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I'm having an issue with vignetting on my microscope images, and I'm not really sure how to troubleshoot it, or if there is anything I can do to fix it. Below is an example image that should illustrate the issue I'm seeing fairly well - the design is completely uniform in the horizontal dimension. Viewing the image through the binocular eyepieces, the color and light intensity appears uniform.

scaled example image

This isn't an issue with focusing, as the image looks good in all portions of the captured image. The subject is flat, and somewhat reflective. The camera is basically an image sensor with threading for a microscope and a USB connector, and doesn't appear to have any focusing optics, or a mechanical shutter.

My goal is to take images at this magnification and stitch them together in post processing, but the vignetting results in visual artifacts that remain. I'm not sure what information would be helpful, but I can certainly answer questions

  • Is there anything preventing you from using dedicated stitching software, such as hugin, which would provide correction as part of the process? – junkyardsparkle Apr 29 '15 at 20:22
  • Also, is it possible that some of the vignetting may be due to the light source? There does appear to be a shift in color as well as brightness... – junkyardsparkle Apr 29 '15 at 20:41
  • @junkyardsparkle I haven't specifically tried hugin yet, I've been using using GIMP to align the images. One issue with some stitching software I've found is that they assume a spherical or cylindrical mapping topology, while my topology is almost perfectly flat. – W5VO Apr 29 '15 at 20:45
  • Hugin can do flat scans, it's just a little more work, initially, but if your image gathering process is consistent, you could probably create a template or script once you have things figured out. – junkyardsparkle Apr 29 '15 at 20:50
  • You might take a photo of a blank slide to calculate a correction factor across the image, effectively "subtracting" the aberrations. – DrMoishe Pippik Apr 30 '15 at 0:21
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Having had similar problems with an enlarger light source, I made a correcting mask. In the dark, I affixed a sheet of black & white film and diffuser so that the light would play on this material. The film material has a low ISO. I turned on the lamp for as short a duration as I could. I developed the film in a very dilute developer. The resulting negative replicated the vignette but its tones are reversed (a negative image). Thus I was able to use this film as a mask. When affixed to the lamphouse, the mask evened out the light. Yes it takes some fooling around.

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Here are some ideas you can try...

  • Use a Fresnel Lens at the light source to distribute light more evenly. This may already be built into the microscope.

  • Use a convex-plano lens at the sensor. This will affect magnification, sharpness, and focus.

  • Use flat-field correction software. This is a digital version of the film technique @AlanMarcus describes.

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