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Old April 9th, 2006, 06:04 PM   #1
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Watching DOF in movies

I noticed that when watching DOF in movies, it is not always so razor sharp. A lot of time, many different layers are in focus. Are they using different types of lenses to achieve this? I am curious because the 1.8 Canon FD that I have seems to make things very sharp, but if the actor moves slightly forward or back, they go out of focus. Is this because of the inherent lack of resolution with mini dv?
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Old April 9th, 2006, 06:10 PM   #2
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One of the problems of using 35mm still camera lenses is that the depth of field is much narrower than on a 35mm motion picture camera (because the image plane is much larger). You can widen the area that is in focus by using a higher aperature. There should be markings on the side of your lens indicating what's in acceptable focus for a number of aperatures. The distances listed on the focus ring that fall between each pair of markings are what's in focus for that aperature. It's a hard thing to explain, but it's not really hard to understand.
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Old April 9th, 2006, 08:52 PM   #3
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not quite right...

Quote:
Originally Posted by Marco Leavitt
One of the problems of using 35mm still camera lenses is that the depth of field is much narrower than on a 35mm motion picture camera (because the image plane is much larger). You can widen the area that is in focus by using a higher aperature. There should be markings on the side of your lens indicating what's in acceptable focus for a number of aperatures. The distances listed on the focus ring that fall between each pair of markings are what's in focus for that aperature. It's a hard thing to explain, but it's not really hard to understand.
Actually, the depth of field characteristics of all lenses of a given focal length are identical -- it does not matter whether the lens was intended for still of motion picture photography.


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Old April 9th, 2006, 09:05 PM   #4
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Then is it what I said about the lines of resolution? More with film then with dv which enhances the sharpness of the foreground and background making it 'look' like it is more in focus?
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Old April 9th, 2006, 09:24 PM   #5
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Leo.

Two things :-

Relative to the 35mm still-image camera which many adaptors emulate, the 35mm motion-picture camera field of view is narrower for a given focal length.

Where a DOP might choose a 50mm lens for a certain shot on a still-image camera, he might choose something closer to 28mm for a motion picture camera. (That is not my own idea - it came from a recent article I read somewhere.)

You have heard of a focus pull?? Marks?? etc., in motion picture production.

These shots are generally rehearsed. The camera operator aims the camera. Another human adjusts the lens from one pre-determined setting to another in accord with the rehearsed moves so the moving subject stays in focus.

If you get two separated planes of sharp focus in a single image, there is a fairly good chance this is a shot composed of two different images, ie., blue screen/green screen - subject over a background plate. there is a fair chance somebody has been incompetent or the perfect shot was simply not technically achievable if two planes of sharp focus are visible in a composited image. - don't take my word for it however.
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Old April 10th, 2006, 02:26 AM   #6
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Quote:
Originally Posted by Leo Mandy
I noticed that when watching DOF in movies, it is not always so razor sharp. A lot of time, many different layers are in focus. Are they using different types of lenses to achieve this? I am curious because the 1.8 Canon FD that I have seems to make things very sharp, but if the actor moves slightly forward or back, they go out of focus. Is this because of the inherent lack of resolution with mini dv?
It's all about Your currrent aperture size. Close Your Canon down, say 8-11
You'll almost loosing shDof effect. Somewhere in between is situation You described - there seems to be shDof but not so cut-throat as we used to see here from DIY and so called "manufacturer" footages. They using most often apertures somewhere in between, say 5,6.

Phototools.xls may help to understand this. Check sheet named DOF.
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Old April 10th, 2006, 07:47 AM   #7
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Leo, it's a factor of three things:

1. How fast is the lens? An F1.2 has shallower depth of field, fully open then an F1.8. This is just basic physics related to the larger rear aperture openings, and circles of confusion.

2. How close to your subject are you? The closer they are, the more critical is your plane of focus with respect to your subject.

3. What f/stop are you using on the 35mm lens? That's already discussed here.

At 1.5 feet, a 50mm F1.4 lens fully open, will have what I would consider unmanageablely short DOF. At F5 it is much better, as are the optical properties of the 35mm lens. Same thing on bokeh. At F8 I've found out of focus specular highlights look way better. The problem is if you have a fairly lossy adapter, you'll never be able to use F5 without adding a ton of light. As more and more of the adapters are used, and their light loss is reduced, so to will the accepted best practice on their use.

