Adobe Photoshop CS5 for Photographers - Martin Evening [236]
Scanned image limitations
The chromatic aberration and vignetting adjustments are always applied relative to the centre of the image circle. In the case of digital capture images the auto correction adjustments will work precisely so long as the source image has not been cropped. In the case of scanned images things get a little trickier since the auto corrections will only work correctly if the source image has been precisely cropped to the exact boundaries of the frame.
Lens Corrections
The Photoshop Lens Correction filter has been promoted, so that instead of residing in the Filter Distort submenu, it now appears at the top of the main Filter menu with its own shortcut ( ). You will notice there are now two panel modes and the Auto Correction panel is shown by default (Figure 10.9). Here you'll find various options that are designed to correct for common lens problems, starting with the Correction section. The ‘Geometric Distortion’ option can be used to correct for basic barrel/pin-cushion distortion. Next we have the ‘Chromatic Aberration’, which can be used to counter the tendency some lenses have to produce color fringe edges towards the corners of the frame. Then there is the ‘Vignette’ removal option, which can be used to correct for darkening towards the edges of the frame, which is a problem most common with wide-angle lenses. Lastly, if you check the ‘Auto Scale Image’ option this ensures that the original scale is preserved as much as possible in the picture.
Figure 10.9 The Lens Correction filter dialog in the default Auto Correction mode.
Basically, if you are about to apply an auto lens correction to an image that contains full EXIF metadata, this information will be used to automatically select an appropriate lens profile in the Lens Profiles section below. If the image you are editing is missing the EXIF lens metadata (i.e. it is a scanned photo) and you happen to know which camera and lens combination was used, you can use the Search Criteria section to help pinpoint the correct combination. It is important to appreciate here that the camera body selection is important. This is because some camera systems capture a fullframe image (therefore making full use of the usable lens coverage area, while the compact SLR range cameras mostly have sensors that capture a smaller area. The lens correction requirements will therefore vary according to which camera type is selected. Once you have the correct lens profile selected you can use the Preview button at the bottom of the screen to compare a before and after view of the auto lens correction.
Custom lens corrections
In Custom mode (Figure 10.10), the Geometric Distortion section contains a Remove Distortion slider that can correct for pin-cushion/ barrel lens distortion. Alternatively, you can click to select the distort tool and drag toward or away from the center of the image and adjust the amount of distortion in either direction. Overall, I find the Remove Distortion slider offers more precise control.
Figure 10.10 The Lens Correction filter dialog in Custom mode.
The Chromatic Aberration section contains color fringe fixing sliders that are identical to those found in the Camera Raw Lens Corrections panel (see page 204), except there are three sliders for fixing red/cyan, green/magenta and blue/yellow fringing.
The Vignette section contains an Amount and a Midpoint slider which are also identical to those found in the Camera Raw Lens Corrections panel and these can be used to manually correct for any dark vignetting at the corners of the frame.
The Transform section controls let you correct the vertical and horizontal perspective view of a photograph, such as the vertical keystone effect you get when pointing a camera up to photograph a tall building, or correct the horizontal perspective where you haven't been photographing a subject straight on. You should only need to use these controls where you need the subject to appear perfectly square on,