Revisiting Sony AF performance (a580)

Started Jan 6, 2013 | Discussions thread
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chlamchowder
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Revisiting Sony AF performance (a580)
Jan 6, 2013

I recently came across Rob Galbriath's analyses of how various AF systems handled action. I don't quite have the time to conduct controlled tests like he did, but I've shot a lot of sports games on the Sony a580. And after shooting quite a few sports events with the Nikon D600 and recently the D3, I think I have a good reference to look back at how the a580 performed. Obviously, I can't compare raw hit rates. I used a fast focusing 80-200/2.8 AF-S lens on the Nikon cameras, and took advantage of options like dynamic area AF. Burst sequences also can't be directly compared, because the a580 is always in focus priority mode. If the a580 doesn't think it has focused right, it doesn't fire, slowing down the framerate. In contrast, I used the Nikon cameras in release priority mode. So, I'm going to try to comment on the character of the a580's AF. All of this applies to the center point only, unless I specify otherwise.

The center point is drawn to high contrast like a magnet - even if the subject is far out of focus, the center point will confidently drive the lens to the proper focus position. The top/bottom cross points are also good, but the center point is better at detecting very defocused subjects. Given high contrast, the a580 is also capable of locking in extraordinarily low light - sometimes even outperforming the D600 in that regard. However, this can also be bad. Focus points seem to prefer higher contrast subjects, regardless of whether you want to focus on them. The focus points are also rather large, and actually cover more area than what the viewfinder markings would suggest.

That means high contrast backgrounds can often make the a580 frustrating to use. That's what I think drove Kai (digitalrevtv)'s remark on the a99 being more willing to focus on the background. Of course, careful focus point placement can mitigate this, but the D600 and D3 seem to pick up the background less often. Perhaps the relatively smaller size of the Nikon cameras' focus points contributed to that.

The a580 prefers subjects that move at a somewhat constant rate. As long as they aren't too close to the camera, hit rate is rather high with running sports like soccer, football, and lacrosse. Sudden acceleration or deceleration, though, makes the AF system struggle. In low light, having to deal with lots of acceleration or deceleration really turns the situation into a train wreck. When shooting a poorly lit indoor tennis match, I don't think I got a single perfectly focused action shot. Tennis in daylight was better, but the hit rate was still nowhere near what the camera can do with more predictable subjects.

In low light, lens hunting speed slows down. With slower focusing lenses like the 70-210/4, hunting can really take forever. It's essential to plant the focus point on a high contrast target.

Using AF-C on static subjects is risky, especially with subjects that have a lot of fine detail. Often, the focus drive will jerk around while being pointed at a complex still subject, even though there's no movement. With more ideal high contrast subjects though, AF-C is quite stable, and could be used as a substitute for AF-S if you don't mind keeping the selected point on target.

Wide area AF (all points) is rather useless for action shooting. In addition to slowing down focus acquisition time, wide area almost unfailingly picks the wrong point every time. There's almost always something with higher contrast (easier to focus on) in the frame than your intended subject, and the a580 really likes high contrast. It might work for a bird against a blue sky, but that's about the only situation I would use it for. It's too risky otherwise.

Focus acquisition time is very good, and probably the camera's strongest point. AF is fast. Locking onto a single target, the a580 is just as fast as the D600. The D3 is almost imperceptibly faster. I have limited experience with other entry level DSLRs, but I'd confidently say that the a580's AF speed compares very well with the competition.

Overall, Sony's 15 point system is very good. It doesn't track like a high end camera and doesn't always keep up, but for someone getting into sports photography, or someone who occasionally shoots action, it's an excellent choice. Within the segment it competes in, the a580 really hits hard against competitors in the AF department. I'll echo DPR's comments here with one addition: AF is precise and speedy for its class with static subjects. With moving subjects, it doesn't always keep up, but it does a better-than-expected job of keeping up for its price point.

For dedicated sports shooters, though, the lack of a release priority option is pretty limiting...they'll want to look elsewhere.

Hope this makes sense. Anyone have comments? I already upgraded to the D600, but I'm still rather curious about how the new 19 point system (a77/99) compares to the now old 15 point system in the a580/560/55/33/65, and how the 15 point system compares to the even older 9 point system in the a550.



Where the a580 really struggles - low light and unpredictable movement. I had a less than 20% hit rate, and even shots that I counted as 'hits' weren't really sharp.



High contrast + semi-predictable movement = decent tracking even in low light



Almost keeping up. Definitely usable at a smaller size though.

Where the a580 really shines - outside, running sports, good light

The a580 can still do a decent job with sports like basketball, if you can keep it from focusing on the background. It doesn't always keep up, but that's more because of the lens I used than limitations of the AF system.

 chlamchowder's gear list:chlamchowder's gear list
Sony Alpha DSLR-A580 Nikon D600 Sony DT 18-55mm F3.5-5.6 SAM +8 more
Nikon D3 Nikon D600 Sony SLT-A55 Sony SLT-A58 Sony SLT-A99
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