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Pulling focus

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Guest Andrea altgayer

Hi there guys,

 

Please can someone explain to me the basics of pulling focus, and also if there are any differences in pulling focus for film and video/HD.

 

Cheers,

 

Andrea

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Hi there guys,

 

Please can someone explain to me the basics of pulling focus, and also if there are any differences in pulling focus for film and video/HD.

 

Cheers,

 

Andrea

 

The bare basic: On the camera side there's a mark that shows where the filmplane is. You need to know the distance from the filmplane to the subject (actor, wall, screw, whatever) that needs to be in focus. The lens is marked with distances. Set the lens to the according distance and keep adjusting it if the distance changes.

 

I'm not sure if that answers your question but to me that's the basic thing. Details about follow focuses, depth of field, aesthetics and all that can be found in various threads.

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Hi there guys,

 

Please can someone explain to me the basics of pulling focus, and also if there are any differences in pulling focus for film and video/HD.

 

Cheers,

 

Andrea

 

 

Ideally, you need to have the lens focused exactly at the point which coincides with the plane out in space that you'd like to be in focus. Not many people can do it precisely at every instant, so you have to rely on depth-of-field to provide a margin of error.

 

The longer the lens, the less DOF you have. The wider the aperature, the less DOF you have.

 

Essentially, prior to a take, you want to know where the lens should be set in order to keep an actor in focus. This can be done by running a tape measure out from the film plane to the Actor's eye. It can also be done by eye focusing through the lens. Having a zoom lens on the camera makes this easier as you can zoom all the way in, find the focus mark, and then return the lens to its original size. Back focus is critical with zoom lenses as the focus will change if the backfocus is not set correctly.

 

During the take, you want to position yourself so that you can see the lens and watch the Actors at the same time. Usually holding the focus knob with your left hand (if you are on the Operator side of the camera) helps to accomplish this. Hold the knob with your right hand if you are on the "dumb" side of the camera. Sometimes it is helpful to lay out marks on the ground to give yourself some kind of reference as the Actors move around the set. Some marks are helpful while too many manage to just confuse things. The wider the lens, the wider the shot and the easier time you'll have. Very tight shots require more concentration as even the slightest movement forward or back by the Actor means significant "pulling" on your part. I found that on those kinds of shots, it is sometimes helpful to hook up the wireless focus transmitter (Seitz or the like) and get way off to the side of the action. That way you can see exactly how much forward and back movement is going on and perhaps better judge where to pull the lens. Different people have different preferences. Overall, it's best to just become an excellent judge at distance. Those Steadicam shots require that you have that skill. Something like a Panatape can be a good guide, but you can't rely on it to give you the distances exactly. Plus, by the time it gives you a distance (assuming it is accurate), by the time you see it and then pull to that distance, you're already soft. You've got to move WITH the Actor and even slightly before him if possible. There is definitely an art to it.

 

As mentioned, ideally you want to hit those marks precisely so that the Actor is never soft. Film can be somewhat forgiving relative to HD. Standard Definition video is slightly more forgiving than 35mm, but again, you still need to be in the neighborhood regardless. Soft is soft, it's just a matter of degree at which it is acceptable or not.

 

The disclaimer here is that I no longer do that job. As a Video Operator, I pull my own focus and find it immensely easier when I can also see the frame. I've noticed First ACs who pull on HD shows now have video monitors set up so that they too can watch the frame to make their jobs easier. For the book I'm writing, I've talked with a lot of people and I think I can say without much hesitation that the Focus Puller probably has the most difficult job on set. Nearly everything else that gets done can be fixed, but focus is one of those things that just isn't one of them. You're either there or you're not and post can't save it.

 

Good luck!

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I would like to add one thing: as a focus puller, you are as much a camera operator as the one who aims the camera itself. Both have to know/decide where the attention of the frame will be, where the viewer in front of the big screen will look at. And both have to work in sync with each other. Especially when the actor(s) start improvising..

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Hi there guys,

 

Please can someone explain to me the basics of pulling focus, and also if there are any differences in pulling focus for film and video/HD.

 

Cheers,

 

Andrea

 

 

Andrea -

 

Check out my book: THE CAMERA ASSISTANT: A COMPLETE PROFESSIONAL HANDBOOK (Focal Press, 1996). Available from Amazon, ASC website, Filmtools website, Focal Press, many Camera Rental Houses, sometimes even eBay.

 

Doug Hart

1st AC, NYC

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Andrea -

 

Check out my book: THE CAMERA ASSISTANT: A COMPLETE PROFESSIONAL HANDBOOK (Focal Press, 1996). Available from Amazon, ASC website, Filmtools website, Focal Press, many Camera Rental Houses, sometimes even eBay.

