A77 AF Drive speed menu option.

[mikeyp2000]

Hi again Sony,

I've been puzzling over one of my A77 menu options.  It's on Custom Menu 4 at the bottom, called AF Drive Speed and is described in the user manual (p.66) as "Selects the focusing speed in AF mode".

I can't for the life of me see any difference between Fast and Slow.  I've tried Sony SSM, Sigma HSM (both 'real' and micromotor versions of Sigma's HSM), Sony SAM and Screw drive lenses.  The Fast/Slow settings don't make any difference.  During AF, the lens focus drives seem to operate at exactly the same speed, whatever the setting.  I'm talking about still photography, not video.  I've not really tried the video funtion at all except when I film my feet by accident while flipping out the rear screen and accidentally pressing the movie button. (annoying!)

Am I missing something here?  I think my A700 had a similar setting that also appeared to have no effect.

Cheers,

Michael

[Mick2011 · Accepted solution]

Offcially, 'slow' mode is for critical focussing, as in macro photography. The implication is that 'slow' is more accurate, which is a little misleading. It's more to do with the speed at which the camera acquires ctirtical focus, either when the lens has traversed a big change in focus distance, or when the subject/conditions are such that it may 'hunt' quite aggressivley when trying to to lock on.

You might find different lenses behave differently – a wide prime lens has much less travel, for example, than a telephoto. Some lenses may even sound different, as the motor makes more noise the faster it attempts to acquire critical focus. But the main advantage of 'slow' mode is to cut down on hunting in low light and with tricky subjects/conditions – like macro work.

Overall focus speed should not really be affected at all, as you've noticed. Slow mode just takes a tiny bit more care, when approaching focus lock, not to overshoot or miss minor details in certain conditions. With this in mind, it may pay to keep Slow as a default, as it will tend to be kinder on lens motors and elements, especially with regard to aggressive hunting.

Cheers

Mick

[mikeyp2000]

Thanks for the very informative answer.  If I understand correctly, the AF Drive Speed option only really affects the AF algorithm, not the motor speed.  Therefore, it applies to all kinds of lenses (HSM, SSM, SAM and screw drive)

Would you advise against using Slow when using C-AF mode to keep a moving subject in focus?

Cheers,

Michael

[Mick2011]

Hi Michael

Would you advise against using Slow when using C-AF mode to keep a moving subject in focus?

Yes I suppose sports, press work etc might benefit from using Fast. However I see no reason why switching to AF-C would necessarily mean switching the AF drive mode from slow to fast. Slow may even soften the 'chattering' you get with AF-C and the payoff in terms of focus acquisition may be in the order of microseconds, which (depending on the application) may be insignificant.

The only significant payoff might be reduced stress on the lens, which does tend to be greatest using AF-C.

Worth an experiment to see, for sure.

Cheers

Mick

[mikeyp2000]

I tried a little test - I sest up a high contrast 'focus target' on an engine on my son's toy train track (!!!)  The train is hardly fast moving, but at short focus distances (1 to 2 metres), the lens has to work hard.  Also, at 50mm, this represents realistic distances and movement speeds for a toddler at play.

I thought this might give a repeatable moving target.  I used my A77 in AF-C, Manual exposure (1/200, f5, ISO1600, window light on an overcast day). 8fps or AutoExposure in the 12fps mode (Iso 1600 and f3.5 for the Sony lens or f5 for the sigma).  I tried bursts of 3-4 shots using the centre AF point with Slow AF drive mode then fast AF driver mode.  Just for kicks, I used my Sigma 18-250 EX DC OS HSM and my Sony 16-50/2.8.   I had similar hit rates with both lenses but the Sony had a higher proportion of 'good hits' than the sigma due to it's better optics.  (On static shots, the Sony is visibly better than the Sigma, every time)

It's only a small sample - approx 16-18 shots for each configuration, shot in mini bursts of 3-4 shots too keep the buffer going.

FPS  AF Drive  Hit Rate
               16-50/2.8  18-250HSM
8    slow      69%        53%
12   slow      76%        75%
8    fast      83%        88%
12   fast      61%        59%

In each configuration, both lenses perform similarly but I was surprised to find the effects of the drive mode - in 8fps, the camera performs better with AF Drive set to 'Fast'.  In 12fps, the best result is AF Drive set to Slow.

Interesting, but hardly scientific due to the possible effect of different Apertures on DOF.

I rattled of another series of shots 12FPS mode gave around 1/500s f3.5 ISO1600 so I also used M mode 1/500s f3.5 and ISO1600 and found that again, for 12FPS AF Drive "Slow" was the winner and for 8fps, AF Drive = Fast was the winner.

Curious stuff.

Cheers.