This blog deals with how the autofocus (AF) system found on most non-DSLR digital cameras works. These are the cameras that have a continuous live preview of the subject, either on LCD or EVF (electronic viewfinder).
The sensor is basically a videochip, which is used for many tasks amongst which also AF.  These cameras normally use the contrast detect AF, as opposed to the more expensive phase detect AF.

The following discussion is based on my Nikon Coolpix 5700, but the conclusions should be valid for most contrast detect AF cameras.

How does contrast detect AF work?

Perhaps surprisingly, not by evaluating your whole scene. The AF works by evaluating a row of a few or perhaps even only one pixel high. Length varies probably per brand but 200 pixels could be a close figure. Very important is that this row is afaik allways positioned horizontally.
Where in the viewfinder this row is situated depends on the camera and your settings.


Test your cameras AF.



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This shot shows my viewfinder, pointed at a white door with a tiny black plastic strip taped on it.
The 5 bracketed areas are the selectable AF areas. By changing the camera position you can place the black strip anywhere in the viewfinder.
Doing a halfpress will show if the camera is able to focus.

The blue strip is the approximate size of the AF sensor for the selected bracket. (of course not visible in the viewfinder)

I advise to play with your camera using this setup to see how your AF works and get a feel for the accuracy of your AF brackets (if available).


This is how the AF system  "sees" the plastic strip:


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Out of focus



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Focussed


The camera focusses by changing the lens untill the maximum contrast has been reached in the row of pixels it is evaluating.


One of the consequences of the horizontal placement of the AF areas is that the camera can NOT detect horizontal contrast.


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In this shot the shutters provide only horizontal contrast if you frame like the GREEN rectangle.
Try a halfpress to lock AF and find the AF will not lock at all.
To make this demonstration work be sure not to include any vertical items like the background showing through the shutters or including the vertical ropes etc.

By rotating the camera towards portrait orientation somewhere around the RED position the camera will have enough vertical component to lock AF.

In the YELLOW position AF lock is immediate.



How to use the AF

If possible control where the camera is looking to focus. If available use AF brackets, and do not let the camera use the whole screen to search for contrast.
(on Nikon Coolpixes use  "AF Area Mode - Manual")

With contrast detect AF it's advisible to let the camera lock focus first before taking the shot, so:
- half-press, wait for AF lock
- recompose
- take the shot.

Make sure the AF bracket covers the spot you want to be sharp in your shot while doing the halfpress.
In faces the eyes are the most important.

If possible avoid having items at different distances in the AF bracket. The camera has no way of knowing what is nearby and what is far away, it is only looking for maximum contrast. In a portrait eg this means do not let the AF bracket partly cover the face and partly the background, even if this seems to give good contrast. The camera AF may very well lock on the high contrast background iso the face.

If unable to lock focus consider rotating the camera to try for a AF lock.

Low light also means low contrast, giving most AF systems problems to lock.
Consider switching on some lights, or look for a light reflection on your subject to lock focus.
Also staying at wide angle on most cameras allows more light through the lens, hence a better change on AF lock.
Options: Contact Ben Wouters
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