Adobe Photoshop CS5 for Photographers - Martin Evening [99]
Figure 3.50 The top screen shot shows a 200% close-up view of an image where you can see strong color fringing around the strong contrast edges. In the lower version I used a Chromatic Aberration correction to remove the color fringes.
Lens Correction filter
The Chromatic Aberration controls discussed here are also available in the Photoshop Lens Correction filter, where there is also now a third, Magenta/Green slider.
To correct for chromatic aberrations you do need to be viewing the photo at a magnification of 100% or higher. The Red/Cyan Fringe adjustment works by adjusting the scale size of the Red channel relative to the Green channel, and the Blue/Yellow Fringe slider adjusts the scale size of the Blue channel relative to the Green channel. The net result is that with careful manipulation of both these sliders you should be able to remove all signs of chromatic aberration across the whole image, even though you are only analyzing one small section of the photo.
Figure 3.51.
If you hold down the key as you make adjustments to the Blue/Yellow Chromatic Aberration controls you can hide the Red/Cyan color fringing (but you must be viewing at 100% magnification or higher). You can do the same with the Red/ Cyan Chromatic Aberration controls to hide the Blue/Yellow color fringing
Defringe
The Defringe options provide an extra level of defringing in addition to the manual slider adjustments. To be honest, the Highlight Edges and All Edges settings usually have a very subtle effect, but if you are going to use this, I would suggest choosing the All Edges option ( Figure 3.52 ), as it can also sometimes help clean up any remaining color fringes.
Figure 3.52 The top half of this Camera Raw preview shows the ‘before’ version and the bottom half view shows the preview with the ‘All Edges Defringe’ option selected. I have to say that with the new Camera Raw 6.0 demosaicing, the difference is very slight now.
Lens Vignetting control
With certain camera/lens combinations you may see some brightness fall-off occur towards the edges of the picture frame. This is a problem you are more likely to encounter with wide angle lenses and you may only notice this particular lens deficiency if the subject contains a plain, evenly-lit background. The Lens Vignetting Amount slider can be used to correct for this by lightening the corners relative to the center of the photograph, while the Midpoint slider can be used to offset the rate of fall-off. As you increase the Midpoint value, the exposure compensation is accentuated more towards the outer edges.
UV filters and edge detail
Fixing a UV filter over the lens is generally considered a good way to filter out the UV light when photographing outdoors, plus it can also offer a first line of defence against the lens getting damaged. However, this is not such a good idea for wide-angle or wide-angle zoom lenses since the light entering the lens from the extreme edges is forced to go through the UV filter at an angle and this can cause the image to degrade more at the edges of the frame since the light passes through the filter glass at an oblique angle.
Vignetting is not always a result of the lens used. In the studio I am fond of shooting with extreme wide-angle lenses and the problem here is that it's often difficult to get the backdrop evenly lit at all four corners. In these kinds of situations I find it sometimes helps to use the Lens Vignetting slider to compensate for the fall-off in light towards the corners of the frame by lightening the edges (as shown in Figure 3.53 below).
Figure 3.53 This is an example of the Lens Vignetting sliders being used to compensate for light fall off on a studio backdrop, to produce a more even-balanced white.Client: Clipso. Model: Lucy Edwards @ Bookings.
1.
Here is an example of a photograph shot with a wide-angle lens, where lens vignetting