 

Doug Hart

1st AC, NYC

 

Doug, can I just say thanks for that great book! Any plans on a second edition?

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I read your book years ago Doug when I first started in camera and it was great. I always tell people who want to get into camera they have to read your book. Its really is a great book.

 

Travis

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Doug, can I just say thanks for that great book! Any plans on a second edition?

 

 

Thanks for the kind words.

Focal Press has been pushing me for a second edition, but I haven't done much about it yet.

What it really needs is a "High Def" chapter, but I haven't done enough HD work to feel comfortable writing about it.

Someday soon, I promise.

 

Doug Hart

1AC, NYC

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.

 

The longer the lens, the less DOF you have.

 

Brian,

 

I hope you don't write that in your book!

 

If the image size on the film or sensor is identical and at the same F stop, DOF will be the same regardless of lens focal length!

 

Stephen

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Brian,

 

I hope you don't write that in your book!

 

If the image size on the film or sensor is identical and at the same F stop, DOF will be the same regardless of lens focal length!

 

Stephen

 

 

:) My book isn't a "How to." It's more like a "What's it like?" I've got no intention of replacing the many excellent "how to" books already out there, like Doug Hart's, which got a lot of us started. Mine is meant to be the bridge between the theory of film school and the real world.

 

Anyhow, I was intentionally simplifying the situation by suggesting that pulling focus on a longer lens is more difficult than on a shorter (wider) lens. While technically you might be correct, the "experience" of the A-camera First who is pulling focus on a 24mm is going to be significantly different than the B-camera First who is pulling on a 75mm on the same action. "A" can relax a bit while "B" has to keep his eye on the ball throughout. Generally speaking, we won't have a CU shot with a 35mm and then the same with a 75mm. I guess it could happen, but that isn't traditionally a normal scenario that I've run into. Pretty much anything less than 50mm is reserved for wider establishing shots and the longer you go, the more the shot is about zeroing in on the talent.

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I hope you don't write that in your book!

 

If the image size on the film or sensor is identical and at the same F stop, DOF will be the same regardless of lens focal length!

 

Stephen,

 

Sorry to to contradict, however depth of field relies on three elements:

The Focal Length, (Longer equals less)

Aperture, (Wider equals less)

Subject distance to film plane. (Nearer equals less).

 

Depth of field should never be an insurance, even with large DoF there is only one point that is truley in focus, the rest is acceptable.

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I hope you don't write that in your book!

 

If the image size on the film or sensor is identical and at the same F stop, DOF will be the same regardless of lens focal length!

 

Stephen,

 

Sorry to to contradict, however depth of field relies on three elements:

The Focal Length, (Longer equals less)

Aperture, (Wider equals less)

Subject distance to film plane. (Nearer equals less).

 

Depth of field should never be an insurance, even with large DoF there is only one point that is truley in focus, the rest is acceptable.

 

You're both right - the point is that keeping an object at a fixed size and then either going wider and closer or tighter and further away shouldn't change depth of field - the two factors will cancel each other out. Therefore, the image size (determined by focal length and distance) and the aperture are the only two factors. This is why you need to stop down to increase DoF in a fixed framing - no combination of lens length and subject distance will change the DoF.

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You're both right - the point is that keeping an object at a fixed size and then either going wider and closer or tighter and further away shouldn't change depth of field - the two factors will cancel each other out. Therefore, the image size (determined by focal length and distance) and the aperture are the only two factors. This is why you need to stop down to increase DoF in a fixed framing - no combination of lens length and subject distance will change the DoF.

 

Are you basically saying that relative distances and focal lenghts that produce a 'similar' sized subject in the frame (Be it a different angle of view) will result in the same DoF.

 

For example, 50mm at 5ft is the same DoF as a 100mm at 10 ft (Same stop for both).

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Are you basically saying that relative distances and focal lenghts that produce a 'similar' sized subject in the frame (Be it a different angle of view) will result in the same DoF.

 

For example, 50mm at 5ft is the same DoF as a 100mm at 10 ft (Same stop for both).

 

 

Hi,

 

Thats exectly right, assuming a physically thin lens. Due to lens design the image size may be slightly different when you measure from the film plane. DOF is calculated from the front nodal point of the lens, with zooms and phicically large lenses this should be accounted for. Custom DOF charts will be exact for the lens in question and T stop Corrected. DOF is derived from F stops.

