joe_hunt1 Posted April 21, 2003 Share Posted April 21, 2003 Can anyone help with this? I'm using a SA XL 38mm lens with centre filter for 6x12. It's very wide and, if the sun is anywhere this side of my shoulder I get flare in the image. Even a shallow efficient hood would make a difference, with care. I'm on my second attempt to make an improvised lens hood out of thin plastic (10cm square, 1.5cm deep, will fine-tune it if I get it on the lens). I hope to slip it on the 72mm thread behind the filter. Camera (Cambo) has a lens guard so not too much clearance around the lens. Is there any product ready-made to suit? Is there anything which could be used as a starting point (e,g a thin flat surface surrounding an 82mm thread (front of centre filter)? Thanks for any help. Link to comment Share on other sites More sharing options...
george_jiri_loun Posted April 21, 2003 Share Posted April 21, 2003 Joe, what's the actual diameter of the lens (the diameter of the front element glass)? I will then tell you exactly the lens shade's horizontal and vertical dimension for a given depth of the lens shade with your format (or for several depths, the deeper the lens shade is the more efficiently it shades). Link to comment Share on other sites More sharing options...
wieslaw1 Posted April 21, 2003 Share Posted April 21, 2003 Joe, take a thick, black, matte paper and cut a circle. Its diameter should be such that it fits the lens. Than cut a square, or rectangular opening in it. If rectangular, than the proportions should correspond with the negative format, for example 4x5. The size of the cutout has to be determined experimentally, so the �hood� does not vignette. Screw a filter on top. This is the best flare protection you can have. Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 21, 2003 Author Share Posted April 21, 2003 George The front very convex element of the lens is approx 35mm diameter, thread is 72mm and centre filter thread is 82mm. Angle of coverage is 120 degrees, perhaps about 114 degrees to corners of 6x12 (I have a formula for that somewhere). Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 21, 2003 Author Share Posted April 21, 2003 Wieslaw - that's a circular filter-sized mask with rectangular cutout that's placed under the centre filter directly over the lens? Could be an elegant solution if I'm careful not to scuff the lens. Link to comment Share on other sites More sharing options...
geoffrey_swenson Posted April 21, 2003 Share Posted April 21, 2003 Joe, Wiewslaw either cannot explain his idea or is pulling your leg. I would like to see sketch on what he means! Cheers, G. Link to comment Share on other sites More sharing options...
wieslaw1 Posted April 21, 2003 Share Posted April 21, 2003 Hi guys! No I am not pulling anybodys legs. (Although I love to do it). I have first used it for my Carl Zeiss 50mm Flektogon years ago and it works like a charm. The idea is very simple - if the negative format is rectangular (or square) you do not need the surrounding portion of the lens which is cylindrical/circular. In principle, the lens could be machined into a square tube, it would be much lighter. For practical reasons lenses are made cylindrical, but you can block those "unproductive" portions of glass with a 2-dimentional cardboard, preventing the extra light. I use such cards with several of my leneses. Can make a photo and show here shortly. Anybody sending me a donation for the idea? Link to comment Share on other sites More sharing options...
howiepete Posted April 21, 2003 Share Posted April 21, 2003 Well, Wieslaw, suddenly my legs are feeling longer! You are forgetting that your mask will reduce the light transmission of the lens and will not shield against light entering the lens at large angles with the axis, outside the field of view of the lens. These rays are the ones that cause image degradation and a proper hood blocks them. There, my legs feel better already! Howie Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 21, 2003 Author Share Posted April 21, 2003 Joe again - I thought of Howard's point earlier. But if the mask is placed below the oversize centre filter (which is a big source of non-image degrading light) maybe much, if not all, of that unwanted light doesn't get past. I don't follow the argument that the mask will reduce the light transmission of the lens. The final image on film is rectangular. Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 21, 2003 Author Share Posted April 21, 2003 On second thoughts, maybe it will reduce light to the image using a wide aperture Link to comment Share on other sites More sharing options...
