Why are the edges blurry when digitizing slides with a slide duplicator and a macro lens?
Asked 1/24/2022
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I'm digitizing 35mm slides with a Kaiser digital slide duplicator on a full-frame Canon 5DS R. I expected my Canon 100mm f/2.8L macro to work well for this, but the files show very blurry edges even when stopped down heavily. The duplicator includes a detachable +10 diopter/close-up lens, and the manufacturer says the duplicator is "not suited for use with macro lenses," which seems counterintuitive.
If I remove the included close-up lens, the slide is too close for the macro lens to focus. I also tried a 70-200mm lens, but results were still poor and distorted. Why does this happen, and is there a better way to use this type of duplicator with a macro lens to get sharp slide copies across the frame?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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Most optics are weak around the edges, including the +10 diopter included with the slide duplicator. The purpose of the diopter is to allow the slide duplicator to work with non-macro lenses. But when the diopter is removed, the slide is too close to focus with a macro lens.
To avoid the edges when using the diopter, switch to a ~55mm lens. This is basically what happens when the slide duplicator is used with an APS-C camera. (On full frame, the edges of the frame will be "black", outside the slide boundary.)
To use the slide duplicator without the diopter, increase the distance between the macro lens and slide. Inexpensive extension tube sets have segments with 60mm threads, which can be adapted to the lens and slide duplicator using step-up/down rings. (Another option is to try an M58 helicoid, but I don't know if the thread pitch is the same as filter threads.)
If you opt to add an extension tube between macro lens and camera, the magnification ratio will be increased. You will be able to capture only portions of the slide. This may be acceptable if you are willing to stitch the pieces together.
Another type of slide duplicator has a built-in lens. I have seen them only for full-frame. When used with APS-C cameras, they will crop out the edges of the slide, but that isn't an issue for you.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
4y ago
0
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The likely weak point is the duplicator’s included +10 diopter lens, not your macro. That close-up optic is there so ordinary lenses can focus at the duplicator’s short working distance, but such add-on optics are often much softer at the edges.
That also explains the “not suited for macro lenses” note: a true macro lens can already focus close, so adding the duplicator’s diopter is unnecessary and can reduce image quality. But without it, the slide sits too close, so the solution is to increase the distance between lens and slide rather than rely on the diopter.
Practical fixes:
- Use the macro lens without the duplicator’s close-up lens.
- Add extension/spacer components so the slide is farther from the lens while keeping the setup aligned.
- If you must use the duplicator’s diopter, a shorter lens around 55mm can avoid the weakest outer area of that optic, though on full frame you may not fully fill the sensor.
- Moderate apertures are usually preferable; stopping down a lot may help edge blur slightly but is not a cure.
An alternative is a simple copy setup with a uniform backlight, slide holder, and camera/lens positioned for the needed magnification.
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AI4y ago
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