DSLR autofocus system design and operation

[shaneando]

Hi folks

This thread is a continuation of a discussion that started in the General Help forum but strayed off the original topic. The original thread is http://www.ausphotography.net.au/for...ens-choice-Why

The discussion is centred around the question of whether a lens with a large maximum f-stop e.g. f/2.8 or better, helps an autofocus system to focus faster. This seems at first glance to have a simple answer - yes it does, based on the two key facts that (1) autofocus sensors are basically very small image sensors, and (2) lenses with larger max f-stops definitely allow more overall light to hit the main image sensor.

Over on the dpreview forums, there is a well known contributor by the name of Marianne Oelund, who is an engineer and photographer, and has a very deep knowledge of the design and operation of most Nikon DSLR's. Marianne has written a fairly long and technical series of posts, based on her test bench setup that includes the autofocus module from a Nikon D300. Link to the thread - http://www.dpreview.com/forums/thread/3713509. Her empirical results seem to show that for the autofocus sensors, there is no light advantage, and therefore autofocus performance advantage, by using a lens with a maximum f/stop greater than f/5.6. However there also seems to be some theoretical evidence that this should not be the case.

I have tried some very basic testing with my D750, my 85mm f/1.4D and my 16-85mm f/3.5-5.6 lenses. At 85mm the 16-85 maximum aperture is f/5.6. This gives the 85mm f/1.4 a four stop advantage in terms of the light transmission. In theory this should be a large advantage but I couldn't see any noticeable difference in autofocus acquisition time. I was focusing on a scene in my study which equates to a light level of about 1 EV.

Doing this type of comparison is not easy because by and large the higher end lenses with faster f/stops also tend to have more powerful focus motors and more highly tuned focus electronics. It's hard to find two lenses that are close in spec except for f-stop. You'll note in my test the zoom is an AFS lens and the prime is a D lens so it uses the body focus motor instead.

A couple of other points to note:

(1) This discussion is mainly centred around Nikon cameras. I believe in some Canon bodies the autofocus system does have some sensors that are designed to take advantage of f/2.8 lenses.

(2) The term autofocus sensors as used in this discussion refers to the sensors that are part of the autofocus module that is buried inside the DSLR, usually at the bottom of the mirror box below the mirror. It does not refer to the array of autofocus points that can be seen in the viewfinder.

I welcome any discussion on this.

Cheers

Shane