What Is Vignetting?
Q: When my Nikon D70's 18-70mm lens is at its wide angle (18mm) setting, I'm getting dark corners on my skies. Is this normal or is something wrong?
A: This problem is common with many lenses and it's called vignetting. It's caused by a reduction in light at the edge of the lens. The barrel or sides of the lens become visible, resulting in dark corners in the image. The use of teleconverters can also result in vignetting. It's an optical problem that, unfortunately, goes hand-in-hand with making lenses affordable. The only true cure is to buy extremely expensive professional lenses.
This image of a plain background shows the effect of vignetting - the darkening in the corners of an image. Where it exists, on most photos it's not noticeable but wide-angle zoom lenses set at their widest field of view may exhibit it, especially if filters are attached.
Vignetting is usually worst when the lens aperture is wide open, so switching to aperture-priority mode and stopping down the lens to f/5.6 or f/8 will help somewhat. Another solution is to zoom in slightly but that of course means you can't use your lens's widest setting.
If you've attached filters (e.g. an Ultra Violet or a Polarizing filter) to your lens, that can also lead to vignetting. So your choice in this case is to remove the filter(s) or zoom in slightly to remove the vignetting.
If you shoot digitally and in RAW mode, a software solution to vignetting can be found in the RAW converter in Adobe PhotoShop. In the Advanced mode, there's a Vignetting slider which allows you to increase the exposure at the edges of the frame, thus removing the problem. [On Nikon digital SLRs RAW files are called .NEF files]
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