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I have a large amount of slides and negatives I want to digitize. I'm thinking about using a slide copier to do this, but own Nikon d5100- a crop sensor. After de-magnifying the image, how will this affect the original image's appearance (from the slide) in digital form?

This will drop my resolution, but will it do anything else too, like cause lens distortion? What type of lens would I want to use?

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There are a few different types of slide duplicator designs:

  • Some attach to the filter threads of a lens. This type of slide duplicator is easiest to use on crop-sensor bodies. Image quality depends mainly on the quality of the lens. Color quality depends on the light source.

    The duplicator is basically a slide attachment and a tube with a diopter filter inside. The macro filter may introduce some barrel distortion, which can be corrected in software. Any loss in image sharpness won't be significant as long as you are able to visualize and focus on the film grain.

    slide copier

    Use a zoom lens to find the appropriate focal length, then switch to a sharper prime, if you have one available. Use the minimum ISO available on your camera, and stop down to the sharpest aperture of your lens (usually F5.6 to F8). You don't have to worry too much about long exposure times since the attachment is motionless relative to the sensor. If your camera or lens has image stabilization, turn it off.

  • Some attach directly to the camera. All necessary optics are built into the duplicator. Some allow zooming from 1x to 2.5x. You can zoom in on sub-miniature formats, but you won't be able to zoom out to capture entire 35mm frames on crop-sensor bodies. You can capture and stitch multiple sections of the film.

  • Some are intended for use with bellows. Lens selection and bellows operation may require some trial and error. As the bellows is used to move the lens away from the sensor, sharpness is decreased and a "glow" may appear.

    bellows attachment

    You can use the thin lens formula to figure out approximate distances and focal lengths you'll need for a given reproduction ratio (magnification, m = v/u).

    1/u + 1/v = 1/f

    u = subject distance
    v = image distance
    f = focal length

    For 1:n, u = (n+1)f, v = (n+1)f/n. So for a 50mm lens with a reproduction ratio of 1:1.5, u = 83mm and v = 125mm.

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As long as you use a proper focal length lens, it shouldn't matter if the sensor is APS-C or full frame unless you hit diffraction limiting issues (which you can avoid by choosing your aperture to balance sharpness and diffraction, ideal value varies from lens to lens.) You simply don't need as much magnification to fill an APS-C sensor with the image, so the focal length you use will be smaller.

  • Slide duplicators tend to incorporate lenses ... not great lenses ... and lenses that might be un-great enough to yield dissatisfying results given the resolution requirements of an APS-C sensor.... – rackandboneman Nov 20 '18 at 12:02

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