Problem with bright fringe/halo

<p>Occasionally when shooting sharply defined dark subjects against a bright background (e.g., dark tree branches against a bright sky) I get thin (maybe 2-4 pixels) bright fringes around the dark object. An example is attached, greatly zoomed and tweaked in Photoshop to exaggerate the effect. It was taken with a Canon 50D with a Canon 24-105 F/4L zoom, my most common combo. It was shot at F/11 and 24mm.<br /> Questions: How can this be avoided? How can it be corrected in post-processing?</p><div></div>

<p>Can you provide a crop of the same area as it does come out of the camera, rather than the exaggerated version? </p>

<p>Are you shooting straight to JPGs? If so, how hard are you telling your camera to sharpen, as it creates those files?</p>

<p>Rob, it's attached. I can assure you the fringe is there, although it's almost impossible to see because now it's the same color as the sky. So of course this wouldn't be an issue if I hadn't wanted to darken the sky. Thanks for your thoughts.<br>

Gary</p><div></div>

<p>Looks like a processing effect, probably too much sharpening. Try shooting RAW and adjusting the sharpness parameter in DPP.</p>

<p>[[i can assure you the fringe is there,]]</p>

<p>I have no doubt that 100% views of digital images can show you all manner of interesting things. Adobe Lightroom has a nice tool for reducing chromatic aberrations that may help. Canon's Digital Photo Professional should also help here. The latest versions from both should have a profile for this lens.</p>

<p>Stepping back, I believe question should be be: how is it affecting the final product? If you're making 22x34 prints this is may be more of a concern. But just staring at 100% views without considering output is not a useful exercise.</p>

 

<p>I agree with Bob, the white fringe is almost certainly coming from a too aggressive unsharp-mask filter.<br>

Is this a "from camera" JPEG?</p>

<p>The image was shot direct to highest quality JPEG with the in-camera sharpening set at two notches above mid-point/default. It comes out of the camera with the fringe, so I'm going to knock off the in-camera sharpening.<br>

But it's not just noticeable at 100% zoom. It can be seen on reasonably close inspection on a few of my post-processed 12x18 inch prints.</p>

<p>From the original post:<br>

"An example is attached, greatly zoomed and tweaked in Photoshop to exaggerate the effect."</p>

<p>Which is why I asked for the original version that Gary was able to provide.</p>

<p>Rob, perhaps I wasn't clear. The "tweak" was to darken the sky and make the fringing more visible. It did not add the fringing.</p>

<p>This is what sharpening does. For instance, on a border between a light area and a dark area, it makes the edge of the light area lighter and the edge of the dark area darker, to accentuate the contrast between them. The higher you crank up the sharpening, the more pronounced the effect. (In something like Photoshop's Unsharp Mask, you have more controls than just "amount" so you can, for instance, determine how many pixels on either side of the edge get adjusted.)</p>

<p>This is the result of a combination of issues. Firstly, the chromatic abberation-which can (and should be) addressed first in Canon's DPP then with Photoshop if the problem still persists. The downside to correction strong abberations in this manner is that you are then left with a thin colorless area which then has to be dealt with using any variety of color fill techniques. Secondly, Using DPP to sharpen images often creates hardline sharpening halos in high contrast or backlit areas. I try not to use DPP for sharpening, rather relying on LR3, Photoshop and various masking methods to help avoid thses issues. No easy one size fits all solution.</p>

<p>There are a series of issues that might contribute here:</p>

<ul>

<li>The image is grossly oversharpened - that is the cause of the very obvious halo effect along all edges in the image.</li>

<li>It is hard to see through the halo effect, but it looks like you might have some relatively normal amount of chromatic aberration (CA) that is contributing to the purple fringe. Some types can be easily and quickly corrected in post.</li>

<li>Sometimes "purple fringing" between highlights and darker areas will occur, especially when the lighter area is very bright. This is difficult to fix in post and is best handled, if possible, by reducing exposure.</li>

</ul>

<p>Dan</p>

<p>Dan, perhaps we are just quibbling over semantics, but I can't accept that the image is "grossly" over-sharpened. It's had a modest amount of in-camera sharpening and nothing else. And that amount does not give the appearance of over-sharpening to the image overall. In fact, I almost always apply a little more sharpening in Photoshop without harm. There is, as you said, some "purple fringing" in addition to the white fringe I posted about.</p>

<p>Dan<br>

This is a classic issue when your radius is set too high. I am sure that you did not do this purposefully, but if your radius is set too high, (usually more than 1 and change) you get these halo effects. It is a software issue not a camera issue. Recheck your settings and see if lowering your radius helps.<br>

Good luck!</p>

<blockquote>

<p>Gary Anthes wrote:<br /><em>Dan, perhaps we are just quibbling over semantics, but I can't accept that the image is "grossly" over-sharpened. It's had a modest amount of in-camera sharpening and nothing else. And that amount does not give the appearance of over-sharpening to the image overall.</em></p>

