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Hey gang! This is Deke McClelland. Welcome to Deke's Techniques. This week's technique is for you photographers out there. I'm going to show you how to defeat transverse chromatic aberrations, and I'm going to show you what these aberrations are and why they occur inside the movie. But first, I want you to get a sense of what they do to your images. Here is a photo that I captured of Ponte Rialto in Venice, and it looks pretty good in so far as things go, but that arrow points to a big problem right there. And I've gone ahead and zoomed in to that region of the image the old-fashioned way by printing the image at a low resolution.
Now, see that color fringing, the red and the green around the windows, and the pink and the blue around the statue? That color wasn't really there. It was invented by the camera. But fortunately, because these are transverse chromatic aberrations, you can get rid of them in Photoshop. Here is how. All right, before I show you how to correct for chromatic aberration inside Photoshop and Camera RAW, I want you to understand what it is and why it occurs. So the specific kind of chromatic aberration we'll be discussing is transverse chromatic aberration, or transverse CA.
This is a kind of chromatic aberration that you'll run into all the time. It's very, very common, and it's also extremely correctable, as you're about to see. Now, check out my very complex diagram here, and you'll see that we have this blue lens. That's what this thing is supposed to be over here. That's the camera lens. And then this gray rectangle, that's the image sensor. And this white line is light coming into your camera. So the white is focused by the lens, and in this case, when it goes right through the center of the lens, it's accurately focused onto the image sensor.
But as the light moves toward the outside of the lens, then it starts to break apart. And what's happening here is that the lens is focusing the wavelengths of light differently--and by wavelengths of light I mean color. So that color is beginning to break up right there. And then finally, as we move out toward the perimeter, the effect becomes even worse. Now this is a gross exaggeration, by the way. This would be a diagram of what happens if you were to pull a frame from a VHS movie that was shot using a plastic lens that you found in the garbage can. But still, this is the kind of problem that occurs, and it becomes worse as the lens becomes cheaper, essentially.
So, inexpensive cameras have more problems with transverse chromatic aberration than state-of-the art cameras with achromatic lenses and so forth. And then just for fun, I'll go ahead and show all the different varieties of light going into the camera, different placements, that is, where the lens is concerned. Now something I want you to understand about this. You might hear tips and tricks for avoiding chromatic aberration. For example, one school of thought has it that you want to avoid radical focal lengths. So, extreme wide angle or extreme telephoto is going to make your transverse chromatic aberration worse, and that's true.
However, transverse CA is not affected by aperture, so it does not matter what you have the aperture set to. Taking the aperture down a few stops does not reduce the effects of this sort of color fringing. However, Photoshop does. So let me show you how that works. I'm going to switch over to this image, which I shot in Venice. And if you take a look at the image, it looks just fine from this far away. But if we start zooming in, particularly if we start zooming in on one of the edges of this image, like I'll zoom into the top of the image here, and you'll start seeing that we have some color fringing. And this is transverse CA that we're seeing right here.
So basically, this brick up here, this terracotta, whatever it is, it's uniform in color--it's a uniform beige-- and yet we have got these weird green edges going on, and some hints of red edge as well. And then if I go to one of the other extreme edges of this image--I'll just go ahead and scroll to the far left side of the image-- you can see that we have really severe color fringing problems. We've got this green edge around this window, this red edge, and this little figurine right here has basically something like a violet face, and it's got this cyan thing behind it, and so forth, and that's what we're trying to eliminate.
Well, when I'm correcting for chromatic aberrations inside of Photoshop, what I like to do is split up the image a little bit, so I can see some of the extreme portions of the image. By the way, if I were to scroll to the middle of the image, you would see that we don't have any chromatic aberrations going on in this region, or very few chromatic aberrations. I think I have a little bit of green there, a little bit of red there as well. But as we move toward the center of the image, the effect goes away. Anyway, I'm going to move back outside here, and I'm going to go ahead and create a split view by going up to the Window menu and choosing Arrange, and then I'll choose this command, New Window for image X here, the image that I have opened.
