DSLR Autofocus Modes Explained

« Both the total number of focus points and their types are very important. However, those are not the only two things that are needed to get accurate results. The quality and the amount of light is another important factor that can seriously affect autofocus performance. By now, you probably already know that your camera autofocus works great when you shoot in daylight, under bright sun and starts to suffer when you move indoors to challenging light. Why is this the case? Because in low-light conditions, it is much tougher for your camera to detect contrast. Remember, Passive Autofocus completely relies on light that passes through the lens. If the quality of that light is poor, so is autofocus performance. »

« Talking about quality of light – lens condition, its quality and maximum aperture are other important factors that affect AF performance. If you have an old lens with all kinds of physical problems such as mold, dirt, too much dust or back-focus/front-focus problems, your AF performance will surely suffer. In terms of lens maximum aperture, there is a reason why pro-level f/2.8 lenses focus much faster than f/5.6 consumer zoom lenses. All modern digital cameras focus while the lens aperture is wide open, so whenever you change the lens aperture to a higher number like f/16, the aperture actually gets changed only when you take a picture. Therefore, larger apertures are better than smaller apertures for better autofocus operation, with the exception of very fast f/1.4 prime lenses. Ideally, the lens aperture should be between f/2.0 and f/2.8 for best autofocus performance. Smaller apertures like f/5.6 mean less light passing through the lens, making autofocus operation more difficult and very large apertures like f/1.4 have a very shallow depth of field, which makes it difficult for AF systems to get accurate focus. »

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