Well, it's been eight months since I got my Canon EOS Digital Rebel XT, so I've ordered a new lens for it. It's the Canon EF-S 17-85mm f/4-5.6 IS USM. Uh, okay, that's not very intelligible to most folks, so I'll break it down.
First of all, it's a Canon lens. I could have gone for another brand, but I decided against it for now.
It has an EF-S lens mount, the actual mechanical interface it uses to attach to the camera body. This is specifically for Canon's newish lower-end digital SLRs, and it's designed for the smaller sensor that they have. The camera will still accept other EOS lenses (the EF ones), but EF-S lenses won't work on most other EOS camera bodies.
Okay, next is 17-85mm. Yeah, that's focal length. It's a 5x zoom lens (85/17 = 5) which covers a fairly wide angle. Of course, the numbers aren't converted for this specific sensor size. Since the sensor is smaller on my camera than a normal 35mm frame size, you multiply that number by 1.6 to get the equivalent 35mm focal length (even though this lens only works on cameras with small sensors, they don't use a different numbering scheme). 27-136mm equivalent, I guess.
Then we get to the f number. I guess this is the widest that the aperture gets, though it varies depending on the zoom level. I would have liked to get a "faster" lens with a wider aperture, mostly since it lets you focus on a single small point and then get a pleasing background/foreground blur, but I know I'd be annoyed if there were moments when I wanted a small aperture, allowing everything to be (nearly) in focus.
Another benefit of a "fast" lens is that the wide aperture lets you take pictures in low-light conditions. However, the "IS" on this lens means it has image stabilization and lets you keep the image in focus despite jittery hands. You can still take low-light photos—the only drawback is that there can be more motion blur with an IS lens as opposed to one with a wide aperture simply because the shutter has to be open longer. Since I like to zoom in on things, I figured this would help in those situations.
USM stands for "ultrasonic motor". I'm not really sure what the benefit of this is though. One thing is that it's virtually silent, not that I think regular lenses make a whole lot of noise to begin with. Besides, if you're using an SLR, you've still got the big "click-clack!" from the mirror anyway. The other benefit is a bit intriguing, though—apparently you don't have to flip the little switch to go from autofocus to manual focus.
Since this is a general purpose lens, it will replace the 18-55mm "kit" lens that came with my camera. The main thing is that it improves my zoom from 3x to 5x, and image quality on the whole should be better. It's a chunk of change, but I felt it was the most rewarding thing for me to put some money into right now.Posted by mike at January 23, 2007 03:10 PM | Photography