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Originally Posted by Murkar
First off...what is a full frame sensor and how does it differ from other sensors? I've heard it quite a bit, but don't really know much about it.
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A full frame sensor is a sensor that's the same size as a frame of 35mm film: 36mm x 24mm. It's considerably larger than entry level dSLR sensors.
The larger sensor doesn't actually make the DoF shallower, but to get the same composition on a larger sensor, you'll use a longer lens, and that will give you a shallower DoF than if you used an (equivalent) shorter lens on a crop body. Larger sensors also have a lower pixel density (i.e., the individual pixels are larger), so they have better
high iso noise performance. Most of the effects of sensor size on digital images is described in
this article.
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(Also, what is 'The L glass?')
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The
L designation is one that Canon puts on its professional line of lenses. It can stand for "luxury" or "low-dispersion glass" depending on which Canon literature you read. Typically, an L will be the most expensive lens of a specific type, have the largest max. aperture, and the most features (IS, USM, etc.) The telephoto zooms are often weather-sealed and white. Technically, an L lens will have at least one exotic glass element in it, either flourite glass, or something else with low dispersion characteristics, which means the lens will exhibit high contrast (which in turn increases sharpness) and low chromatic aberration. There are only three L lenses under $1000: the 17-40 f/4L USM (~$750), the EF 70-200 f/4L USM (non-IS) ~$650, and the EF 200mm f/2.8L USM (~$750), so just assume a four-figure price tag when someone mentions an L lens.
Some folks on boards will often say something idiotic like, "you should only buy Ls". Ls are terrific, they do amazing things other lenses don't do (which is why I own three). But they're
really expensive, they're big, they're heavy, and sometimes they only offer a marginal improvement over a much cheaper USM lens. And in the case of the 85L vs. the 85/1.8, the autofocus performance is much slower on the L. Ls, like all lenses, have individual characters, and specific tradeoffs. Sometimes an L is the only answer to a specific shooting problem. Sometimes, it's completely unnecessary. You don't
need L lenses to take good pictures, and learning to stop down and to properly post-process images can considerably narrow the gap between consumer and pro glass results.