magenta or blue halo

[crockny]

What causes a magenta or blue edge in the details of some of my photos and not others? How do I get rid of it? I shoot in RAW and work in CS4 ... when I make adjustments the colored edge gets more pronounced.

[OsmosisStudios]

A sample would be nice, but Im going to guess what you're seeing is chromatic abberations that show up in certain lenses at certain apertures under certain conditions.

What happens is that, through the lens, the different wavelengths of light dont hit the sensor at the same time, so you end up with "fringing", or what looks like magenta/cyan coloured halos. These are most common on fast prime lenses when used at large apertures and on very wide lenses when used near max aperture. They're also only seen where you have a sharp difference between bright and dark (tree branches in front of an overcast sky, for instance).

You should be able to adjust it in ACR: There's a "lens" tab, the top two sliders of which control chromatic abberations. I know that for my 10-24 I often need to dial in -25 or so for the Cyan/Magenta slider. Just adjust it so that you can see it anymore (or you remove it as much as possible). You may not always be able to remove it.

[crockny]

Great info - thanks -- I'll look for that slider -- I shoot with a Tamron 18-270 and the photo I'm talking about was at almost maximum lens length and 22 aperture -- and had great contast between the sky and mountaintop -- so chromatic aberration is exactly what it is ...!