Vignetting (Light Falloff) With Canon Lenses
What is Vignetting?Vignetting or light falloff occurs when more light reaches the center of an image and less light reaches the outside edges. As a result, the corners of an image become dark and diffuse. There are three types of vignetting; optical, pixel and mechanical. Optical vignetting is caused within a lens and will be the primary type considered here. In short, when light enters a lens at certain angles, it may not reach the image sensor in the camera at the same brightness due to the way in which lens optics are constructed and aligned. The pixel type is caused by light not reaching the bottom of photon wells in the image sensor of a camera body. Light angles are also the culprit here. Mechanical vignetting is caused by an obstruction between the light and the lens. This is often a lens hood, filter, lens barrel or other object.
Is Vignetting in Canon Lenses Good or Bad?
Optical vignetting is almost impossible to fix without unwanted problems. Canon could produce a lens that completely minimized light falloff, but the lens would be very large, heavy and impossible to carry. Smaller apertures (higher number ex. f/22 vs. f/2.0) significantly reduce vignetting. Canon could therefore just make lenses with only one aperture. These lenses would be fairly useless as well. Therefore, vignetting will always occur in lenses.
Vignetting can be a good thing. Many photographers and lens designers plan for it as an effect meant to improve image quality. When a subject is centered in an image, vignetting can help draw a viewer"s attention to the center of a frame. Light falloff in Canon lenses is not always a problem. Most times, if vignetting optical rather than pixel or mechanical, it is very slight and can be fixed by adjusting to a less wide aperture.