What causes this cat-eye shaped green flare in backlit portraits?
Asked 1/19/2017
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When I use an older SMC Pentax-F 50mm f/1.7 adapted to an APS-C camera, I sometimes see a bright green, cat-eye shaped light artifact in strongly backlit portraits. In these shots, the setting sun is in the frame or very close to it, and I also see a flare ring.
The shape changes slightly with framing, but seems to be the same phenomenon each time. I don’t usually notice this kind of artifact with newer lenses.
What is this type of flare called, what causes its shape and color, and is it more common with older lenses or can it happen with any lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
4
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.
Originally by user7212. Source · Licensed CC BY-SA 4.0
user7212
9y ago
0
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This is lens flare, specifically a ghost flare/internal reflection rather than a separate defect or chromatic aberration.
What’s happening:
- A very bright light source (the sun) is in or near the frame.
- Light reflects between lens elements and other internal surfaces instead of going straight to the sensor.
- Those reflections can form distinct ghost shapes near the image plane.
Why it looks like that:
- The cat-eye shape likely comes from the reflection being clipped near the edge of the lens image circle or by the mechanical edge of a lens element, so it isn’t perfectly round.
- The green/magenta tint comes from the angle-dependent behavior of lens coatings and glass reflections.
Is it specific to old lenses?
- No. Any lens can do this if a bright point source is near the edge of the frame.
- Older lenses often show it more readily because coatings and flare control are usually less advanced than on modern lenses.
A lens hood can help reduce flare, but with the sun in frame some flare and ghosting is normal.
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AI9y ago
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