I couldn’t really think of anything to photograph last night, so I figured I’d check out the reports that Sigma glass tends to be darker in general than Canon glass. I happen to have two lenses both with 50mm focal lengths and a tripod. The Canon lense in question is their inexpensive 50mm f/1.8, and the Sigma is their “digital” 18-50mm f/3.5-5.6. Given that I can’t go below f/5.6 at 50mm on the Sigma, I set the camera body (Canon Digital Rebel XT, or 350D) to f/5.6 with a shutter speed of 1/3sec.


The photo through the Canon 50mm f/1.8 lense.


The photo through the Sigma 18-50 f/3.5-5.6 lense.

With the Sigma lense being designed for the smaller crop sensor, and the Canon being an EF lense, rather than an EF-S probably makes a difference as to how much light gets through even at the same aperture. Anyway, in my test the Sigma is in fact darker, by about a third of a stop.