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Started by keagle, 06/14/2005 10:13PM
Posted 06/14/2005 10:13PM Opening Post
Hi everyone,

I have a newbie imaging question that I hope someone will help me out with. I have an SV TMB-105, and I was attempting some imaging with a Canon Digital Rebel. I got a T ring and adapter, and I was trying to do some imaging at prime focus. I took a few terrestrial pictures and they turned out fine, except that there was some vignetting. That is, the central part of the picture was very nice and clear, but outside of a central circle occupying about 80% of the field,there was considerable darkening.

Can someone explain what is going on here, and if there is something I can do to avoid it? Is this why people buy scopes with larger focusers, or what? Thanks for educating me about this!
Posted 06/16/2005 10:35PM #1
Excerpt from the Optics4Birding website:


"Selecting a digital camera for digiscoping is different than picking one for general use. Excluding the professional digital SLR cameras, the best cameras for general use have large objective lenses with large apertures. For digiscoping cameras, however, large objective lenses lead to one of the biggest problems in digiscoping - vignetting.

Vignetting is the effect caused when the entire frame of the image is not illuminated, leaving a circular image with surrounding black. It occurs when either the objective lens of the camera is larger than the exit pupil of the scope, or when the curvature or mounting of the objective lens of the camera is farther away from the eyepiece of the scope than the scope's eye relief. For this reason, most digiscopers prefer cameras with small objectives, which can be brought very close to the scope's eyepiece. In general, pocket-sized digital cameras with large zoom ranges are also problematic, as their zooming lens elements recede from the eyepiece when zooming to larger magnifications.

The only way to solve the vignetting problem without changing the physical parameters of the camera and scope is to crop the image. Cropping can be done in a photo-editing program, which reduces the size of the image, or by zooming in with the camera while taking the photo. Zooming in maintains full resolution of the image file, but lowers image brightness."
Now for my own experience. I do not yet have a an SLT digital.
My Olympus 4040 is notorious for vignetting problems. Buying a Scopetronix Maxview 40mm eyepiece went a long way towards ameliorating the problem. The remaining vignetting disappeared entirely by using the optical zoom in the camera. Zooming in takes the circular shadow out of the picture.

I am not sure whether you are attaching the camera directly to the scope without a photo lens. You probably are. In which case, I am not sure if you can use the optical zoom in the camera to reduce vignetting.

You may want to post the same question in the astrophotography forum instead of in the equipment forum. You are more likely to get an answer there.

Rey Cordero