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Measuring the distance from lens


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#1 Malik Sajid

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Posted 25 September 2008 - 03:01 AM

I am a student, studying here in an arts school with majors in video making. Well......its seems quite a non-sence question but being a student i have to ask this.

I have seen on a lot of shoots that people take a measurment of the distance from the lens to the object. Why do they measure that distance.
Is it about the focus? Means, for a specific distance measured, the lens would be on that specific stop.

Kindly explain in detail. I'll be grateful

Saj

Edited by Malik Sajid, 25 September 2008 - 03:03 AM.

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#2 David Rakoczy

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Posted 25 September 2008 - 06:26 AM

Yes Focus. We measure from the Film Plane (Film) to the Subject desired to be in focus. These distances are most often changing as the Camera or Subject or (both) are moving.. so the Focus Puller adjusts the Focus during the shot.. or 'Pulls' focus as we say... a good Focus Puller is worth their weight in gold and has probably the most challenging job on Set. As a DP, I always admire my Focus Pullers and their ability to perform such a unique job skill.

God Bless the Focus Pullers!
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#3 Steve Absalom

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Posted 25 September 2008 - 09:49 AM

But how does it work? I mean, say you measure the subject, a little scotty dog named Lupus who is eating a treat in the middle of a wide field. I have the camera lens say 25 feet (or do we measure in meters here?) away. How would I then calculate that into getting the best focus?
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#4 David Desio

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Posted 25 September 2008 - 11:14 AM

There are distance marks on the barrel of the lens that would be read and dialed to the correct distance. On some prosumer dv cameras, you can set the focus measurement to read in feet, you would use that to dial in your focus if need be.
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#5 Malik Sajid

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Posted 26 September 2008 - 04:06 AM

arrrrright.........seems an easy task....

simpley take the distance and set the focus ring according to that. Is that it???
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#6 Ian Cooper

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Posted 26 September 2008 - 04:30 AM

...simply take the distance and set the focus ring according to that. Is that it???


That's about it.
The skill comes into play when the subject starts moving either towards or away from the camera during the shot. At that point the focus puller has to adjust the lens focus ring whilst the subject is moving, without the use of a tape measure!
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#7 David Desio

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Posted 26 September 2008 - 08:11 AM

Like Ian said, it gets more complicated as the shot gets complicated. This is why during a rehearsal you would give your actors marks, not only for them, but for the focus puller, as well as others, but thats another story.
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#8 Brad Grimmett

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Posted 26 September 2008 - 08:18 AM

arrrrright.........seems an easy task....

simpley take the distance and set the focus ring according to that. Is that it???

It sure sounds easy doesn't it! And it is pretty easy when you do a lockoff with a wide lens. But when you suddenly want to do a 5 minute steadicam move with 8 actors and 20 or 30 marks that moves hundreds of feet, you realize the skill that is needed. Those are two extremes, and most shots fall somewhere in between. There are reasons I never wanted to be a focus puller, and my second example above is one of them.
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#9 K Borowski

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Posted 26 September 2008 - 09:48 AM

But how does it work? I mean, say you measure the subject, a little scotty dog named Lupus who is eating a treat in the middle of a wide field. I have the camera lens say 25 feet (or do we measure in meters here?) away. How would I then calculate that into getting the best focus?


You use whatever the hell you want, unless you live in a socialist country that has a silly law against using customary units :rolleyes:

Actually one advanatage of using metrics (although a lot of metric focal lengths are rounded from fractions of an inch, but I digress) is that you can actually see the relationship between the focal length, frame size, and distance from the lens. But hey, who actually uses lens formulae besides myself these days. Anyone, anyone?

Unfortunately, or fortunately, all the leg-work regarding depth of field and critical focus is done for you by lens manufacturers.

If you don't want to give your poor focus-puller a hard time, you won't shoot your lens at the widest F/stop and instead choose a smaller aperture so they won't have to ask you "Mr. Kubrick, which eye do you want to be in focus?" The DOF was so shallow in one of Kubrick's movies, "Barry Lyndon" and the focus puller so good that he could actually keep the focus on only one eye at a time.

It really is a difficult job. I have a hard enough time with manual focus stills cameras. Now imagine, with manual focus, keeping your subject in focus in real time throughout the scene, sometimes coupled with camera movements, often without even being to actually *see* through the lens, but instead having to adjust focus by feel and intuition.

