Autofocus issues

S

sbaechler

Member
Hi

We are shooting a feature with the HVX 200 and I noticed that the autofocus is not really usable. During a take with a fixed camera on closeup of the actor it suddendly goes out of focus and back in. Does anyone encounter similiar issues or is this a problem with our camera?

We were shooting low light but the key on the actor was still at 50% IRE.

Regards
Simon
 
Works fine for me. I use it to film wildlife all the time that move to fast for me to keep fiddling with it. Course, I'm shooting at 60fps too so I'm sure the focus is more quick to adjust but I've never had it go into search mode on something it had locked onto.
 
I am not an expert so this is just my 2 cents.

one of the reason that autofocus works well in day light / better light is even distribution of light. In low lighting areas, the light might not be even and hence the autofocus has to constantly adjust. Also from what i understand the "autofocus" feature is not truely Auto in other words you have to be fairly slow and study in the camera movement for the camera to figure out the light intensity to correctly focus the scene especially in low light area.

Again this is based on my own experience.

cheers,

-Kris
 
The HVX uses a contrast-maximising method of auto-focus, I believe. This, as stills photography learn long ago, is a slow and unreliable method, which is why those boys developed phase-detection methods for the higher-end cameras. On the HVX, it looks to me that the part of the image processor in charge of auto-focus polls or checks the signal as often as the frame is being read off. This is why AF is slow at 24P and better at 60i.

I've not used auto-focus much on my HVX but my experience bears out that the camera looks over quite a large part of the image field for the highest contrast, and tries to maximise it over an edge. Because it's not a phase detection system, when it encounters unsharpness, it doesn't know which way to turn the lens so it tries one way and if the contrast goes down, it knows it got it wrong, and tries turning the other way - so it's hunting for focus.

Now, if you consider that the circuits are checking out some 1440 lines to find edges, it's not surprising that it's rather slow to work, especially in low light. I think it's pretty amazing it works at all, myself. Anyway, I'd expect AF to shift in low light close-ups when an actor moves only a little, because a momentary loss in edge contrast (more like acutance) will cause the camera to go hunting for focus, so it can lose it momentarily.

Not surprisingly, most of us focus by good ole eyeball. Though n8ture sure gets cracking results with his camera.

cheers
Tom
 
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