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-   -   Rack Focus (https://www.dvinfo.net/forum/open-dv-discussion/7415-rack-focus.html)

A. Goldberg March 4th, 2003 04:56 PM

Rack Focus
 
How would one go about racking focus in/out on a GL2? I'm shooting some interview footage and would like to add some bells and whistles to it.

Thank you!

Nathan Gifford March 4th, 2003 07:39 PM

Its kind of tough to do with the lens most of these cams have. Lens focus, as well as zoom, are fly by wire systems. That is focus and zoom commands actually drive servos not lens elements.

Therefore, there really isn't a way to mark the lens focus ring marking distances. This really takes a manual lens like those available for the JVC and XL-1.

I have read where someone says you can simulate this in After Effects by applying filters and carefully controlling depth-of-field. This takes time and probably some computer time.

Rick Spilman March 4th, 2003 07:39 PM

It's not impossible to rack focus a GL2. Close maybe, but not impossible.

The biggest problem are the 1/4" ccds. The smaller the ccds the greater the depth of field. The standard advice is to get the camera as far away as possible and zoom in on the subject. Open the iris as wide as possible. Use a neutral density filter to let you shoot at a lower Fstop. (And hope for the best). Oh yah, the focus ring will drive you crazy as well.

But it is not impossible.

Rick

Jeff Donald March 4th, 2003 09:14 PM

Increasing the camera to subject distance and zooming into your subject at the same time will have no effect on DOF. Opening up the aperture is the best way to control DOF under almost all conditions. DOF is explained in this thread and this article.

Henrik Bengtsson March 5th, 2003 01:16 AM

You know, me and a electronics wiz at work has been toying around with the idea of actually try to build a rack focus Lanc controller for a camera. The problem with remotely controlled focus is that you do not have any absolute value for where the focus is (that i have found so far anyway :) in the Lanc protocol. To solve this the idea would be that you would have to calibrate the unit to 0 (or max zoom) in the beginning, and the unit would then itself calculate a "position" figure based on the focal speed and the time. The setup could even record focal speed changes when you slide the focus control slider and playback that with the actual camera.

So what does this give us? Well we would have made a speed control (ie. the focus ring) which gives no fixed positional value into a fixed slider, where every position on the slider confirms to an actual focal position. This also means that you would be able to put down focus markers on the sliders for your rack focus.

What do you guys think? Should we stop smoking or get on with it :)

/Henrik

Charles Papert March 5th, 2003 03:33 AM

Er ah ahem. It might be a good moment to clarify the verbiage used in this thread, just so we are all talking about the same thing...

"Rack focus" means to make a deliberate and substantial focus change, i.e. racking the subject out of focus would involve starting with an in-focus image and then using whatever mechanism the camera allows to result in a substantially out-of-focus image. Or, racking between two subjects means the focus shifts from one to the other.

"Follow focus" is as the name says, maintaining focus on a subject that is shifting distance by following along with the focus mechanism.

Of course there are crossovers between the two terms, but in the context of the original post, my interpretation of the desired effect of rack focus meant throwing the interviewee well out of focus and/or bringing same back into focus. Some of the subsequent posts dealt more with the hassles of attempting true follow focus with the GL2.

Yikes. Tedious nomenclature lecture over!

Henrik Bengtsson March 5th, 2003 07:02 AM

True =) And the problem remains if you want to rack from one focus subject into another. Such as for example shooting through a wire fence to a subject behind it, and rack the focus from the subject in the background (thus having the fence invisible due to no focus) and into the fence int he foreground.

To do this without the possibility to mark the different focal points is very hard. And something that is not just evident on the GL2 but rather all small DV cameras without a proper manual focus (or servo focus).

Please correct me if im wrong,
Henrik

Charles Papert March 5th, 2003 05:29 PM

All good, Henrik. So I guess to be absurdly correct about all this, one would say that you are looking to build an electronic follow focus device to allow you to rack focus...

OK, I'll go away now!

P.S. good luck with your project, hope it works out!

Cosmin Rotaru March 7th, 2003 06:09 AM

"you would be able to put down focus markers on the sliders for your rack focus"
that would be nice! Helpfull!

How much? :)

Bill Pryor March 9th, 2003 06:57 PM

The problem with shifting focus with an electronic lens is that the focus ring isn't a mechanical focus ring--it's electronic. It could very well be a button; they just put the ring there to make you feel good. So, you can't have an assistant put marks on the lens and return to the same spot each time. You could, but it wouldn't work, except maybe on occasion if you lucked out.
What you can do is do it yourself by guessing...ie., say you're focused on a subject in the foreground...start shooting and when you're ready to shift, just do it slowly and stop when the background object is sharp. It takes some practice, but eventually you will be able to hit it most of the time. I do this a lot with a DSR250, which has a high res B&W viewfinder--I don't know if the viewfinder on the GL2 is good enough to allow you to see the focus coming at you before it's too late.
Also, as already mentioned, the smaller chips give you greater depth of field, so you can only do this when you're shooting pretty tight and there is a lot of distance between your subjects.

Rene Legaspi March 10th, 2003 12:43 AM

If reasonably close to the object to be in focus first, rack focus is relatively easy. Say a meter away first, then swing the focus barrel to focus on the far away object. Here's an example of a well done rack focus I downloaded from 2-pop.

http://www.tentimesnothing.com/24prackfocus.mov

This was done on a pana dvx100 in 24p mode.


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