Factors affecting autofocus beauty photography

The photographic subject can have an enormous impact on how well your camera autofocuses—and often even more so than any variation between camera models, lenses or focus settings. The three most important factors influencing autofocus are the light level, subject contrast and camera or subject motion.

An example illustrating the quality of different focus points has been shown to the left; move your mouse over this image to see the advantages and disadvantages of each focus location.

Note that each of these factors are not independent; in other words, one may be able to achieve autofocus even for a dimly lit subject if that same subject also has extreme contrast, or vice versa. This has an important implication for your choice of autofocus point: selecting a focus point which corresponds to a sharp edge or pronounced texture can achieve better autofocus, assuming all other factors remain equal.

In the example to the left we were fortunate that the location where autofocus performs best also corresponds to the subject location. The next example is more problematic because autofocus performs best on the background, not the subject. Move your mouse over the image below to highlight areas of good and poor performance.
In the photo to the right, if one focused on the fast-moving light sources behind the subject, one would risk an out-of-focus subject when the depth of field is shallow (as would be the case for a low-light action shot like this one).

Alternatively, focusing on the subject’s exterior highlight would perhaps be the best approach, with the caveat that this highlight would change sides and intensity rapidly depending on the location of the moving light sources.

If one’s camera had difficulty focusing on the exterior highlight, a lower contrast (but stationary and reasonably well lit) focus point would be the subject’s foot, or leaves on the ground at the same distance as the subject.
In the photo to the right, if one focused on the fast-moving light sources behind the subject, one would risk an out-of-focus subject when the depth of field is shallow (as would be the case for a low-light action shot like this one).

Alternatively, focusing on the subject’s exterior highlight would perhaps be the best approach, with the caveat that this highlight would change sides and intensity rapidly depending on the location of the moving light sources.

If one’s camera had difficulty focusing on the exterior highlight, a lower contrast (but stationary and reasonably well lit) focus point would be the subject’s foot, or leaves on the ground at the same distance as the subject.

This entry was posted on Sunday, January 10th, 2010 at 9:09 am and is filed under Camera, Photography Tips. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.