 

 

Stephen

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Usually, what I do when pullling focus is to wing it. Actually, I'm only half-kidding. Certainly I measure start/stop/end points, but often I just use my wingspan gauge. I just imagine my arms spread and use the fractions or increments accordingly. This is possible because your fingertip-to-fingertip distance is apporximately your own heigth (or just measure it for determine) I find that this is an easily visualizable distance for me (at 6 feet).

 

Another trick that no one has mentioned yet is to use a "choir boy", which is basically any flashlight with an exposable bulb (preferably small, like a MagLite) which can provide a very distinct point of focus when the lens is wide open. Just hold the bulb at the appropriate distance and focus by eye. This can be done before the take and also after the take to check if the end point was in focus.

 

Other good focusing points: Focus chart, writing on slate, eyelight, wrinkles on face, etc...

 

Another thing, which I guess was kind of mentioned, but just wanted to clarify (no pun intended...ha ha!): DoF is determined by 4 factors, not three: lens length, aperture, distance, and size of format.

 

Thank you,

 

JSV

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Format size doesn't have any effect on the DoF, but it does have bearing on which focal length you'll use, which is a factor. 16mm and 35mm cameras shooting the same subject side-by-side with identical lenses will have identical DoF. The 16mm image, however, will look tighter than the 35mm one.

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Format size doesn't have any effect on the DoF, but it does have bearing on which focal length you'll use, which is a factor. 16mm and 35mm cameras shooting the same subject side-by-side with identical lenses will have identical DoF. The 16mm image, however, will look tighter than the 35mm one.

 

Format size doesn't have a direct effect. But it does affect the the image reproduction ratio. E.g. when you use a 50mm lens on a 16 and a 35mm camera side by side you have the same DOF, but a different field of view and thus, a different image reproduction ratios. So to get the same field of view in both cameras you'll have change lenses or the distance to your object and, thus have different DOF. For more info http://en.wikipedia.org/wiki/Depth_of_field

 

So, summing it up, 3 factors influence DOF:

  • image reproduction ratio (a combination of distance and gauge)
  • focal length of the lens used
  • aperture

Cheers, Dave

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Format size doesn't have a direct effect. But it does affect the the image reproduction ratio. E.g. when you use a 50mm lens on a 16 and a 35mm camera side by side you have the same DOF, but a different field of view and thus, a different image reproduction ratios. So to get the same field of view in both cameras you'll have change lenses or the distance to your object and, thus have different DOF. For more info http://en.wikipedia.org/wiki/Depth_of_field

 

So, summing it up, 3 factors influence DOF:

  • image reproduction ratio (a combination of distance and gauge)
  • focal length of the lens used
  • aperture

Cheers, Dave

 

Hi Dave,

 

Great site! I need to spend more time in Wikiland, so much good info. Here's another one I like. The part with the images of Grommit shot with two different focale lengths but at the same magnification is interesting. As always when using Wikipedia be sure to verify facts and formulas as errors have been known to creep in.

 

http://www.vanwalree.com/optics.html

 

Chuck

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Sorry to reopen the DoF debate, but there's only one factor that controls it and that's f-stop. Focal length and distance give the appearance of shallow DoF, but it doesn't actually change it. This article goes into great detail about this and gives examples:

 

http://www.bluesky-web.com/dofmyth.htm

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Sorry to reopen the DoF debate, but there's only one factor that controls it and that's f-stop. Focal length and distance give the appearance of shallow DoF, but it doesn't actually change it. This article goes into great detail about this and gives examples:

 

Joey, the link you posted explains the two things that "influence DOF:

 

f-stop and image reproduction ratio

 

That's precisely what I'm saying. I should have incorporated focal length into image reproduction ratio (=image size). The three factors for image reproduction ratio thus are: distance, film gauge and focal length. So each of these has an indirect effect on DOF.

 

And the f-stop of the lens has a direct effect.

 

When you're shooting a one person interview, and you zoom in, DOF gets more shallow. Why? Because the image reproduction ratio changes.

 

When you move back & then zoom in to frame the shot EXACTLY as it were before you moved, DOF does NOT change. Why? Because you image reproduction ratio did NOT change!

 

A quote from the web link:

 

"In what is considered the bible of cinematography, "A Hands-On Manual for Cinematographers", David Samuelson states at the conclusion of his section on depth of field : "Depth of field remains the same, regardless of lens focal length, so long as the image size (and f-stop) is the same. There is no point in changing to a shorter focal length lens and moving closer, because if the image size remains the same so will the depth of field." (Focal Press, London, second edition, 1998, p.218)"

 

[my emphasis]

 

Cheers, Dave

Edited by David Auner

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