harry_martin Posted December 13, 2004 Share Posted December 13, 2004 I know that I can add negative single element lenses greater than my primary lens FL which is a Schneider Symmar-S 6.8/360mm. But a -3 lens FL is 333mm, less than the 350.5 actual FL of my primary lens. Based on Blaker's book, Field Photography, my lens will not focus. Can I add a +1/2 positive lens to give me a 400mm FL and, hopefully a total combined FL of around 1400? I can't test it yet because I need to buy another 24 inches of 8x10 bellows. Harry Link to comment Share on other sites More sharing options...
Glenn McCreery Posted December 13, 2004 Share Posted December 13, 2004 It's easy enough to add (or subtract) lens elements to change focal length. The formula is 1/f = 1/f1 + 1/f2 +...1/fN, where f is combined focal length and f1, f2 through fN are the individual focal lengths of the components. The hard part is controlling aberrations; chromatic, spherical, astigmatism, coma, field curvature, and distortion. And, field of view may be reduced by adding components. Controlling aberrations is where lens design computer programs come ito play. However, you can often be sucessful by just trying various combinations until you find one that works. And use small apertures, this typically reduces aberrations. Link to comment Share on other sites More sharing options...
ole_tjugen Posted December 13, 2004 Share Posted December 13, 2004 Seems to me you're trying to make a telephoto lens? Many years ago, most lens makers had a "telenegativ" on their lists. This virtually died out around 1920, since the problems with corrections, spacing, focal length and aperture calculations made it too difficult for most potential buyers. The few that survived were coupled to specially designed lenses, or were redesigned to FL doublers for cameras with a fixed film/flange distance - like 35mm SLR's. In other words it's possible to do what you suggest, but getting any usable lens system (sharper than a wad of cotton) is very, very difficult. Link to comment Share on other sites More sharing options...
harry_martin Posted December 13, 2004 Author Share Posted December 13, 2004 I did realize after my post that I do have a means to test a similar setup on my 4x5. I have adequate 4x5 bellows to use a minus(-) 2 single element lens (instead of the -3 lens that would provide a 1400mm combo) combined with my positive(+) 1/2 single element lens to give me approx. 500mm combo. Has anyone tried this? Harry Link to comment Share on other sites More sharing options...
ole_tjugen Posted December 13, 2004 Share Posted December 13, 2004 No, haven't tried it. But I would be interested to hear what you get out of it! With this kind of setup the spacing between the elements gets critical. Many of the early tele lenses had variable focal length ("zoom"), the FL was adjusted by changing the distance between the positive and the negative elements. Link to comment Share on other sites More sharing options...
andrea_lee3 Posted December 13, 2004 Share Posted December 13, 2004 Am quite interested in what you are actually trying to do, especially given that you're monkeying about with a rather expensive bit of glass. You are unlikely to get stunningly correct results, considering that overtightening around a shutter can make a critical difference, but being able to monkey about with what we want is one of the great things about LF. Why not cheat a bit and check out the combinations of the convertible plasmat sets? As for focusing, I work at night and roll a large sheet of thin cardboard into a conic section. Do keep us updated as to your results. Link to comment Share on other sites More sharing options...
harry_martin Posted December 13, 2004 Author Share Posted December 13, 2004 Thanks for the comments. I thought it might just be a crazy idea to mix (-) and (+) simple lenses. I am encouraged! I will run the tests after the first of the year and report back. The element spacing is a big question for me as well. I have a (-)1 and (-)2 lens for my primary lens 120mm dia. front element and (-)1 and (-)2 lens in series 9 (82mm dia.) for my primary lens rear element so I will be able to test element spacing and location either way. My series 9 positive lens will have to be rear element mounted. I Just acquired the series 9 negative lenses from eBay and I bought the large negative lenses from Surplus Shed. Harry Link to comment Share on other sites More sharing options...
ole_tjugen Posted December 13, 2004 Share Posted December 13, 2004 I was wondering if I should have mentioned this earlier... A Tele lens has a positive group in front, and a negative one behind. If you turn that around, that's how retrofocus wide angle lenses are made! Link to comment Share on other sites More sharing options...
