A technical term that you are likely to hear a lot is lens vignetting. Lens vignetting (sometimes also referred to as light fall-off) is a phenomenon that causes light to not reach the entire sensor after travelling through the lens aperture. A number of reasons are attributed to the phenomenon of lens vignetting. They are mainly segregated into two – mechanical and optical. The result however is the same, dark shadows in the shape of a circle at the corner of the image.
Reasons for lens vignetting – optical vignetting
There are a number of reasons why lens vignetting is caused, primary among them is the reason that you are using a wide open aperture. Wide open apertures tend to accentuate the effect of lens vignetting. It is noteworthy to mention here that lens vignetting happens on all lenses regardless of the build quality and whether it is used wide open or stopped down.
You would think that using a wide aperture should allow more light to come through and that it is an unlikely cause for lens vignetting. The thing is at wider apertures any light that comes straight on is allowed to pass through without problems. But, on the other hand if light hits from an angle that causes it to be bounced off. Thus dark areas form around the corners where light is unable to reach. This is an example of optical vignetting.
To counter optical vignetting all you need to do is stop down the aperture by one or two stops and the problem should go away. In some situations stopping down the lens may not be an option. Let’s say when you need to deliberately shoot an image with wide open aperture for blurring out most of the frame. The solution in that case is to correct lens vignetting in Photoshop by using lens profile correction.
Simply open the RAW image in Photoshop, camera RAW (in all latest Photoshop versions) should take over automatically. Click on lens > correction. Check Enable Lens Profile Corrections. Drag the Vignetting slider all the way to the right (or depending on your preference to the exact degree) to make the image devoid of vignetting.
Mechanical reasons – lens hood
Mechanical reasons are more to do with design imperfections and other simpler issues. Let’s say you are using a third party lens hood for your wide angle lens. The lens hood can obstruct a great deal of light in certain conditions simply because the hood manufacturer makes one generic size for a lot of lens makes. In most likelihood if the filter thread matches you would buy it, not giving much thought as to whether it might have any adverse impact on your images. In all probability it would. This is because proprietary lens hoods are designed so that it does not interfere with the light gathering ability of the lens. In other words the problem of lens vignetting is marginalized. Third party lens hoods often don’t prevent that.
Filters, especially circular polarizing filters are another reason for mechanical vignetting. Filters are made of thick glass, which are designed to stop or polarize light. Either way they stop light in varying degrees. Some lenses tend to react to that rather well, i.e., vignetting is not that noticeable. However, not all lenses are that well behaved and do show up nasty vignetting in the images produced. If you are buying lenses for outdoor shooting and mean to use them in combination with filters, check the reviews and find out more about them to establish whether they suffer from vignetting.
Vignetting can be artistic
Reading thus far you are no doubt convinced that vignetting is necessarily a bad thing. Not always. Vignetting can be and is commonly used when photographers want the viewer to focus on a very specific aspect of the image. Vignetting, if it is not there already, is often artificially added in post-processing.
Both Lightroom and Photoshop make it very easy to remove or add (as the case may be) vignetting in your images. Whether you wish to leave it uncorrected or add it or remove it completely, however, depends on you and your artistic sense.