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Thread: lens info needed

  1. #1

    lens info needed

    i have just finished modifying a canon scanner for a scanner camera project (taped led, removed pinhole array, ground out channel, and re-assembled). now it is time to get serious about the camera itself, but i am in unknown territory here- i can barely understand how a pinhole works!

    i have scavenged an old lens from some sort of reproduction machine, a 300mm rodenstock apo-gerogon (the opalescent coating seems to have pooled near the edges- what is up with that?). My question is what kind of image circle i can expect from this lens? will it cover the 8.5x12" scanner bed? My most naive question of all- does 300mm mean the fl should be fixed at that distance, or should it be adjustable for focusing? un-lensically yours, staft.

    ps i also have 135mm and 185mm repromaster lenses- will those be usable, too?

    pps any suggestions for ground glass? will any frosted glass work? can i make my own with sandpaper?

  2. #2

    lens info needed

    Staft, I'd set things up to make the lens adjustable for focusing, if I were building one and had the necessary technology. A sliding pair of nesting boxes, just under 300 mm at minimum extension and extendable to, say, 500 mm, should let you focus from horizon down to 3-4 feet. Your scanner's bed may measure 8.5 x 12, but it probably only scans up to A4, which is a smidge narrower and a bigger smidge longer than 8.5 x 11.

    The 300 mm may vignette a bit or get quite soft at the corners, but ought to be okay. That lens *should* cover 8x10 with some margin, unless it's an odd, specialty design (not impossible at all for a repro lens, of course); 325 mm is "normal" for 8x10, and there's generally a bit of leeway designed into lenses for covering "normal". Not sure what you mean by the "opalescent coating", however. The shorter Repromaster lenses almost certainly won't cover at infinity, but of course, they were probably made for copying 1:1 or close to it, and even a 135 mm will cover 8x10 at 1:1.

    The cheapest way to make ground glass is to get a suitable piece of glass (by cutting down what comes in a $3 Goodwill picture frame) and coat it with frosted tape on the side that will go toward the lens. This may not be even enough if you're directly scanning the ground glass image, however, as opposed to picking up the aerial image on the scanner and using the ground glass only to focus.

    Second option is to grind your own glass -- glass as above, plus a tube of fine valve grinding compound from a parts store, and a glass mason jar. Spread a little of the compound on the glass, and rub with the bottom of the mason jar evenly over the whole surface. Replace the compound when it starts to sound smooth instead of gritty. Wash the surface periodically with dish soap (to remove the oil that's the base of the compound) to check your progress, and stop when you have an evenly frosted surface.

    Or, if you don't mind some delay and spending a remarkably small amount of money, you could order a glass from Satin Snow; they'll make it to you exact dimension, and will be happy to quote a price before you commit. Their glass is better than anything you can make yourself (most likely), and their service is incredible, but their turn around time may not fit your project schedule; they usually take a few weeks to deliver since all their glass is made up to order.

  3. #3

    lens info needed

    thanks, im. as usual, i start these things to learn something, but end up getting more learning than i bargained for. the opalescent coating i refered to is what looks like oil refraction rainbows, like optical coating, but shows up in blobs near the edge of the glass. i have tried to find this condition mentioned online with no luck. i don't know if it will affect the image in any way. the lens sat unused for quite a few years, based on the surrounding grime, and may have been subjected to highish (florida summer) temperatures.

  4. #4

    lens info needed

    Whatever the blobs are I wouldn't worry about it, as long as they're near the edge they shouldn't matter much.

    I don't know what to expect in the way of sensitivity from your scanner, but if you can stop the lens down to maybe f/11 you can mount it at the hyperfocal distance and leave it there. If you focus with the lens wide open on something 15 - 20' away, when you stop it down you'll have reasonable focus from maybe 8' out to infinity, without the headache of adjustable focus. The position will be something more than 300mm away from the film plane, which is about the separation at infinity.

    You might use a variation on a room-sized camera obscura to do some rough initial testing. If you mount the lens in a window cover, then make a "ground glass" with some waxed paper mounted in a cardboard frame, when you darken the room and hold the waxed paper up behind the lens you'll see an image. Start with the waxed paper around 300 mm from the lens to get rough infinity focus. It's as crude as it gets, but you can get a feel for how it will focus by moving the waxed paper and watching different parts of the scene outside come into or out of focus.

    I would also consider using the shorter lenses. So what if they don't cover the whole scanner bed? When I scan a 4x5 neg it doesn't cover either. Most scanner software allows you to set a crop window to scan only part of the bed, and one of the shorter lenses will make the beast a lot less unwieldy. Obviously it will be more of a wide-angle camera with the shorter lenses.

  5. #5

    lens info needed

    good advice, moot. i think some tests are called for, even though it contradicts my typical m.o. of "build first, then ask questions"! i suppose the ability to use different lenses is an argument for adjustable fl.

    does anyone see the need for bellows movements with a camera essentially built for time distortion? i am not sure how i would go about it, but it would be smart to plan ahead, maybe, if that is something ultimately useful.

    and is there a simple device from which i could scavenge an adjustment knob/track system for the focal length? i am planning to rig something up with wood blocks, but a more reliable mechanical system would be great.

  6. #6

    lens info needed

    boxcamera that is the scanner as the filmplane?? I would be interested in making one if only to prove the concept for myself.

    boxcamera with variations

  7. #7

    lens info needed

    Movements? I'd say start with getting actual images, then put sophistication into the next iteration if you like the preliminary results.

  8. #8

    lens info needed

    From Agfa Gevaert

    - Helioprint lens 1:8/150 Aperture range 1:8 - 1:64 At f/8: Angulare coverage 65, image field diameter and magnification 1:1 380mm At f/22: Angulare coverage 68, image field diameter and magnification 1:1 400mm magnification range 25% - 400%

    OK, I didn't remember right: 1:4 - 4:1

    I must have only posted half the data (150 mm only); I retrieved this from the web.

    I'll look for 210 mm data.

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