AF speed and accuracy

[Mark K]

I am an alpha mount fanboy but also happen to have Canon and Nikon. Despite Minolta was the first to introduce AF into SLR bodies in large scale (Dynax 7000), the AF speed and accuracy in digital bodies remain much to be desired. Most of the camera reivews do not include focusing as an item but to me this is equally important as other part. Is there a scientific explanation to technology used in SLR system.

[David Kilpatrick]

There are plenty of reviews with AF data. Sony (currently) is very fast, often matching or beating individual Canon or Nikon camera and lens combinations, and Minolta/KM was by no means behind the others - the Dynax 9 and 7 cameras were best in class at the time of launch. What affects results most is the type of lens used, and what affects perception may be the level of visible focus taking place, or the noise from the system.

The issuing of the 18-70mm lens with most bodies up to now has resulted in some slower figures than, for example, Nikon and Canon fitted with ultrasonic motor 18-55mm basic lenses. Since Sony had no comparable lens, unmatched comparisons were often made. However, if you put even an old Minolta 50mm standard on a body, and compared it with (even) a brand new standard on other makes, the result could favour Sony. Ultrasonic type lenses are not actually all that fast, they are just very quiet and good at making tiny focus adjustments. Experienced owners have found that some old lenses like the 80-200mm f2.8 Minolta HS apo actually focus much faster than the latest 70-200mm SSM (supersonic motor) Sony.

There are really no rules, because each body model will vary slightly in the way focus is handled, and every lens will be different. What I can tell you for sure is that Sony's A700 and A900 will focus on targets of low contrast, in very low light, which no current Nikon camera recognises as a focusable situation - but Nikon's AF is highly praised. Also, there is a general rule that speed is a trade off against accuracy in AF. On the whole I find Canon AF faster than Sony, but not as accurate. Nikon - all I will say is that the AF on the old F5 (the basis for a Kodak DCS760 I use) knocks spots off the new all-dancing 51-point system. We did some quick tests with a new D3 and the old DCS760, just 'walking at the camera' stuff to see how they compared, and ended wishing it was possible to put the D3 sensor into an F5 body instead.

There are a few lenses know for very fast and precise focus with the Sony bodies (especially the A900 and A700 which have both double cross hair and f/2.8 centre sensors). Examples are the old 135mm f/2.8 and 28-135mm zoom, the 80-200mm HS, the 50mm standards, most wideangles, the 28-75mm Tamron/KonicaMin, the CZ 16-80 and SAL 16-105mm, the CZ SSM 24-70. There are others which are famous for being slow - 70-300mm Tamron, 75-300mm Sony, oldest type 35-105mm AF zooms, Sigma 28-105mms etc of various dates.

David