Nick Zentena <[EMAIL PROTECTED]> wrote: > I've been thinking of how to enlarge 5x7 and 8x10 without buying > an 8x10 enlarger. > > It would seem a plywood box with a light source at one end and > lens at the other would be all that's needed. I spent some time > with googles usenet archives and saw people mention everything > from 1100watt monster light sources to 150watt work lights. > Anybody know if a halogen work light would actually work? The > price is right. They generate alot of heat but I'm guessing > almost anything bright enough does. Some sort of diffuser between > the light and the negative holder would seem the easy way. > Avoiding condensors. A couple of fans to handle the heat? One > pulling in room air the other venting out hot air. Could I leave > the top open? It would seem with the lamp pointing down the > plywood box not much light is getting out the back. It would make > cooling easier. > > > xxxxxxxxxxxxx > xxxxxxxxxxxxxxxxxxxxx > x x > x x > x x > x xxxxxx > x x x > x x x > x xxxxxx > x x > x x > x x > xxxxxxxxxxxxxxxxxxx x > xxxxxxxxxxxxx > > > Pardon the ascii art. Thats the basic shape. A negative holder in > the middle. A filter holder above it. Mounted on a table. With > the easel mounted on a wall. You'd move the enlarger forward/back > to change print size. Or I guess I could attach the easel to the > table some how and make the easel moveable. > > Now the math part. Lets say I'd like to make a 24x30 from an > 8x10. Is that a 3x magnifaciton or is it 1/3? > > The formula I have for figuring out bellows extension is > > bellows_extension = (1 + magnifaction) * focal_length > > But that's for the camera. An enlarger goes the other way. I'd > like to use my 240mm process lens. It covers 5x7 at infinity but > I'm hoping it'll cover 8x10 in this application. If not it's > either the 210mm or the 360mm which both cover 8x10.
Horizontal enlargers similar to your conception were used almost a century ago with a self-contained light source, or backed up to a masked window with an outside mirror or white board to reflect light to the diffusers. A few sheets af ground glass may be used as diffusers, with a slide-in glass carrier. The distance from the light source to diffusers should be at least 2X the format diagonal, or 24"+ for 8x10 to avoid excessive fall-off. In addition to providing cooling fans, consider making the light box out of sheet metal to reduce the danger of fire, and use fixtures and wiring which are appropriate for high temperatures and loads. Using two timers in series wouldn't be a bad idea - the first could be a lab timer set for 10 minutes which would cut power if you left the room with a timer set in the focus mode. For VC printing, the source color temperature should be around 3200 K. Large deviations (household bulbs or fluorescent lamps) will compress the contrast response, and those sources should be filtered to 3200 K for grade 2 contrast w/o vc filters. A bank of fluorescent tubes should be left on all the time with at least a 1/2 hour warm up time, since output veries with temperture. A deep red filter at the lens may be used as a shutter, or a lens cap. Another important aspect is perfect alignment and parallelism between the negative, lens and easel. Ansel Adams has a good description of a home-made horizontal enlarger in the "Print". The box between the negative carrier and lens should be large and well baffled, or flare light will reduce high value contrast. With respect to "math", you are simply taking a macro photo of the negative when enlarging. The "extension" is the lens to image (easel) distance, (1 + magnification) x focal_length The lens to subject distance (lens to negative carrier), is (1 + 1/magnification) x focal_length. The enlarger bellows is actually on the subject side in an enlarger, unlike most cameras, so using the term "bellows extension" may be misleading. To determine the format coverage circle of your enlarging lens, multiply the circle at infinity by (1 + 1 / print_magnification). You'll need 326 mm to completely cover an 8x10 negative. If your 240 just covers 5x7, the circle would be around 218 mm. Therefore, you print magnification from 8x10 would have to be m = 1/(326/218 - 1) = 2X or smaller (16x20" print or smaller). For 5x7 or 8x10, it's usually cheaper and more efficient to find an old enlarger and modify it if required. ________________________________________________________________ Sign Up for Juno Platinum Internet Access Today Only $9.95 per month! Visit www.juno.com _______________________________________________ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
