NEX 6/RX100 focus faster without assist light

[Johnsansom]

When I first got my NEX 6 I was not very impressed with the hybrid focussing - getting a lot of big green squares on the screen - until I turned off the AF illuminator - after which the focussing was a lot faster, particularly in low light conditions. I have previously posted a query about this on the excellent www.depreview.com NEX Forum and whilst several people agreed with me, no one was able to explain why this should be, particularly of course because the illuminator light is there to help focussing in low light conditions. It does help when things are very dark, but otherwise seems to slow down the focusing considerably. Does anyone out there have any suggestions as to why this might be - perhaps a Sony engineer? Many thanks, John

Message was edited by: -Cass-

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[Mick2011 · Accepted solution]

Hi John, welcome to the Sony Forums

It's most likely the extra 'processing' required.

With hybrid AF, you're basically getting both phase- and contrast-detect at the same time, as the incoming image is effectively split into two and averaged out to get a fast lock. This all happens in a fraction of a second. If you could measure the additional fraction of a second it takes to activate the AF illumination lamp prior to this, you will probably find it's at least doubling the time it takes to lock.

It's just a guess but I think that's probably what's causing it.

Cheers

Mick

[Johnsansom]

Hi Mick - and thanks very much for your reply - which sounds logical.

The only thing is that I found that, with the focussing assist light ON I wasn't getting any focussing lock at all under lower light conditions with lack of contrast, no matter how long I waited. Turn it OFF and in most cases I got a pretty instant lock. So I don't think that can be down to time it takes the focussing light to come on and be registered by the system. Needless to say I now keep the light OFF on both the NEX 6 and RX100.

Any other possibilities?

[Mick2011]

Could well be that the lamp activation switches the camera to CDAF. Certainly the lack of contrast inherent in low light has always caused contrast-detecting systems to falter, and an AF lamp has been the answer to that in many cases. PDAF is faster in low light, so when you switch the lamp off the camera doesn't default to CDAF and allows the relatively faster detection method to kick in – often with better results than with the lamp on, as you noticed.

Again, just off the top of my head, but it would make sense.

Cheers

Mick