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Re: NARA Seattle
Hi Richard,
vignetting is the fall-off/loss of light intensity that radiates out from the centre of an image captured with a lens. In the case of a microfilm reader, it can also be caused because the illuminating light source (bulb) is positioned in the centre of the frame, and the light intensity will decrease towards the edge of the frame, visibly showing as the centre of the frame being brighter than the edges. My initial experiments some years ago in photographing off a microfilm screen, using a 4 MP camera, produced strong vignetting, but I found a year or so ago that this could be easily corrected during post-production in Adobe lightroom. I haven't seen the problem occur with my DSLR. I believe that the problem was compounded by using the 4 MP camera at the widest aperture; the vignetting being a result of the poor optical performance of the lens at the widest aperture.
Cheers
Rod
PS - I have found a DSLR to be worth its weight in gold during visits to various archives. Like yourself, because of the costs associated with visiting archives, I want to maximise the amount of information obtained. One trip, to Australia, resulted in 4,500+ pages of photographed documents, while another trip to TNA, London, resulted in 11,500+ pages of photographed documents. Any identified document where the reproduction was critical (such as plans, maps, diagrams) could always be re-copied properly (i.e. flatbed scan or photocopy) later.
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