Lens vignetting is a problem that’s commonly encountered with wide-angle lenses and is particularly noticeable if the subject you are photographing contains what should be an even shade of tone or color. For example, you’ll become more aware of such lens vignetting problems when you are photographing a landscape with a large expanse of clear blue sky, or you are photographing a subject against a plain, light colored backdrop. It is in these types of situations that you are more likely to notice a darkening of the image toward the corners.
The Lens Corrections panel (formerly known as the Vignettes panel) consists of two sections. The Lens Vignetting section contains the Amount and Midpoint sliders. By adjusting these two controls, you can usually find an optimum setting that will correct for the light falloff in a photograph, such as in the landscape photograph example shown on the right. Most of the correction is done by first adjusting the Amount slider, followed by a fine-tuning adjustment made using the Midpoint slider to balance the vignette adjustment from the center to the edges. With these two slider controls, you should be able to precisely correct for unwanted lens vignetting in almost any photograph. Once you have found the setting that is right for a particular lens, you might want to save this Lens Corrections panel setting as a preset that can be applied to other pictures that have been shot using the same lens.
The Lens Vignetting Amount and Midpoint sliders can also be used to compensate for the light falloff in studio lighting sets. In Figure 6.54, you can see an example of a studio shot in which the model was photographed against a white background using a wide-angle lens. Although I tried to light the background and foreground as evenly as I could, there was inevitably some light falloff toward the edges of the frame. In situations like this, it can be useful to adjust the Lens Vignetting sliders so that the darker corner edges of the frame are lightened slightly. Here, I would adjust the first photo in a series to get the Lens Vignetting balance right and then copy the Lens Corrections setting across all the remaining photos that were shot using this particular lighting setup.
Figure 6.54 The vignette controls can also be used to compensate for light falloff in a studio shot.
Just as you can use the Lens Vignetting sliders to remove a vignette, you can use them to apply a vignette. I often like to use the Lens Vignetting sliders to deliberately lighten or darken the corners of a photograph. This is also something that you can achieve using the Post-Crop vignettes in the Effects panel. I’ll be discussing these controls a little later, along with the new Lightroom 3 Post-Crop vignetting options.
- Vignetting is always more noticeable in photographs where there is a large area of flat continuous color or tone, such as a deep blue sky. The increase in darkness toward the corner edges is quite noticeable here.
- Here I applied some Lens Vignetting adjustments via the Lens Corrections panel, in which I used a positive Amount setting to lighten the corners and fine-tuned this anti-vignetting adjustment by tweaking the Midpoint slider.
Chromatic aberration adjustments
Chromatic aberrations are caused by the inability to focus the red, green, and blue light wavelengths at the same distance along the optical axis. As a result of this, where some color wavelengths are focused at different points, you may see color fringes around the high contrast edges of a picture. This can be particularly noticeable when shooting with wide-angle lenses (especially when they are being used at wider apertures), where you may well see signs of color fringing toward the edges of the frame.
The sensors in the latest digital SLRs and medium-format camera backs are able to resolve a much finer level of detail than was possible with film. As a consequence of this, any deficiencies in the lens optics can be made even more apparent. To address these problems, some camera companies have designed “digital” lenses that are specially optimized to provide finer image resolution and, in the case of non-full-frame-sensor cameras, lenses that are optimized for a smaller sensor chip. However, many of the lenses in common use today were computed in the days of film, when the film emulsion focus distance was different for each wavelength. Digital lenses, on the other hand, are specially optimized to take into account the fact that the filtered sensors on a typical digital SLR CCD all now lie in the same plane of focus. However, one exception to this is the Foveon chip, where the CCD sensors are placed in layers and it could be argued that lenses computed for film are more appropriate for this particular type of sensor.
Where lens aberrations are a problem, the Chromatic Aberration sliders in the Lens Corrections panel (Figure 6.55) are able to address and correct for such optical lens deficiencies. The Red/Cyan and Blue/Yellow Chromatic Aberration sliders are able to correct such lens problems by very slightly expanding or shrinking one of the RGB channels relative to the other two. So, if you adjust the Red/Cyan slider, the red channel expands or shrinks relative to the green and blue channels. If you adjust the Blue/Yellow slider, the blue channel expands or shrinks relative to the red and green channels. If you hold down the Alt key as you drag the Chromatic Aberration sliders, you will see a more neutral color image in which the color fringe edges are easier to detect. As with other panel controls, such as the Detail panel, you must inspect the image at an actual pixels view size in order to gauge these adjustments correctly. In particular, you will need to scroll the image to carefully examine the corner edges where the color fringing will be more obvious. As I just mentioned, these problems are more common with wide-angle lenses, so you may find you want to share the lens correction settings made for one image across others shot using the same lens (as long as they were all shot with the same lens at the same focal length and lens aperture).
Figure 6.55 The Lens Corrections panel controls include the Chromatic Aberration sliders.
- I don’t usually expect to see color fringing when I shoot using my 70–200 mm lens, but this is a genuine example where color fringing was seen around the edges of the bright pink flower petals.
- This was resolved by adjusting the Chromatic Aberration sliders in the Lens Corrections panel. Here, I adjusted the Red/Cyan and Blue/Yellow sliders to reduce the blue/purple fringe.