Vignetting wide open isn’t uncommon among fast lenses. Broadly, there are two types of vignetting that are inherent in a particular lens design: “natural” and “artificial” or “optical”. Vignetting introduced by filters or lens hoods can be referred to as mechanical vignetting.
Natural vignetting is, not surprisingly, a natural property of the lens that is related to the angle of incidence of rays (the Cos^4 law), and the designers will try to overcome it. As far as my limited understanding goes, they do this by selling their souls to the devil. The wider the lens, the more natural vignetting will occur.
Artifical or optical vignetting is caused by things in the lens getting in the way of the oblique rays but not the axial rays, as mentioned by Dave and Dan. These could be deliberate, like field stops, or they could be mismatches between the location of the entrance pupil and the iris*, or the physical size of the front and rear elements – the effect being to obstruct the entrance pupil when viewed obliquely. As the iris is closed down, the obstruction to the axial rays matches the obstruction to the oblique rays and the vignetting disappears. Poof! It’s gone.
The full cure may be worse than the disease. So it’s not so much a sign of cheapness, more a sign of how much the design is balanced between different goals. One opinion is that a little vignetting at f/1.4 is not such a bad thing in a lens intended for pictorial use, because of the typical circumstances in which f/1.4 is used. The Leica Noctilux (50 mm f/1) has a lot of vignetting wide open, but who would notice it in blurry snaps taken in the gloomy dungeons that typical Leica users frequent?
*Which is the answer to Dan's question "...when the lens is wide open how can the diaphragm cut off the outer part of the cone of light that the lens projects?"
Last edited by Helen B; 09-13-2005 at 04:15 PM. Click to view previous post history.
Reason: Paragraph arrangement in original was misleading.