Is strong color fringing at f/1.8 normal on the Pentax DA 50mm f/1.8?
Asked 8/15/2014
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I’m seeing strong purple/green color fringing in close-up shots with my Pentax DA 50mm f/1.8, especially wide open at f/1.8. The example was shot on a tripod at ISO 100 with no shake reduction or in-camera chromatic aberration correction. The subject was near the center of the frame, but it appears it may not have been exactly on the plane of focus.
Is this a defective lens, or is this kind of fringing normal for this lens at wide apertures? Reviews describe its chromatic aberration as negligible, so I’m trying to understand why my results look much worse.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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I think what we see here is not the traditional lateral chromatic aberration. This is appearing in the center of the image, in more or less blurred area, the color is blue and purple and is dependent on an f-stop. I think it is a combination of axial chromatic aberration and spherical aberration.
Some thoughts:
- This is probably shot at very short distance. If that's the case, I would suggest doing more tests at portrait and landscape distances as the lens may behave better there.
- It does not look pretty at this abnormal enlargement, but may really be negligible in prints or computer display presentations
- In perfectly in-focus situations, which does not seem to be the case with the toothbrush, the CA may be really just a few pixels wide.
- I don't think this is abnormal for an inexpensive 50mm lens, especially the spherical aberration.
- Stopping down the lens 1/2 stop or so seems to improve image quality considerably. I had the same issue with several $1000+ fast lenses and learned to use them wide open only if I absolutely needed the shallow depth of field or the light.
I think what you see is pretty much normal. I'd recommend shooting the lens more, try stopping down 1/2 stop, making actual prints, trying different conditions and shooting distances. The lens may have other features that are great and may be well worth this little annoyance.
Originally by user27944. Source · Licensed CC BY-SA 4.0
user27944
11y ago
0
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What you’re seeing is most likely not normal lateral CA, but longitudinal chromatic aberration (also called axial CA) and/or spherochromatism. That kind of fringing often shows up as green behind the focus plane and magenta/purple in front of it, especially with fast lenses used wide open.
A few key points:
- This effect can appear even near the center of the frame.
- It becomes much more visible when the subject is slightly out of focus.
- It is usually worse at f/1.8 and improves as you stop down.
- Close-focus shots can make it look worse.
So based on the answers, your lens is probably not broken. This behavior is considered normal for an inexpensive fast prime, and reviews measuring “low CA” may be referring to in-focus lateral CA rather than out-of-focus longitudinal fringing.
Try testing again at different subject distances and with very careful focus. If the in-focus subject still shows severe fringing under normal use, then it may be worth comparing with another copy, but the example shown sounds consistent with the lens design rather than a fault.
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