17-85IS & CA.

In the true spirit of this forum, I've always tried to post my honest opinion when the question "Whats your opinion of" has been asked of any kit I've owned. When that question has been asked of the 17-85IS, I've always said it's a superbly versatile lens that I love, but it can occassionally suffer from CA. I've been to the Lake District today, where it's been a high pressure day, rarely going above freezing point and many places have been been coated in frost all day. Itook about 100 shots with the 350D / 17-85IS, many of which are superb, but one was a classic example of CA with this lens and I thought I would share it with you all. Frankly, this just shouldn't occur with a lens costing £400+ in the UK. I overexposed the sky for sure, but CA like this is bad news.

I don't think the attachment worked 1st time!

It should be here now

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In all honesty, I've seen worse from the 24-70L. How does it look when printed?

I don't know, my printer is out of cyan and my pizza just arrived! All I know is the raw file is a big purple mess around that area. I'll change the ink after dinner and report back.

That's not chromatic aberration, it's "purple fringing",which is more a sensor issue occurring when shooting very contrasty objects with wide angle lenses. Overexposing the sky certainly didn't help. To be honest, the picture looks somewhat oversharpened, too. What were your settings and what have you done in post-processing? CA is the result of the lens being unable to focus all wavelengths of light to the same point, and will look like a miniature spectrum with red and blue-green edges.

Back aprez pizza. In fairness on my printer (Canon i560 and not really photo quality), it looks OK, but the raw file............

Jon, My apologies, but I thought Purple fringing was CA. The 350D was set to save as; Raw+jpeg at Canon default; ISO 100; EV + 1 stop; f6.3@1/13thsec; focal length 22mm. I basically save everything RAW now and only use jpegs for a quick view. all I did in post processing was levels & brightness adjustment in PS7.

Purple fringing is widely non-understood (not "misunderstood", noone's entirely satisfactorily pinpointed a single cause). For further confusion on the topic,see here. But it's most common with extremely bright areas next to dark objects, and with wide angle lenses especially at wide apertures.

Thanks for that Jon, I learned something new!

There's an article at PCWorld online that shows how to remove purple fringing:
http://www.pcworld.idg​.com.au …1015;fp;2;fpid;​1585691688

You have to work at it a bit but it does the job.

Ed