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I have an old SMC Pentax-F 50mm F1.7 that I like to use with an adapter on my APS-C camera for portraits. It looks great, even though it has somewhat heavy CAs at f/1,7. On some backlit portraits, I get this curious artifact:

light artifact

I don't want to share the entire image as I haven't cleared it with my model yet. This photo was taken at sunset, the setting sun is visible in the image, so it shines directly into the lens, producing a flare ring and this light artifact. Here's another photo at a slightly different angle:

light artifact 2

It looks different, yet seems to be the same phenomenom.

I have never seen this type of flaring on newer lenses. The green hue seems to suggest it's some kind of combination of a flare and a Chromatic Abberation, if that makes sense? I was just wondering what causes this specific kind of flare/leak/artifact, if it has a name and in what (type of) lenses it occurs. Thank you!

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That's just an ordinary flare. It is cat-eye shaped probably because it is reflected from the edge of the lens element - edge of the lens mechanically blocked part of the otherwise round shape. Green-reddish color is because of angle of light hitting lens elements. That same kind of flare can happen with any lens, you just need point light source near the edges of lens image circle - inside it, or just slightly outside, doesn't matter.

I have the same lens, and yes, wide-open it isn't the best, as expected from large-aperture old lenses. CAs are easily cleaned in post, and I've seen much worse CAs from much more expensive lenses, though.

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All camera lenses show flare, especially so when a bright light source is in or near the field of view. We can mitigate somewhat by mounting a lens shade. Flare is a scattering of light within the optical system. Flare is thus stray light rays that in due course, baths entire image. Flare is thus devastating as it robs the image of contrast. When focused by the lens near the focal plane a ghost image forms. Ghost images become more pronounced as the lens is stopped down.

The lens maker attempts to mitigate internal reflections by use of a flat black coating on internal parts. However the main contributor is internal reflections from the polished glass lens elements. To mitigate, lenses are coated with a thin layer of minerals. Such a coat reduces reflections by interference. A well coated lens transmits 97% and reflects 3% (or better).

It is the thickness of the coat that does the trick. The thickness is adjusted to handle one specific wavelength. This will be about ¼ of the wavelength of the color to be optimized. The Reflected light we see is that color’s compliment. A coat optimized for blue, reflects yellow, a coat optimized for green reflects magenta (red + blue). A coat optimized for red, reflects cyan (green + blue), etc.

The color of the flare and a ghost is thus dependent on what color or colors comprise the reflection. This will be a variable based on the angle of incident.

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