how to focus properly with video

[Luke McMillian]

Hi,

Another questions has popped my mind that I would welcome a solid answer to. I understand most people will zoom into their subject, then focus on them, and then everything is in focus from the subject to the camera. Is this correct, and is there other tricks like this for obtaining particular depth of field. Generally auto focus should be not used for moving subjets, because the autofocus point is in the centre of the frame, and movement will make try to refocus. How should I set up my shots by focusing? I have a pansonic GS400 and want to make use of all the manual controls, just want to make sure I'm doing as much as I can with them.

Luke

[Jason Knoll]

I am told to either focus on the face in the eyes as tight as you can get.

Or focus behind your subject, and depending on your iris, everything between you and the object you focused on will be in sharp focus, including your subject. I may not be right, one of the more experianced members may be of more help to you!

[Rob Lohman]

The reason to zoom in, set focus, then zoom out again is to make sure the
focus is set to where you want it. However, keep in mind that zooming in might
change the aperture (f-stop).

If you have a shallow depth of field (ie, only a small portion [depth wise] is in
focus) and the camera starts to move or your subjects do then you may need
to compensate on the fly. This can be a difficult thing to master.

The reason you don't want autofocus is that it may focus on the parts where
you do not want the focus or it changes throughout a scene.

I had a camera on a dolly once tracking a guy through a hallway. I didn't see
the camera was on autofocus and you can see on the resulting clip the camera
is hunting for focus as it moves down the hallway (even though the subject
was dead center).

[Andrew Khalil]

in most situations, just zoom in all the way, focus then recompose the shot.

generally, the eyes are what you want to be in focus because they are a point of contact for the audience, so even if the rest of the face is out of focus (which can be the case sometimes), as long as the eyes are in focus you're good.

if you are focused, it doesn't mean that everything between the lens and subject is in focus - if you're using a wide aperture (shallow depth of field) then only a tiny area is in focus and if the subject moves back or forward, they'll move out of that area of focus. In extreme cases, I've done shots where the subject's eyes were in focus but their nose and ears were soft.
I don't think you'll run into that with the Panasonic you're using, but just keep in mind that only a certain area is in focus and it can be really big if you're shooting outside or really tiny if you're shooting indoors with a wide aperture.

[Luke McMillian]

So When i'm wokring with something like mini dv generally their is a lot of depth of field most of the time, so i think 80% of the time I'd be ok. But what about for pro video cameras and film, how do they set up their focus? Any links that you guys have to help me? Do they measure on the floor the depth of field depending on how shallow or deep it is, and then figure out the blocking from their, how far the actors can move? set their marks with tape?

so for shots that have a shallow depth of field and the actors are moving rack focusing must be done..correct?

How would I set focus for a tracking shot, say the camera moving backwards and a subject walking towards the camera and it dollies back?

and I guess when they use a wide angle lens, with a small apeture they have greath depth of field, and no worries.

anything else I should be doing to set up focus for video, and any links are much appreciated. thank you!

[Andrew Khalil]

you're right, for mini dv, you'll usually have a large depth of field so most of the time there isn't much to worry about. However, the sensor size (imaging area) greatly determines how shallow a depth of field you'll have, so a camera such as a small consumer one with a 1/6th inch sensor will have a lot more depth of field than say an XL2 which has 1/3 inch sensors, so it's more important to watch out for focus when using larger sensors. Going a step further to an even higher end cam, the CCDs could be 2/3 inch in which case there is even less in focus which means even more attention needs to be given to focus.

When shooting on film, there are 2 things that are different. First of all, the imaging area is much larger which means a lot less in focus. Also, they don't get to see their footage until it's developed so they make sure things are focused from start in a number of ways.
First of all, film cameras generally have optical viewfinders which make it a lot easier to see what is and isn't in focus.
Secondly, they actually measure the distance from the lens to the subject and focus the lens accordingly based on the numbers written on the lens.
Lastly, they refer to a depth of field chart for each lens which tells them how much dof they have for a given aperture, distance and zoom length (if a zoom is being used).

When doing complicated movements, most serious shoots have a person known as a focus puller whose main purpose is to just make sure things are in focus. Since professional lenses have distance markings on them, it relatively easy to go back to a certain focus point. So, for example, if a specific shot calls for an actor to walk from point a to point b, they will focus for each distance and mark these two points on what's known as a "follow focus" which is a small gear that allows the lens to be more easily and ergonomically focused. Once the markings are there, all teh focus puller needs to do is turn the follow focus knob at the proper speed to make sure the actor is in focus and since he's just going between two points that are marked on it, it's not as complicated as it sounds, but it does take a lot of practice.

Doing all that without all the gear can be more of a challenge, but it isn't impossible. Since pretty much all the prosumer cameras use an electronic servo to control the lens, you need to get a feel for how it responds to you moving it. Once you have that, practice the shot several times and try to keep the subject in focus by turning the ring at the correct speed - marking anything down on the ring won't help because the ring isn't physically attached to the lens. However, some companies have tried to simplify such shots. Panasonic's DVX100 comes to mind since it actually displays a digit based on the lens position which never changes, so these can be remembered and used in such circumstances. Also, on the 20x servo lens for the XL2, it has a button which allows you to save focus points and return to them. While neither are as good as a nice manual lens, they do help a lot.

Hope this all helps - in the end, with this level of gear, I think it's important to just practice a lot and reherse shots a few times.

PS: looking back at all this now, I realize I wrote an essay, so sorry to all of you who don't like long posts, but I hope it's informative and someone gets something out of it:)

[Luke McMillian]

No worries about essays man, that answered everything, thanks very much!

[Andrew Khalil]

no problem, glad it helped

[Rand Blair]

Andrew, I for one appreciated the essay and am able to share it with someone here in Kampala that had a similar question.

Rand






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