Now, I'll go ahead and choose that command and now we have a new view into this image. And now I'm going to go ahead and zoom in on that image by pressing Ctrl+1, Command+1 on the Mac, to switch to the 100% view, and 100% or larger is the zoom ratio that you want to use when you're trying to evaluate chromatic aberrations. And then I'm going to go up to the Arrange Documents icon up here in the applications bar and I'm going to switch to a 2 Up display like so, so that I can see both of my images at the same time, though I want to move them. I don't know why Photoshop always does that and puts images in the wrong windows. But this way, with this guy over here on the left-hand side and this guy over here on the right-hand side, I have a pretty good sense of what's going on.
And I'll press Shift+Tab to hide my right-side panel so I've a little bit more room to work here. Then go up to the Filter menu and choose Lens Correction. Now the Lens Correction command is the command that allows you to compensate for chromatic aberrations inside Photoshop, but one big important note before you choose that command: do not crop your image yet. This needs to happen before you crop or straighten the image. You have to be working from that original image file at its original proportions for the command to work. All right, so I'll go ahead and choose the command and of course, you need to do some more zooming, so don't think you're done zooming inside the software.
You've got to zoom in inside the preview as well. While you work, you're only going to see the in-dialog-box preview. We'll just be able to use the other views we created a moment ago to get a sense of how good of a job we did after we click the OK button. Now switch to Custom, because this way you can dial in your own corrective modifications, and then notice that we have three Chromatic Aberration controls. We've got Red/Cyan, red versus cyan, because they're complementary colors. We've got Green/Magenta and we've got Blue/Yellow. Well, when I'm working with these controls, what I like to do is take a look at the image and get a sense of what's going on.
And the worst offenders, where this image is concerned, I think, are these aberrant colors around these window frames right here, around these shutters. And notice that we've got a green edge on the left-hand side and a kind of reddish edge over on the right-hand side. So that tells me that I should be using the Green/Magenta control, and notice they're in the same order as the control name, Green/Magenta. So if I were to increase the value, I'll increase make the edges worse. Notice that. I'll increase the green on left and the magenta on right.
What I want to do is counteract, so I should go with a negative value, and of course, I can see that as I work through the control. I'll go ahead and take this value down to an even number. I'll take it down to -30, let's say. You can see the detail jump around like crazy, because what Photoshop is doing is independently scaling the color channels. It's actually doing a kind of spherical distortion. All right, now I need to see what kind of edges are left here, what sort of aberrant colors are left. I'm noticing that where my figurine is concerned that he's got a purple face and the cyan in the background.
Well, I don't really have a purple/cyan control, so I'm going to have to somehow split the difference between Red/Cyan and Blue/Yellow, but I am noticing that the cyan appears in back, so it's actually opposite order. So I need to apply a positive correction. And I ended up finding out that +15 worked out pretty nicely. And then I'll go ahead and tap down to Blue/Yellow, and I would go ahead and take it up as well. Obviously, you can just play with these controls and see what ends up working for you. But in my case, I also took this one up to 15.
So I've got 15 for Red/Cyan, I've got -30 for Green/Magenta, and I've got +15 for Blue/Yellow. And then basically what I would do is click the OK button in order to apply my modifications. But before I do that, I just want you to see one more thing. You can turn off this Preview checkbox if you want to, to get a sense of a before and after, but you can also do that by pressing the P key. So press the P key to turn the Preview off, so you'll see the original problem image and then press the P key again to turn the Preview on, and you'll see the image jump to a different location like you just did, and you should see your problems correct.
All right, now click the OK button in order to apply my modification, and I can see now that it did correct the image quite nicely on both the far left-hand side of the image-- It's not 100% corrected, because this figurine still has a purple face, but you know what, I don't know, maybe the figurine did have a purple face. I never got that close to it, but based on this line right here which has some purple and cyan around it, it's pretty apparent that I didn't totally correct the problem. But that's about as good as I'm going to do with lens correction. Meanwhile, if I press Ctrl+Z or Command+Z on the Mac to undo the effect, I can see that I had quite a bit of fringing around this window right here, a bunch of green and a bunch of red there, and I'll go ahead and zoom in so you can see that problem in more detail.
So there it was before. If I press Ctrl+Z or Command+Z again to reapply the filter, you can see that the windows are in much, much better shape. That, my friends, is how you correct for transverse chromatic aberration inside Photoshop.
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