Keeping Focus

The place for questions about shooting with Blackmagic Cameras.
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Blaine Russom

Keeping Focus

PostWed May 22, 2013 1:05 am

How do you guys keep your focus ring to stay in place? I don't use a follow focus because its not neccessary for what I do.

However, I'm finding that if I leave the focus ring to do something else, sometimes the thing falls out of focus. Do I need a follow focus to keep it in place????
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Peter J. DeCrescenzo

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Re: Keeping Focus

PostWed May 22, 2013 1:19 am

If you're talking about a situation where the subject/camera distance doesn't change, but the focus is shifting (going out of focus) spontaneously, then there's apparently something wrong with your lens. The BMCC doesn't have any control over lens focus. Focus is manual; a lens function.

But if the subject/camera distance is changing then, as expected, you'll need to adjust focus periodically to keep the subject in focus. Use the camera's double-tap magnify feature on the touchscreen. Shooting with a relatively high aperture value will make it easier to hold focus, because the depth of field will be deeper.

A follow focus mechanism can make manual focussing easier, but it doesn't make it easy. Keeping a moving subject in focus is a skill; like other skills, it usually improves with practice.

A high quality external EVF or monitor can also be a huge help for focussing, but of course can be expensive.

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Blaine Russom

Re: Keeping Focus

PostWed May 22, 2013 2:01 am

Well, I've been doing this (regularly) for over 2 years every week. So it's not a skill thing.. it must be a bad lens or something.

I usually keep my aperture at about f2.8 or f4, I just noticed a lot of play with the focus ring. Before I got my lenses for the BMCC I was using the kit lens for my old Canon Rebel XT. Naturally, that lens has a lot of play. But when I got the better lens (Tamron 70-200mm VC USD) it doesn't move at all.. however when I am filming, I do notice that it actually does move. Not as much as the kit lens, but it's significant enough to notice the subject getting out of focus. And the subject does not move much.

I'm not really sure if this is a lens issue or what, but the lens definitely does not always stay where I left it.
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Jaime Amunátegui

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Re: Keeping Focus

PostWed May 22, 2013 3:13 am

Blien wrote:Well, I've been doing this (regularly) for over 2 years every week. So it's not a skill thing.. it must be a bad lens or something.

I usually keep my aperture at about f2.8 or f4, I just noticed a lot of play with the focus ring. Before I got my lenses for the BMCC I was using the kit lens for my old Canon Rebel XT. Naturally, that lens has a lot of play. But when I got the better lens (Tamron 70-200mm VC USD) it doesn't move at all.. however when I am filming, I do notice that it actually does move. Not as much as the kit lens, but it's significant enough to notice the subject getting out of focus. And the subject does not move much.

I'm not really sure if this is a lens issue or what, but the lens definitely does not always stay where I left it.


If it is while zooming since the lens might not be par-focal, i doubt it will maintain the focus as the same distance, even more with such long ranges it will change the depth of field too, so you might consider a follow focus and keep marks calculated in the follow focus with a whiteboard pen, that might be your solution during the zooming, and focus changes... hope that helps...
Jaime

Blaine Russom

Re: Keeping Focus

PostWed May 22, 2013 3:56 am

I wasn't zooming, just to clarify a bit this is what is happening..

I focus on my subject, the subject is speaking and staying in the same position. Not moving back or forward, pretty much like an interview. The camera itself is on a tripod so I'm not moving either.

I take my hand off the focus ring and as I'm looking at the monitor and its out of focus.. just like that. So I played with the ring very gently to see how much will cause it to jump out of focus and it's not much of a movement. And the depth of field is not too shallow, I have it at about f 2.8 most of the time.

So my question is, is there anything that can keep it exactly where I left it? i.e. a follow focus or anything of that sort..?
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Peter J. DeCrescenzo

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Re: Keeping Focus

PostWed May 22, 2013 4:11 am

Blien wrote:... I focus on my subject, the subject is speaking and staying in the same position. Not moving back or forward, pretty much like an interview. The camera itself is on a tripod so I'm not moving either. I take my hand off the focus ring and as I'm looking at the monitor and its out of focus.. just like that.

... So my question is, is there anything that can keep it exactly where I left it? i.e. a follow focus or anything of that sort..?


A follow-focus won't make you lens work correctly. A FF simply does exactly the same thing your hand does. A FF allows very fine focus adjustments, but it doesn't "lock" focus.

If your lens is "falling" out of focus on its own, a mechanical FF won't prevent that.

Your lens' focus mechanism is apparently so sloppy it's loosing focus no matter what you do. Use a different lens.

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sean mclennan

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Re: Keeping Focus

PostWed May 22, 2013 2:59 pm

Is your lens level or is it pointing slightly up or down? Lens creep is an effect of either the zoom element or focus element inside the lens moving under it's own weight. Gravity.

If it has it, switch the lens to Manual focus and put a piece of gaffer tape across the lens to stop the ring from moving.

Blaine Russom

Re: Keeping Focus

PostWed May 22, 2013 3:22 pm

Ah ha! Yes the camera was on a slight tilt downward. I'll check it out and see if that helps.
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Theodore Prentice

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Re: Keeping Focus

PostWed May 22, 2013 4:47 pm

Peter J. DeCrescenzo wrote:... A FF allows very fine focus adjustments, but it doesn't "lock" focus.

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:roll:

You most certainly can "lock" focus with any follow focus that has lock-off stops. They arent just there for saving focus position for racking.
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Peter J. DeCrescenzo

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Re: Keeping Focus

PostWed May 22, 2013 6:15 pm

Theodore Prentice wrote:
Peter J. DeCrescenzo wrote:... A FF allows very fine focus adjustments, but it doesn't "lock" focus.

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:roll:

You most certainly can "lock" focus with any follow focus that has lock-off stops. They arent just there for saving focus position for racking.


Hi Theodore: Understood. You're correct in reference to well-designed, fully manual & cine-type lenses in good working order, but I was referring to his DSLR-type lens with an apparently sloppy, loose, focus mechanism. With his lens, a FF will keep the lens' focus ring from moving, but that doesn't address his issue.

But yes, you're correct: Good FF systems include a lock feature. Cheers.

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