Autofocus
Image: Even in low light using ISO 12,800, the ten frames per second with AF shooting rate performs well
A new 15-point AF system is featured in the Alpha 55, of which three of the AF points are of the more sensitive cross type. Combined with the phase-detection AF system, this camera offers some of the fastest AF speeds I have come across in the Alpha range.
When used in Live View and video modes, phase-detection AF is particularly impressive. I photographed single-subject tracking scenes, such as my brother running on the beach or sitting on a merry-go-round, and found that the AF is responsive to lock on the subject and then maintain focus. For slightly faster-moving subjects, such as dogs, it can lag behind by a fraction of a second.
For the most part the subject was in focus in around eight out of ten frames. I also took the camera to a floodlit football training session. At times the continuous AF struggled when groups of people were moving within the frame, with an undesired subject in focus. I found that the AF system favours the subject in the centre of the frame, so placing it in this position really helped. There is a slight decrease in the number of in-focus frames when shooting in low light, but the Alpha 55 performs well nonetheless.
There is a catch to using continuous AF in the 10fps mode: it is only available in auto exposure (AE) and therefore you lose control over aperture, shutter speed, ISO and white balance. You can regain these controls by selecting single-frame AF, but in this mode the AF and exposure values from the first frame are kept throughout the sequence.
Losing the constant focusing is a disadvantage when shooting a burst of images, but if the subject is unlikely to move in and out of focus and the lighting in the scene is consistent, then single-AF offers more flexible controls for consistent results. That said, to have constant AF at 10fps is very impressive, especially considering the Alpha 55’s price.