</blockquote>

<p>I'd guess Dan was referring to your "tweaked in Photoshop to exaggerate the effect" version -- the sky is full of artifacts, not to mention the white halos. <br /> <br />Anyway, evaluating the non-adjusted version, it looking to me like chromatic aberration (CA), which usually is easy to remove during RAW development in ACR. Here's a couple of tutorials a quick google search found:</p>

<ul>

<li>http://www.dgrin.com/showthread.php?t=159524</li>

<li>http://www.photos-of-the-year.com/articles/adobe-raw/</li>

</ul>

<p>Don't think there's anything else you could do apart from using a different lens (some are better than others).<br>

<br />P.S.<br />Your comment about "a modest amount of in-camera sharpening" suggests to me you might be shooting JPEGs. While ACR can open JPEGs, I'm not sure whether it can still remove CA fringing; doubt it. Shoot RAW!<em><br /></em></p>

<p>Gary asked:<br>

"Questions: How can this be avoided? How can it be corrected in post-processing?"<br>

Answer: Shoot in RAW and convert to jpeg with a competent program like Lightroom or DxO Optics Pro, using only moderate sharpening and radius at around 1, with the Lens Correction mode turned on.</p>

<p>Thanks to all for your thoughtful replies. The advice is not entirely consistent, but I guess one lesson is to shoot in raw, then you have more control over this kind of problem. Of course I knew that already, I'm just lazy and, most of the time, shooting in high-quality jpeg is just fine.</p>

<p>It's over-sharpening that's causing the bright halo.</p>

<p>What you need to do to minimize this is :</p>

<p>(1) Shoot RAW - no sharpening applied.<br>

(2) Convert from RAW and don't sharpen.<br>

(3) Do whatever you want in photoshop.<br>

(4) Sharpen LAST and target the sharpening at your final display size ( e.g. your intended print size ).</p>

<p>Your problem is that you are applying aggressive sharpening ( two notches above mid is aggressive ) to the JPEG in camera and then trying to adjust in Photoshop. This is guaranteed to exaggerate any haloing from in-camera sharpening.</p>

<p>Incidentally you can sharpen using edge masks to reduce the effect of halos. Again you need to do this from un-sharpened RAW conversions. You can also try using a layer blending like darken only, although this isn't as effective as an edge mask.</p>

<p>If you want to shoot JPEG then set in-camera sharpening to the absolute minimum and use a neutral tone curve. Again adjust things in Photoshop applying sharpening last. The neutral tone curve ( and reduced contrast ) will help capture data that's better for processing.</p>

 

I was referring to the first image at the start of the thread. It is, indeed, "grossly" over-sharpened. Even at 100%

magnification we should not see such a large radius nor so much sharpening.

 

Dan

<p>The fringing you describe and which appears in your photo is in my experience nothing to do with you, your post processing or even your camera, its just a common weakness with some types of lens. The 24-105 is a fantastic all round lens. It performs a wide variety of tasks with very good results, however it does have some shortcomings and this example of chromatic fringing on high contrast edges is one of them.<br>

I use a 5d mk ii and when I first started using this lens on it I noticed the fringing immediately. I should add I gave up on jpegs years ago and only shoot raw. The fringing is clearly evident when you zoom in 100% in the raw dialogue box before any changes have been added.<br>

Happily if you're shooting raw, go to lens corrections in the raw dialogue box ( on the right hand side), and you'll find a couple of sliders that reduce the problem in a moment.<br>

Hope this helps.</p>

<p>The thing often called "fringing" is probably not what we are seeing in the first example. This, also called "purple fringing" is often found where a dark area abruptly meets a very bright area that is frequently overexposed. You can find that effect when branches are photographed against an open sky or when a bright reflection on a human subject (perhaps on clothing) is next to a much darker area. The focus of the area where it occurs can also play a role. (If possible, the best way to deal with this is to try to avoid it by not blowing out the highlights - though that isn't always easy or possible. Fixing this in post is not easy, though sometime a "defringing" setting can help.)</p>

<p>I think that this is simply the usual chromatic aberration that most often is seen as red/green "halos" on opposite sides objects with sharp edges. If you look at images at 100% you will find some amount of CA more often than not, even when you shoot with fine lenses. Fortunately - since virtually all lenses do this to some extent - it is easy to fix in post in almost all cases.</p>

<p>As to sharpening, there are a variety of approaches that can work. Because I usually work toward a print, my approach is maybe a bit more than most people might want to try - though it isn't a complicated as it probably looks when presented in a text format, as it is <a href="http://www.gdanmitchell.com/2009/10/15/how-i-sharpen-a-quick-overview">here</a>.</p>

<p>Generally, a smaller radius and less "strength" would be good here.</p>

<p>Dan</p>

<p>"I use a 5d mk ii and when I first started using this lens on it I noticed the<br /> fringing immediately."<br>

Andrew, that is most interesting. I've only been using the 24-105 about six months but it seems to me that when I do get this "fringing" it's with that lens, not with my two other L Series Canon lenses. I didn't mention this in my post because I haven't been able to do a rigorous enough comparison of the lenses to blame the 24-105 (as opposed to the camera sensor or some other thing.) I remain convinced that although sharpening makes the problem worse, and over-sharpening makes it much worse, sharpening is not the root cause of the problem.</p>