push autofocus jumping
 

I'm having problems with the push autofocus button. I'm in manual, if I hit the push autofocus button a couple of times without reframing, the focal distance it sets will jump - from 5m, to 10, all the way to infinity if I press it often enough. (not in 5 meter jumps each time necessarily, but that's fairly typical). Surely the push autofocus button should give me the same focal reading each time? Is this a glitch or something in the camera's functionality I'm not aware of? Or maybe is it because I'm sometimes in low light environments?

All autofocus devices have a harder time in low light. In low light, camcorders often hunt and focus on objects that are at different distances. That is possible even if the camera is locked down and the scene is not changing. But if it is doing consistant jumps from 5m to 10m to infinity like it is going through steps and those distances do not correspond to something in the scene, then I have no idea about that one.

Hi Leonie.........
 

Push AF/ Auto Focus needs a specific target to work correctly, by which I mean:

If an object occupies a certain percentage of the centre frame (varies from camera to camera) AF/ PAF will always lock to that target.

The smaller that target gets in the frame, the less likely the AF/ PAF will lock to it.

Rack the camera into full wide angle so that your target is now just part of the landscape, AF will most likely go to Infinity. PAF doesn't really know what to do because everything in the frame is a valid target and everything is at a different distance.

Whatever PAF sets, if you press the button again with no obvious target, it thinks it's chosen incorrectly and will have another bash based on it's programming parameters, usually a large (ish) step outwards till it reaches infinity, then back to close up again.

The trick in these circumstances is to zoom right in on your target, press PAF, then rack out to frame and shoot.

Make any sense?


CS

yeah. That makes sense. I'm shooting wide angle the whole time (making a doco). It does seem to work ok when I zoom in. Was hoping to be able to skip zooming but... that makes sense. Thanks.

The zoom-in technique is the preferred method of insuring good focus. However, I will throw out one caution to the "zoom-in" method. Make sure back focus on your camera is good before trusting it with a paying job. My V1 had to be sent into Sony service because the back focus was off. I would zoom in tight, focus, zoom out and the subject would be unacceptably soft. I know a couple of others on here also had the same problem.

In which case it doesn't matter a hoot what distance your lens is focused on. You're working at a focal length of just a few mm, feeding tiny ¼" chips, so even at full aperture you'll have dof that goes from half a metre to infinity. I'd leave the camera on auto-focus in such conditions, but of course be far more careful to use locked exposure.

tom.

Assign function [FOCUS INFNITY] to one of the external buttons. If you're in the wide-angle, far-distance situation where basically everything is in focus (hence the PAF hunting symptoms you mentioned), there's no reason to risk back focus issues: just depress [FOCUS INFNITY] and you're done, secured.

Autofocus is nice too and usually a good bet, but can hunt if you pan or something or someone comes in the way.

Leonie: just to clarify, the "push" auto focus button is more of a "brief hold" to focus.

Not sure if you are using it as a "momentary" (pressing VERY briefly) or holding down until the object is in focus but the button engages the autofocus servo and focus detection for the time it is held down. If you "tap" it, it will attempt to focus on whatever the priority item in the centre of the frame is for as long as the button is held down.

Yeah, have just been tapping it. Using 'focus infinity' on an assign button sounds like a good idea, will do that.

In light of your response Leonie, you're using the push button incorrectly. Hold briefly until focus is achieved, then release.

Righto. Thanks.