wieslaw1 Posted April 21, 2003 Share Posted April 21, 2003 Patent pending! I do not know how it works with shifts. Take it or leave it.<div></div> Link to comment Share on other sites More sharing options...
howiepete Posted April 21, 2003 Share Posted April 21, 2003 Joe, My comment was a general one, not taking into account the center filter. Any lens performs a two-dimensionnal Fourier transform on the light incident on it, which can be stated in simple terms as: "EVERY point on the lens contributes to EVERY point in the image". So cutting out some of the lens edge by a mask removes those contributions and dims the image. There is no one-to-one correspondence between the shape of a mask on the lens and the shape of the image. Ask someone not aware of this what happens to the image if you cover half of the lens and they will respond "You cut off half of the image", or they may, thinking they are very clever, (remembering the image inversion), state: "You cut off the opposite half of the image". (My students were suckers for this!) Then you cover half of the lens with a card and see the image dim by half, but IT IS ALL THERE! Link to comment Share on other sites More sharing options...
matthew_brain Posted April 22, 2003 Share Posted April 22, 2003 Howard, I'm afraid to say that my camera must have been one of your students. When I put a card in front of half the the lens - I see half the image disappear yes the other half does get darker but the first half is definitely gone. When I shade, I typically try to cover as much of the glass that is not in use as possible. I don't know the maths behind it but sure improves image quality by reducing flare (that is more shadow detail - not less from making it darker). Sinar make a product that is similar to the above design but sits further forward it is part of the lens hood see: http://www.bhphotovideo.com/bh2.sph/FrameWork.class?FNC=ProductActivator__Aproductlist_html___47856___SIBHM2___REG___CatID=82___SID=F4EACBA5CB0 This product sits over the end of their lens shade bellows and allows the end of the shade to be tailored to the image being taken. It is effectively what Wieslaw is doing. Wieslaws design would provide more flare protection if it were further infront of the lens but it is a pretty good and cheap start, especially if you don't use movements. Link to comment Share on other sites More sharing options...
andreas_schmidt2 Posted April 22, 2003 Share Posted April 22, 2003 Joe, several makers like Arca, Toyo, Sinar and probably many others (not sure about Cambo) make bellows lens hoods, which attach to the camera itself. They are not attached to the front thread of the lens, which may be problematic with such kind of extreme wide angle lenses. I modified a Toyo hood for my Arca and it works quite well with an SA XL 47mm. Link to comment Share on other sites More sharing options...
george_jiri_loun Posted April 22, 2003 Share Posted April 22, 2003 Joe, here are 3 examples of a lens shade specifically calculated for your lens ans the format you use it on. 1) Depth of the lens shade - 50mm. Horizontal side - 187mm, vertical side 94mm. Excellent shading efficiency. 2) Depth of the lens shade - 25mm. Horizontal side - 110mm, vertical side 55mm. Very good shading coeficient. 3) Depth of the lens shade - 15mm. Horizontal side - 78mm, vertical side 40mm. Average but good shading. The depth of the lens shade means how far the frame (with its horizontal and vertical dimension)is in front of the lens vertex. If you screw the lens shade on the filter thread it must be less deep (some mm, depends on how far the filter thread is ahead of the lens vertex). These dimensions are calculated for a real 60x120mm film format. Thus in your case (56mmx112mmm for ex.) the lens shade will be slightly overdimensioned, which is good as a security margin. If you want more specific details contact me on my e-address. Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 22, 2003 Author Share Posted April 22, 2003 Thanks everyone for a lot of helpful and thought-provoking comments. Howard - my last responses were posted at 2.30am local time after wandering over to the computer with another glass of wine. I really had regressed! Matthew's reference to masking at the end of a lens shade would not be the same issue I think - the mask edges would be in the position of an efficient customised lens hood, and there would be no reflected light off the inside hood surfaces - obviously good, but sounds like a bulky setup. The Cambo Wide DS is a flat "cigar box / pancake" camera with no provision for attaching a bellows hood to the body. It would obstruct lens (helicoid) focussing and settings. The rationale of the camera is a simple platform for wide angle lens, easy to hand hold, quick to use. Thanks George for the sets of figures - they suggest the hood can be a lot deeper than I thought. Actually I hoped to keep the front edge inside and below the crash bars surrounding the lens, which is why my design is 15mm deep and just wide enough to clear the 90mm filter cap. I could try a push-on design next, using the figures! Thanks again. Link to comment Share on other sites More sharing options...
