Regarding halos in images

In post-processing, what is the most common
cause for creating a "halo" on the horizon?
What is the best way to correct it or to keep
it from happening in the first place? I appreciate
your time and help.

Cordially,

Robert Dayton

Without seeing a specific image, I would say the short answer is oversharpening. Each image will require different USM settings and amounts, so you'll need to experiment to see which combination of amount, radius, and threshold gives the best results.

--P

The other major cause is in HDR technique. Halos shouldn't be appearing here either, except if want them there and go a bit extreme.

Could you post an image or two showing the halos?

Thanks

Another cause of halos is improper use of selection tools followed by application of tonal changes (levels, curves, etc.) and especially use of shadow/highlight adjustment with improperly exposed subjects.

I'm posting an example to give you a better chance to evaluate the problem.
It seems to me that the comment from you, Richard, about "shadow/highlight
adjustment" may be the culprit. Although I do sharpen as I did in this posted
example; so, Preston, you may have identified the source of the error. Thanks for all of the help.

Robert, I usually do not apply the same setting for USM on the entire photo. I select areas that will allow me greater range to sharpen such as the center of a flower petal...and avoid sharpening the higher contrasting edges ...such as the edge of a petal or a horizon. These areas I select separately and apply less sharpening because of the halo created.

And remember, you never need to sharpen skies. Keep your edits local and not global when at all possible.

Sharpening is probably the most common cause of halos. You can also create halos without specifically sharpening the photo. In converting from RAW, for example, ACR has a Clarity slider, that probably includes a sharpening element. The halos it makes are usually broader and less distinct than the ones created by final over-sharpening. The halo along the tops of your mountains is a little wider than the typical over-sharpening halo and looks like it may have been created by some sort of auto-enhancer, such as the shadow/highlight you mentioned, rather than just sharpening. If you shoot JPEGs, this could be even be put there by your camera settings.

Here's a question for you Robert:

Is your camera set up to do a little sharpening in-camera? That's the most common source of haloing I see in post processing. It's best to have no sharpening applied in-camera, because it's never as good as you can do with PS, and often seems to conflict with your best efforts there.

This isn't finger pointing, by any means. Many of our clients want in-camera sharpening, especially when we're doing a direct hand off of discs rather than post processing ourselves. Checking in-camera sharpening settings is one of the first things I do when picking up a body, because I sure don't want to pay the price of getting it wrong.

If you have Photoshop CS5, the Refine Edge... tools have been greatly improved and a little work with them can greatly reduce haloes. As others have said, select what you want sharpened (or if you want to apply different sharpening levels, then a subset). Select the Refine Edge command and apply some smoothing to the edge (I find 5-10 is usually right) and then feathering (I usually use .7 - 1.5 pixels). Then apply sharpening.

You'll want to experiment with the amount of feathering and even the smoothing; I'm still learning the right settings. And I can tell you the amounts vary depending on the contrasts and colors involved.

It would be very helpful if one of the really good PS users here could write an article on Refine Edge and its uses...

I'll agree with the average theme of the above posts. When I battle halos, it is usually as a result of trying to increase edge contrast.

Sharpening is by definition an increase in edge contrast, so halos can certainly show up with sharpening. However, most traditional sharpening is low radius with high amount and will cause a tight, bright halo at the edge as apposed to the broad halo in the photo you posted.

Unless you've created a layer mask that also includes the halo, adjustments that broadly increase edge contrast (to improve definition and more broadly blend the effect with surrounding areas) such as RAW's clarity, shadow/highlight, and local contrast enhancement (high radius, low amount sharpening or HIRALOAM) are the most common offenders. The halo is most noticeable on the brighter side of an edge with a high dynamic range.

A quick and easy fix is to create a white layer mask on the offending adjustment in Photoshop, and then to paint along the offending edge with a black brush set to a size slightly larger than the halo and an hardness of 0%. I usually use an opacity of 80% to 100% depending on the photo, or in CS5 just paint at 100% opacity and then reduce the mask's density to taste. I usually paint with the Caps Lock key on so I can keep the center of the brush right at the edge. For a photo like the one you posted, this should take less than a minute of total time.