is autofocus really an advantage with DSLRs?

Started Feb 7, 2012 | Discussions thread
Bilgy_no1 Veteran Member • Posts: 4,880
Moving subjects

Tony Rogers wrote:

The figures about AF speed in these reviews is rather deceptive. With PDAF, the camera measures the distance to the subject in a few milliseconds and then tells the lens to go to the position that will focus at that distance. Let's say that takes .25 second.

With a CDAF camera that can alSO focus in .25 seconds, the process is very different. The image is analysed to get a measurement for contrast in the focus area and the lens moved in one direction or the other. Then the image is analysed again and if the contrast has improved, it keeps moving in that direction. This process continues until contrast is maximised. Assuming that the sensor is read 120 times per second, this gives it about 30 attempts to maximise contrast. Probably less.

This process works brilliantly when the subject is not moving and the camera is not shaking. But, if it is, the samples taken during each iteration may not be of the same bit of the subject, which means that finding the place with maximum contrast may not get the thing you want accurately focused. It may also take longer than .25 second because the contrats does not converge. i.e. the lens "hunts".

Faster AF in CDAF systems does help a lot because there is less opportunity for the image to change between samples.

But how does that differ for PDAF? The measurement is made at t=0 and the lens moves to t=0.25 without any further adaptations. Unless you use continuous AF (in high level DSLRs), which allows the camera to refocus as long as the mirror is down an the AF active.

For CDAF, the sensor readout and processing power need to be improved in order to achieve faster continuous AF. That will come. For single AF, the best CDAF systems are already on the same level as DSLRs

Finally, we should not forget the influence of lenses. This goes for both type of AF systems. The latest CDAF lenses in m4/3 have only one moving element for focussing, which is a lot faster than moving a whole barrel in or out.

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