The aperture of a DSLR lens is wide open when you are looking through the viewfinder and focusing. When you press the shutter the aperture closes to the one you or the metering has selected. You can see this happen if you look into the lens with the camera set at f/22 and press the shutter. You\'ll see the aperture close and reopen.
The significance of a 2.8 vs slower lens, in addition to transmitting more light and producing shallower depth of field (DOF) is how it affects auto focus (AF) performance. The center AF point becomes more sensitive and accurate when lenses which are 2.8 or faster are used.
A difference between the less expense designs and most L lenses is that in the less expensive lenses as you change the focal length the effective aperature changes. With the more expensive L lenses like a 24-70mm 2.8 L the aperture stays constant regardless of focal length. Since aperture affects DOF and exposure the latter is more desirable.
So for your wide zoom the 2.8 Sigma or Tamron should in theory have better AF performance. In practice? At a beginner\'s skill level you probably will have much impact one way or the other. Shorter focal lengths have wider DOF at all apertures than longer focal lengths at the same distance so precise focus, while always desireable, isn\'t as critical. In other works shorter focal lengths are more forgiving.
Not on your list but worthy of consideration is the Canon 10-22mm. I\'ve used it on my 20D and 50D and love the way is creates near/far perspective which is significantly different than what is seen by eye. The difference between 18mm and 10mm might not seem like much, but the 10mm provides a signifcantly wider field of view and allows creative distortion of perspective in situations like these:
In addition to the 10-22mm I carry the 24-70mm 2.8 L and 70-200mm 2.8 IS so I don\'t have any gaps. But I don\'t think a gap between 22mm and 50mm would be too much of a handicap.
Beginners tend to try to include focal point and background context in every shot. That results in compositions were the focal point and background occupy equal space in the photo and wind-up competing with each other for attention. In terms of storytelling it\'s usually more effective and interesting to tell a story visually with the combination of wide establishing shots to set the scene context followed by closer cropped shots of the action. Shooting that way you won\'t notice the gap between 22mm and 50mm much and in most situations can work around it as needed by moving closer or further away, respectively, to cover the gap.
You might find these tutorials of mine related to focal distance and shooting distance helpful in deciding which lenses to buy. A \"cinematic\" approach to story telling: http://photo.nova.org/CinematicApproach/ and deciding what focal length to use for portraits: http://photo.nova.org/LensPortrait/
Jul 21, 2012 at 12:14 PM
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