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Infrared “hot spots”

Sooner or later when processing infrared images (probably sooner), you’ll discover that some lenses are poorly suited to the task because their coatings produce an objectionable flare or “hot spot” in the center of the image. This flare isn’t present in visible-light photographs, but it can be present with some lenses, even some of the very expensive ones, such as the otherwise outstanding Canon EF 70-200/f2.8L IS. Below is an example, sized down (but representing about 80% of the width of the original frame). Note how the grayish letter “A” is significantly lighter than the other letters.

The contrast loss is a serious problem for some images, though in some it can be harder to detect. It is also a problem in that it can “blow out” or “clip” the highlights in a key part of the image—the center. Of course, any uniform image area such as sky or water will show such hotspots readily. I have also observed hot spots in areas other than the center with some lenses.

Post-processing can greatly reduce the visibility of such hot spots, but any clipping of detail can of course not be recovered.

My search is on the for the most suitable lenses for infrared work on the Canon EOS 5D, and I already have a few favorites. In general, primes are better suited to the task, though some zooms are quite fine performers too.

Look for a diglloyd.com article on lens selection for Canon and Nikon in the late August/early September time frame, as I accumulate more experience with more lenses.

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