Difficulty with follow focus on 550d

[bryhoyt]

Hi, I'm relatively new to Magic Lantern, and loving it so far (thanks, all involved!). I'm using it on a 550d, and I would like to be able to use the Follow Focus feature in Movie mode.

However, I can't seem to get it to work right. Here's what I'm doing:
1. I turn on Follow Focus, set my lens to AF, and start a movie recording.
2. I use the up/down/left/right arrows to set focus where I want it. This works great so far.
3. Now that the focus is sharp, I press "menu" to save a focus point. ML reports "Focus point saved here. To return to it, press play."
4. I set the focus somewhere else.
5. I press play to return to the focus point saved in #3, however it does not return properly.

I now expect "play" (the > triangle button) to toggle the focus points between #3 and #4. But it just moves the focus around a bit, and doesn't focus anywhere in particular.

What sort of accuracy should I expect from Follow Focus? Or am I using it wrongly somehow?

Regards,
Bryan

[Francis]

I usually can obtain pretty accurate results depending on the lens. Have you tried changing the delay and step size to see if that helps? When using the directional buttons for follow focus is the focus movement fairly smooth?

[bryhoyt]

Thanks, Francis. I've found something that mostly works, now, with your help, though there are still a few issues.

There were two main problems. One was that I needed to set the "Wait flag", otherwise for some reason it wouldn't record/replay the correct number of focus steps.

The other issue was that I was using the FF++/-- buttons between setting a focus point and pressing "Play".. Somehow the Follow Focus feature records those button presses incorrectly. It seems to do double the number of focus steps necessary. If I use the FF+/- buttons instead (left & right), it works as expected, and returns to the correct focus point.

Is this a bug?

There's still a minor accuracy issue, which seems to be mitigated but not completely solved by setting the step size. But it's mostly only noticeable if I replay a focus change repeatedly -- it gets slightly out each time. Is this a bug, or something that's simply not solveable due to the limited ability to communicate with the lens? Is there anything else I can do to improve this?

Increasing the step size helped to track down some of these issues, because it makes it easy to count the number of focus steps replayed. In terms of smoothness, the focus shift seems smooth enough visually, but it still moves the lens in distinct, separate steps, no matter what the step size or delay time is set to. I guess this is by design? Is ML theoretically able to control the lens in a continuous manner?

[Francis]

It is not possible to move in a continuous manner, only in the step sizes set by Canon for use with USB remote control. You have discovered one issue, if you replay the same rack focus, it will drift. Finding the right settings for each lens helps but doesn't completely fix any of the issues you have discovered. The ++/-- moves 10x (I think) as many steps as +/- so that accentuates the drift.