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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 |
#2
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"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? For a lens that is (for example) a 24-70mm 2.8 L, the "2.8" simply indicates that the maximum aperture of 2.8 is available throughout the zoom range...which is highly preferable to most consumer zoom lenses that end up giving you a reduced max aperture as you zoom toward the tele end. Remember, this ONLY refers to maximum aperture, and does NOT indicate a limitation of adjusting to smaller apertures. On lesser lenses, you'll see a range of apertures given, but that only indicates how much the **maximum** aperture changes throughout the zoom range. Most zooms allow larger apertures at the wide end of the zoom than at the telephoto end. You can always reduce the aperture to whatever the limitations of the lens is (usually anywhere from f22-f32, or perhaps f64, up to whatever it's max ap is). The only modern lenses (that I'm aware of) that have a TRULY "fixed" apertures (CANNOT be changed) are mirror lenses--which are NOT a good choice for most people, due to their strange rendition of background out-of-focus elements, and highlights. |
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"Derek Fountain" wrote in message
... Skip M wrote: Fixed aperture, in this case, refers to the fact that the aperture does not change when you zoom the lens This sentence straightens out my confusion. } Thanks to you and the others who pointed out the (now obvious to me) concept. -- 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 You're welcome! -- Skip Middleton http://www.shadowcatcherimagery.com |
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