While Sony’s 24.3-million-pixel, full-frame Alpha 99 has a glittering specification on paper, the true test is how the camera handles a number of demanding situations in the field. Read the Sony Alpha 99 review...
Sony Alpha 99 review – Autofocus
This diagram shows the 19-point phase-detection AF sensor array and the 102-point focal-point array. Sony claims the latter covers 30% of the frame
Sony has introduced some interesting changes to the autofocus set-up of the Alpha 99. It uses a dual phase-detection autofocus system. Like the Alpha 77, the phase-detection AF sensor is made up of 19 points, 11 of which are the more sensitive cross type. These 19 points are grouped together in a small central portion of the frame. For quick and successful focusing across a number of situations and light conditions, it is key for the subject to be covered by the cross-type points. There is a further focal-point sensor with 102 points that is built into the imaging sensor and has an approximate 30% coverage of the frame, which is impressive.
To complement the new dual AF set-up, an AF-D mode has been introduced, which uses both systems. This mode works well with object tracking for fast-moving subjects such as football players, but also for landscapes or subjects that are off-centre as the AF point coverage is wide. It should be noted that the dual AF is only available with six current lenses, including the Carl Zeiss 24-70mm f/2.8 lens used in this test, although Sony promises firmware upgrades for more lenses.
A handy feature is the new AF range control, which offers manual control of the minimum and maximum focus distance. There are a couple of benefits to limiting the focus range – subjects nearer to or further away from these distances do not confuse the AF system, and there is less distance for the camera to scan for a focus point, which means quicker focus in low-contrast light.
For subjects in the centre of the frame, the Alpha 99 performs excellently. I do, however, find the limited coverage of the 19 points to the centre portion of the frame more restrictive for spot focusing.