In looking for reviews and recommendations for a cheap, wide range (greater than 5x) zoom lens I found this gem of information in regard to the Sigma 18-50 f/3.5-5.6 DC lens:
bq. The Sigma is designed for Digital SLR. So its image circle falls neatly inside my Canon DRebel’s APS sized sensor. Therefore an 18mm is really an 18mm.
Let’s break this down a bit. First, yes, the image circle of Sigma DC lenses are designed to cover a APS sized sensor and will not cover a full-frame 35mm sensor. And, 18mm is ALWAYS 18mm. However, what this person is talking about is effective focal length (in regard to field of view) when compared to a 35mm film camera. Just because the image circle produces by a DC lens is smaller, it is still an 18mm lens. Therefore, with a smaller image circle, there is LESS image. That doesn’t mean that the image in the non-DC lens would be reduced to fit in the smaller circle. This CAN be done by shortening the length of the lens. But that would, of course, decrease the focal length. Therefore, in regard to field of view, a multiplier still needs to be considered on DC lenses and, to use his improper terminology, an 18mm DC lens is NOT an 18mm lens on his camera… it’s a 28mm lens just like like a regular non-DC 18mm lens would be and just like any focusing plane of glass held 18mm in front of his sensor would be.
Misinformation sucks. Had I bought this lens assuming I would get the field of view equivalent of an 18mm lens on a 35mm camera, I would have been quite disappointed since I would need a 12mm lens to achieve that and they run 5-10x the cost of this cheap lens.