Again, not an easy job.
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#10 Malik Sajid

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Posted 27 September 2008 - 03:24 AM

Arrright i got some of it now....

well....tell me should it be a good practice to measure the distance and then set the focus according to that??? As we here in our school normally do it without measuring the distance and focus by eye watching in the eye-piece. I normally use DSR-450 or Cannon XL 1.

Should i make it a practice to set focus by measuring the distance? Does this work with dv cams(as they have the zoom lens mostly)???
Or it works only with fixed/prime lenses.
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#11 K Borowski

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Posted 27 September 2008 - 12:57 PM

Arrright i got some of it now....

well....tell me should it be a good practice to measure the distance and then set the focus according to that??? As we here in our school normally do it without measuring the distance and focus by eye watching in the eye-piece. I normally use DSR-450 or Cannon XL 1.

Should i make it a practice to set focus by measuring the distance? Does this work with dv cams(as they have the zoom lens mostly)???
Or it works only with fixed/prime lenses.


Hey Malik. You have to remember that, on Union productions in the United States and Europe, there's often a camera operator and a dedicated focus-puller position. Operator gets to look through the eyepiece. Sometimes there's a video monitor for the focus puller to use to judged distance (as is almost always the case with Steadicam). Sometimes there's no monitor.

So, in essence, the focus-puller is pulling focus "blind". That is why they have to take all of the precautions in the world sometimes, and actors have to hit specific marks, because all but the absolute best pullers can accurately, to within the inch, tell how far somthing is away, just by looking at it.

Obviously, focusing by eye with a monitor or optical viewfinder makes it a lot easier, but having measurements, tape marks at specific distances as guidelines will help a focus-puller double check on the distance, just to make sure everything is where and how far away it should be.

Hope this helps. . .

Oh, yeah, this'll work with everything, although keep in mind that some video cameras measure focus from the front of the ring on the focusing dial, whereas with a lot of film cameras, distance is measured, as it should be from the film/sensor plane at the back of the lens. Measure from the correct spot from your camera or don't measure at all! Finally, with a zoom lens, you want to focus by eye with the lens zoomed all the way in on the subject to be focused upon, then zoom out if need be for the actual shot.

Edited by Karl Borowski, 27 September 2008 - 12:58 PM.

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#12 Dan Diaconu M

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Posted 27 September 2008 - 01:55 PM

Operator gets to look through the eyepiece. Sometimes there's a video monitor for the focus puller to use to judged distance (as is almost always the case with Steadicam).

Operator gets to look through the eyepiece sometimes. There's a video monitor for .......
(I just had to re - place the dot)

The operator uses a monitor when camera is on a crane or steadicam (and dolly sometimes) for framing purposes, so...hardly optical viewfinder (eyepiece) available for any or both operator/focus puller (good for sticks though).

Instinct helps professional focus pullers to react instantly and accurately but rehearsals and marks are crucial. More on the subject at CML- focus pulling. One remarkable (from memory) quote: "the only marks worth taking are the ones that do not change- or when subjects/camera doesn't move" (imagine how often that happens :P ). Even leaning forward while standing can make a difference for close-ups @ wide apertures and short distances. However, some technology IS available :blink: to help those in need. ;)
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#13 Brian Drysdale

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Posted 27 September 2008 - 02:39 PM

Arrright i got some of it now....

well....tell me should it be a good practice to measure the distance and then set the focus according to that??? As we here in our school normally do it without measuring the distance and focus by eye watching in the eye-piece. I normally use DSR-450 or Cannon XL 1.

Should i make it a practice to set focus by measuring the distance? Does this work with dv cams(as they have the zoom lens mostly)???
Or it works only with fixed/prime lenses.


If your video camera has an ENG style lens, the distance measurements are taken from the front of the lens. Cine style lenses are from the focal plane on the camera. Chances are you've got an ENG style, but you should also check if the markings are accurate. BTW It's much easier with a Cine style lens because the distance scale on the focus ring is much larger.

Eye focusing is pretty standard way with ENG style lenses to find the sharps and the focus puller marks them up.
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#14 Malik Sajid

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Posted 30 September 2008 - 03:25 AM

arrrright.....yah atleast pd-170 doesnt have the readings marked on the lens barrel.
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#15 john serrato

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Posted 06 October 2008 - 09:47 PM

I'm not positive with other cameras, but it's not a very good idea to set focus by measuring tapes/lasers when dealing with the HVX. You will only get a ballpark estimation, no matter where you measure from on the camera.

The best way to set focus with the HVX and most (if not all) prosumer HD cameras is to use an appropriate monitor.
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