harry_martin Posted December 13, 2004 Author Share Posted December 13, 2004 Ole, Your last comment went right over my head. I do expect differences whether the negative lenses are mounted on the front or rear elements. How much or why, I don't know? All I do know is that the "d" in Blaker's formula will vary from front to rear mounting. Harry P.S. I will be taping them on with blue painter's tape. Link to comment Share on other sites More sharing options...
darin_cozine Posted December 14, 2004 Share Posted December 14, 2004 A 'retro-focus' lens, as people like to call them, have a longer film-to-flange length than a normal lens of the same focal length. These are used in SLR's to allow a wide-angle lens enough clearence for the mirror to swing. However they have more problems to correct than a standard type lens. Link to comment Share on other sites More sharing options...
mike_rosenberg1 Posted December 14, 2004 Share Posted December 14, 2004 Harry, I was wondering - where do you get the various lenses? Thanks, Mike Link to comment Share on other sites More sharing options...
harry_martin Posted December 14, 2004 Author Share Posted December 14, 2004 Mike I learned about www.SurplusShed.com on this Forum from BoB Nugent(hope I have his name spelled right?. The two large negative lenses were bought there. First page Search: negative lens ...... That should show all that are there. I know they still have the 4-7/8" dia. -2 lens for $15.00. I didn't see the minus (-)0.883 (not a full -1)diopter lens but it did sell for $10.00. Shipping $5.00 max.. US dollars It is 4" square but the corners touch the prime lens rim for taping. The added bellows draw covered my 8x10 GG. The negative series 9 lenses were found on eBay by searching in Camera & Photo for "series 9" . It took six months, all it takes is patiences! Darin Thanks for the great info! Harry Link to comment Share on other sites More sharing options...
harry_martin Posted December 14, 2004 Author Share Posted December 14, 2004 Mike... The eBayer I bought the -1 & -2 lenses is selling a plus 1 lens series 9 size right now. He also has plus 1/2 lens for 72mm I believe. You might email him and ask if he has anymore? His eBay name is flatlander3. Harry Link to comment Share on other sites More sharing options...
harry_martin Posted December 17, 2004 Author Share Posted December 17, 2004 Having some good weather here in Southern California, so I was able to get out and test the (-) and (+) diopter lenses on my 360mm lens combo. Looking through the GG I couldn't tell the difference between (1)Using only the 360 to focus, (2) using the -2 lens on the front element of the 360 focusing and (3) using the -2 lens ... 360 ... and the plus 1/2 diopter lens on the rear element combo. The bellows draw was approx. 14", 25" and 37" respectfully. The film will be processed next week. Haarry Link to comment Share on other sites More sharing options...
harry_martin Posted December 17, 2004 Author Share Posted December 17, 2004 It should be 14", 37" and 25". Link to comment Share on other sites More sharing options...
harry_martin Posted December 22, 2004 Author Share Posted December 22, 2004 Hi I just found out today that my film processing has been delayed until January 5th 2005. Happy Holidays, Harry Link to comment Share on other sites More sharing options...
harry_martin Posted January 6, 2005 Author Share Posted January 6, 2005 Okay, I got my photographs back yesterday and I am very happy with the results. My photos using a -2 lens on the prime lens front element and a +1/2 lens on the rear prime lens element worked beautifully! Next step is to order 30" extension bellows for my 8x10 camera so I can use -3 lens combined with a +1/2 lens for a combined focal length of approximately 1400mm. Harry Link to comment Share on other sites More sharing options...
harry_martin Posted January 25, 2005 Author Share Posted January 25, 2005 I have received my 30" extension bellows from Camera Bellows(UK)in a little less than two weeks. Now the fun begins! Harry Link to comment Share on other sites More sharing options...
harry_martin1 Posted February 19, 2005 Share Posted February 19, 2005 Sorry to say, the bellows had to be returned, nothing else will be done on this matter. Link to comment Share on other sites More sharing options...
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