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Lenses with fixed aperture
"Derek Fountain" wrote in message ... I'm shortly going to upgrade from a compact P&S (Canon S40) to a Canon DSLR and have been pondering lenses. I find a lot of lenses which have a fixed aperture, such as the well regarded 17-40mm f4 L lens. I'd have thought that fixed aperture would be a bad thing... One of the things about a compact camera is that changing aperture from one end of the range (f2.8 for the S40) to the other (f8 for the S40) really doesn't do a great deal in many cases. I was rather looking forward to working, experimenting and learning with a system that allows a good range of aperture adjustment. But now, in my quest for a small number (like 1 or 2) of quality lenses as a starting point, I find myself homing in on lenses with fixed aperture. Am I right to be concerned about this, or is having a single wide lens fixed at f4 a good thing for reasons I don't understand? -- The email address used to post is a spam pit. Contact me at http://www.derekfountain.org : a href="http://www.derekfountain.org/"Derek Fountain/a Fixed aperture, in this case, refers to the fact that the aperture does not change when you zoom the lens, i.e., it will stay at f4, if you set it there, whether you are at 17mm or 40mm, or somewhere in between. Of course, you can set the aperture at anything from f4 to f22 (I believe) and have it stay there. A wide aperture lessens your depth of field, so that backgrounds can be pleasantly out of focus, diminishing distracting details behind your subject. So, the 24-70mm f2.8L can, but doesn't have to, stay at a maximum aperture of f2.8 throughout its zoom range, but the 28-135 f3.5-5.6 IS will be at a maximum of f3.5 at 28mm, f4 by 50mm, f4.5 at 70mm, and f5.6 at 100mm. So you can keep the background more out of focus at the long end with the 24-70 than you can the 28-135. I know it's confusing, I have a "fixed aperture" 400mm (old Canon) FD mount Spiratone "Baseball Bat" telephoto that is permanently set at f8. By the way, small sensors like in the S40 give the lenses a greater depth of field, so apertures don't make as much difference as they do with the larger sensors of DSLRs, or so it seems. -- Skip Middleton http://www.shadowcatcherimagery.com |
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