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Focusing on a small image from a short distance

  1. Oct 21, 2011 #1
    I apologize if the breadth of this question requires a response beyond what anyone is willing to contribute. I've started researching optics ( of which I have little knowledge ) and feel immediately overwhelmed.

    Using a DSLR ( with a DX size sensor ) and preferably the available lens (18-55mm or 18-110mm), what additional optics would allow me to fill the frame and focus on a 35mm wide image at a distance of about 5 1/2"?

    I imagine maybe a longer lens would help. I've been experimenting with macro tubes and close-up filters without much luck. The close-up filters introduce too many artifacts in the image. I've also considered that a tele convertor may help, but have yet to get my hands on one.

    Any advice would be super, and I'm sure I left out some details for this equation, let me know what other info is needed.
  2. jcsd
  3. Oct 21, 2011 #2
    What exactly are you trying to do? Use a small-sensor short focal-length lens on a 35 mm film camera? Image something at infinity, or something close-up? How much do you care about image quality, and angular field of view? A camera lens is designed for a small range of image distances about a particular focal length, and it won't work nearly as well if you use it with a different image distance to try to fill up a larger detector area. You also lose infinity focus if you move the image plane back, and adding other lenses to try to compensate will make the image quality problem worse. You'd be better off finding an appropriate lens for the task.
  4. Oct 21, 2011 #3
    That makes sense, so I'll forget about using a kit lens and attempt a solution that is more appropriate.

    What I am trying to do is take a proper photo of a 35mm mounted slide presented inside of a projector, through the empty "projection lens cavity" of the projector. I measured a distance of about 5 1/2" from the surface of the slide to the outer casing of the projector. The reason I need to shoot it from the outside is because the entry hole is only roughly 50mm in diameter (much smaller than the OD of most lenses), and I can't bore it out.

    Image quality is important, and I don't entirely understand how angle of view factors in
  5. Oct 21, 2011 #4
    If image quality is important, why not use a slide scanner?

    A google of the following should help.
    35mm slide scanner
    35mm slide duplicator
    Macro photography
  6. Oct 21, 2011 #5
    O.K., that's completely different from what I thought you were trying to do - you are in fact trying to take a picture of an object at a distance of 5.5" using a digital camera with a lens. Yes, just scan the slide. Failing that, find a macro lens for your camera. Failing even that, you could possibly kluge something with extension tubes between the camera and lens, but image quality will suffer - and most electronic SLRs these days won't work unless there's an electrical contact between the body and lens so that the computer knows what it needs to know about the lens. This would be easier with a simple mechanical 35 mm SLR, but then you'd probably want to scan the slide or negative anyways, so why not just scan the slide.
  7. Oct 22, 2011 #6
    The issue isn't being able to scan the slide, otherwise I would indeed use my slide scanner to digitize it.

    I'm assembling what will be a much faster method. Being able to index slides with the slide projector will allow for much quicker digitization of a batch of slides.

    I've installed a backlight and built all of the circuitry to automate the advancing and shutter release, now I am working on the optical side of things. I'm leaning towards a powerful diopter.
  8. Oct 22, 2011 #7


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    I admire you for even approaching this subject. When I was writing semi-professionally, long before home computers let alone the internet, I tried to amass reference materials about anything interesting. I made the mistake once of buying a book on optics. That damned thing is pretty much tied with "The Riddle of Gravitation" for the highest ratio of formulae to understandable words in any English-language book that I've encountered. I'd burn it out of spite, but am afraid to because such might cause the known Universe to collapse.
    Anyhow, I'm glad that some people such as yourself are capable of dealing with it, because I'm useless without my glasses.
  9. Oct 22, 2011 #8


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    You can get slide scanners with autofeed mechanisms - even Amazon sells them!

    If you don't want to buy something relatively expensive for a "one-off" proect, there are companies that hire out this sort of equipment by the week.
  10. Oct 22, 2011 #9
    Most cameras have a macro setting for close up photography, but I would not expect the quality to be close to a scanner either in pixels or color accuracy.

    Have you tried using a light table and magnifying glass like most professionals? Adapting a camera for use with a light table might be easy.
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