geoffrey_swenson Posted April 22, 2003 Share Posted April 22, 2003 Wiewslaw, I owe you an apology, you weren�t kidding. Probably you figured out what an impossible geometry you first explanation might have had suggested. Yours is an interesting concept. As for the cutting off half the light rays concerned that is what the compendium hoods do as well. Cheers, G. Link to comment Share on other sites More sharing options...
wieslaw1 Posted April 22, 2003 Share Posted April 22, 2003 No apology needed - the magician pulled the leg while the dentist pulled the tooth. Compendium hoods are great in a studio. But if you practice winter alpinism, alone in backcountry, with Linhof camera and all the gears hanging around your neck, at subzero temperatures, you don�t fool around with complex inventions. Howard, when I analyze the Raman or IR spectra I deconvolute the Fourier transforms to get the exact peak shape. I don�t care about any of it when I am taking pictures. Probably amazing George can take the Fourier transforms into account whilie calculating his hood dimensions! Cheers. Link to comment Share on other sites More sharing options...
jerald_brodkey Posted April 22, 2003 Share Posted April 22, 2003 I can't tell if what uou want is a 72mm or 82mm lens hood. 72's are not hard to find but if it's 82 you want I would suggest the Linhof 82mm screw in hood. B&W has it as Mfr Catalog # 021911 & B&H Catalog # LILH82. It's $69 which is alot cheaper than your lens and filter combo............ Jerry Link to comment Share on other sites More sharing options...
pvp Posted April 22, 2003 Share Posted April 22, 2003 Have you considered the lo-tech path? Wear a hat... :o) Link to comment Share on other sites More sharing options...
howiepete Posted April 23, 2003 Share Posted April 23, 2003 Obviously Joe wants to end this thread. OK by me! But first I need to eat a little crow and then salvage what is left of my reputation. I assumed until I saw Wieslaw's photo of the mask that it was located flush with the center filter. Things change if it is some distance in front. And the little experiment involving covering half of a lens and observing the entire image dimmed by half works only for a single (simple) lens with the card on the lens. (Try it!) Things are different with a complex lens system, as Matthew points out. The real world is so much harder to deal with than the ideal one! Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 23, 2003 Author Share Posted April 23, 2003 No! Not at all Howard! I'm still here. Made a lens hood, looks like it will work, had hoped that I could have fitted a mask to the front but it's already a close fit to the GG image on left and right. George's figures for hood depth seem to match what I got with mine, given that I set it higher than the lens. I suppose I'll be turning off the light here now ..... Link to comment Share on other sites More sharing options...
ken_schroeder Posted April 26, 2003 Share Posted April 26, 2003 At the risk of sounding low tech, have you tried using the dark slide to cast a shadow on the lens? I've used this method for years, either alone or in combination with a hood. Link to comment Share on other sites More sharing options...
joe_hunt1 Posted April 26, 2003 Author Share Posted April 26, 2003 Thanks for your suggestion Ken. I had previously been doing this (with a piece of card). I don't actually carry the slide or leave it slotted in the filmholder. I reckoned that it let light in when being used in bright sun. As I use the camera with optical finder, and don't change lenses in the field when film is loaded, I have made a short false removeable "safety slide" & cover to block the slot. As the camera is hand-holdable with a grip-mounted cable release, it's rarely convenient to check the shadow of a piece of card. As the shutter release is fixed on the hand grip it's also hard to use a card from this side of the camera even when on a tripod. As the lens covers 120 degrees the difference between no shading and actually intruding on the image is quite critical. The advice on this thread has thrown up useful different options, and I've now made a neat hood which stays attached and looks like it covers to the edge of the ground glass. Link to comment Share on other sites More sharing options...
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