Kit lens vs Sigma 18-125 in-store comparison

Started Apr 2, 2005 | Discussions thread
IgorR New Member • Posts: 17
Re: I don't get it, somebody please clarify

As far as I can understand, there are several things that can affect the focusing speed/precision. It's true that the contrast detection circuitry is in the camera. However:

1. The camera uses the lens' maximal aperture when focusing. So, a lens with largest aperture of f/1.8 should focus faster than one with f/4.0

2. The lens glass quality can easily influence contrast, which in turn may help/hinder the focusing system. On a bright day with plenty of shadows that may not be much of an issue. But, in a lighting-wise challenging scene focusing problems may become apparent.
3. The lens' motor obviously plays a great role in focusing speed.

4. Focusing point selection is another major factor, which is frequently overlooked. You should be aware than 300D/350D has 7 focusing points and 20D has 9 focusing points. Either way, leaving a camera to do it's own thinking regarding focusing point selection may produce mixed results. The camera will look for greatest contrast, not greatest importance. Portrait closeup is the best example of this problem. Sharp focus on eyes produces a stunning effect. Eyes have the most detail. Less than perfect skin focus is acceptable and even desired (softer out-of-focus skin looks more flattering). Try setting your camera on automatic and taking 10 portrait shots. Eyes will be in focus in some shots and not in others. I guarantee that you will be concerned that your camera's focus is not working properly (I was with my first DSLR). Now, select a focusing point yourself and position that focusing point on the eyes. Results are much more predictable and desirable. If you are fortunate to own Canon's high-end DSLR such as 1Ds with 35 focusing points, this is less of a problem.

Sorry for this non-scientific rant. This is my understanding of focusing systems. I hope it helps.

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