider lens on a much larger format (e.g., 18mm on a 6x6cm which would be cropped to 24x56 or 24x65mm yielding 110 degrees horizontal coverage, as with the Horizon 202s etc.). You aren't enlarging the image produced by the 28 or 29mm lens on a 24x36mm film slice, you are enlarging the equivalent of an 18mm lens used with a cropped MF negative size of 24x56mm etc. This is why using a DOF calculator designed for fixed lenses doesn't do justice to the swing lens case. They happen to be close, if you fiddle with them and vary CoC etc., because the 28mm lens is close to its hyperfocal distance at any distance past 3.3m (at f/8 on 35mm format per http://medfmt.8k.com/brondof.html). In short, DOF is about image magnification, and the limits for a cropped image covering 110 degrees on 24x56mm (equiv. to 18mm on a cropped 6x6cm format) from a Horizon 202 is going to be different than a fixed 28mm lens on some lesser format. The close convergence between the two DOF for similar swing lens cameras suggests to me that they must have similar fixed focusing distances, viz., circa 7.5m per Noblex camera specifications. Suggesting that one must have a fixed focus distance of 7.5m and the other 280m (!), and that in the process the mfgers have gotten their own camera specifications wrong, isn't convincing to me. sorry, but we just have to agree to disagree on this issue ;-) As I've noted, I do agree with you on other issues/posts, just don't find this convincing, rather, an artifact of hyperfocal convergence in calculator ;-) bobm -- ************************************************** ********************* * Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 * ********************Standard Disclaimers Apply************************* |
#7
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swing lens cameras and focussing distance
Bob Monaghan wrote:
It is just a happy coincidence (or more russian copying?) that the horizon 202 matches the Noblex 135 so closely... Are you suggesting the Russians had a time machine? From Marco Pauck's excellent web site: | The Horizon 202 is in production since ca. 1989. It is a successor of | the Horizont (note the 't') which was produced from ca. 1967-1973. The first Noblex, the model 150 medium format camera, was made in 1992, the first 35 mm model appeared in 1994. Ralf -- Ralf R. Radermacher - DL9KCG - Köln/Cologne, Germany private homepage: http://www.fotoralf.de manual cameras and photo galleries - updated April 29, 2004 Contarex - Kiev 60 - Horizon 202 - P6 mount lenses |
#8
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swing lens cameras and focussing distance
no, but good points dates; however, there are lots of models of both noblex 135/150, some of which are adjustable focus, others fixed focus, and as others have pointed out in this thread, they evidently come in various focusing distance flavors ;-) Ditto, there are models of the Horizon 202 series, such as my horizon 202S, which were aimed at the post-soviet market, vs. the military Horizon/Horizont/Horizon series. Certainly, a lot of changes have been made by various parties in some post-soviet camera models (cf. Kiev88CB/CM series) to appeal to the world market more, adapted other back styles (e.g., hasselblad compatible backs) and so on. Moving the focusing distance between two models is simple in comparison ;-) grins bobm -- ************************************************** ********************* * Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 * ********************Standard Disclaimers Apply************************* |
#9
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swing lens cameras and focussing distance
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