Sunday, July 17, 2011

D7000 Auto Focus – does live view contrast detect work better than phase detect?

By definition, contrast detect auto focus (CDAF) is just about perfectly accurate. After all, when the contrast is at its peak, so is the focus. Period.

But phase detect auto focus (PDAF) is generally much faster because it is based on parallax and can thus guess with near pin point accuracy which way and how far to turn the lens to hit the focus point without the hunting associated with CDAF.

The problem with PDAF is that it is mechanical after a fashion, whereas CDAF is based on image data only. Which is to say that the alignment of the PDAF hardware (image splitters and pass through mirrors and what not) makes a real difference to the accuracy of the whole PDAF system.

This is why Nikon introduced the AF fine-tune feature a few bodies ago. As resolution rises, so does the impact of even a slight difference in the calibration point for the PDAF system. Thus, we see people using back or forward focus numbers on their lenses with the D7000 that were not necessary with the D300 or the D70s …

A number of people have been testing this, but invariably the tests seem flawed by poor technique. For example, just using a tripod does not help if you don’t switch off VR and use mirror up, or at least mirror delay. Mirror slap will introduce enough variability that the results become meaningless.

So I had a little fun this afternoon testing my 70-300VR from tripod with a remote release in mirror up mode. The results are shown below, although you really need to click through to see the crops.

My conclusion is that I got a fraction more contrast with the CDAF image, although it was also darker – another phenomenon that has been associated with live view.

Once contrast is equalized, there remains a tiny difference in the finest of details … basically what looks like dust particles in the wide dark portions of the center target shown at the right of each frame.

Frankly, this difference is so slight as to be irrelevant in practical use. If you have the time and are already set up on tripod with a release in hand, then I would use CDAF. Why not …

But don’t think you have to twist your style to make PDAF work. Just shoot normally. And if you need to adjust your AF fine tune, then go ahead and do that.

A note on mirror lockup mode. Why use it? Because there is a danger zone where the thump of the mirror rising shakes the camera body. This affects all brands and cameras, although the D7000 appears slightly more affected. With mirror up, there is a delay after the thump of the mirror, and the thump of the mirror dropping has no effect because the shutter has already closed.

This is well demonstrated in the following video from Vimeo:

Camera Laser Vibration Detector Revisited from Camera Technica on Vimeo.

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