Once you strap an adapter on though, it's time to brush up on the basics of SLR photography...as the concepts are the same.
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Old April 10th, 2006, 09:45 AM   #8
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Quote:
Originally Posted by Ben Wolf
Actually, the depth of field characteristics of all lenses of a given focal length are identical -- it does not matter whether the lens was intended for still of motion picture photography.
Umm....No. If you had bothered reading his post you'd realize he isn't talking about the lenses at all. He's talking about the image plane, the size of the imaging area, the film size if you will. The imaging area of 35mm motion picture film is smaller than that of 35mm film. Don't believe me? I suggest at least a review of the following articles:

http://en.wikipedia.org/wiki/Depth_o...lm_format_size

http://en.wikipedia.org/wiki/Film_fo...e_film_formats
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Old April 10th, 2006, 10:07 AM   #9
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Right. I was sort of presuming that we were talking about the depth of field for the same angle of view for both formats. Using this handy Mac widget --

http://www.fuerstentum.net/Widget/index.html

-- I checked the DoF for a theoretical 28mm cinema lens and a 50mm SLR lens at 1.4. Not sure if a 28mm 1.4 lens really exists, but wouldn't it be nice? Anyway, with the lens correction factor set to 1 (anybody know if that's right? The Web site doesn't explain it) I checked the depth of field for a focal point of 10 meters. On the 28mm the near focus was listed at 6.9 meters and the far focus was 18 meters. For the 50mm lens the near focus was 8.6 meters and the far focus was 11.9. It would be tricky to pull focus on a scene like that.

One thing I would add about Leo's original question is that sometimes split diopters are used. I have a set of these and they are great fun. Basically they're a single element acromatic diopter that's been split in half. That way you can focus on a distant and near object at the same time, leaving the middle out of focus. They're designed to fake deep focus shots. The endless depth of field that video people hate is sometimes envied by film types. The grass is always greener ...
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Old April 10th, 2006, 10:20 AM   #10
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Here's another more precise calculator from Panavision if anybody is interested.

http://www.panavision.co.nz/main/kba...alcFOVform.asp
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Old April 10th, 2006, 10:44 AM   #11
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<warning: I use the terms filming, videoing and shooting interchangeably! They all hold the meaning to record the presence of light.>

Depth of Field (DoF) is a function of the iris size and the capture plane size (there's more to it, but these are the major factors). The larger both are physically, the shallower the DoF. The DoF in movies is exagerated by using longer lenses to bring the out of focus areas in the background closer to the foreground (longer lenses compress distances). Shorter lenses push the out of focus area away from the camera so your brain can get more information and give you the impression of better focus in the background.

With 35mm cameras, the 35mm capture plane along with a smaller f-stop number (wider iris) will cause the backgrounds to turn into a blur of color. This helps to focus attention where the filmmaker wants. In digital, the capture medium is substantially smaller. My XL1s has a 7mm capture plane, causing the "Everything in focus all the time" digital calling card. You can minimize this by opening your iris as wide as it will go (add Neutral Density filters to compensate for the light flooding in) and putting your camera uncomfortably close to the subject being filmed (the closer to the camera you focus, the shallower the DoF) or much farther away with distance separating the subject and the background and zooming in to fill the frame (This enlarges the out of focus area larger in the frame making the blur in the background seem more pronounced).

35mm adaptors for digital cameras work by forcing the image from the lens to resolve on a 35mm plane and then filming that plane with the camera. This is a cheat, but is very effective!
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Old April 11th, 2006, 12:20 PM   #12
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Guys, you're jumping technically way past the original question...

Leo, the difference is that in film, they rarely shoot with the lens iris wide open, as many do with their adaptors. They shoot whatever aperture get the correct exposure based on the light available. The may be shooting at F2.8 to F4 on interiors, and F8-F11 outdoors. At those apertures, the depth of field is deeper, into feet and meters.

Some have referenced shots from Clint Eastwood films where a gun in the foreground and a person across the street are both in focus. They used a very narrow iris and a lot of light to achieve that shot.

In film, they aren't always going for the shallowest depth of field. And the speed of the film stock is a variable we don't have on our cameras. If they choose to use 500 speed film instead of 250, then they immediately have to close down the iris an extra stop more than they would have with 250.

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Old April 11th, 2006, 12:36 PM   #13
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Movie sets (and locations) also have LOADS of lights, so they can add light to allow the DP to get the fstop desired.

One other thing to note is the use of split diopters.
These are specialized lenses that have more than one focal plane!
Here's an example:
http://www.centuryoptics.com/product...sf_samples.htm

[EDIT: Oops, just noticed that Marco already mentiond diopters ..... still, the sample helps demontrate the dramatic difference they can make..]
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Old April 11th, 2006, 12:53 PM   #14
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When Century was doing their big sale (man, I wish they'd do that again) I was able to pick up a full set for $50 each! It's not something you do a lot, because it's kind of gimmicky, but they sure are neat. I've only actually used them in one movie.
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Old April 11th, 2006, 01:44 PM   #15
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wrong

[QUOTE=Joshua Provost]Guys, you're jumping technically way past the original question...

Leo, the difference is that in film, they rarely shoot with the lens iris wide open, as many do with their adaptors. They shoot whatever aperture get the correct exposure based on the light available. The may be shooting at F2.8 to F4 on interiors, and F8-F11 outdoors. At those apertures, the depth of field is deeper, into feet and meters.




In contemporary films with any production value at all, F-stops are not chosen to adjust exposure, but to control depth of field.

Neutral density filtration and lighting are generally used for exposure.

Read "American Cinematrographer" and you will hear DPs routinely discuss the F-stop they chose to shoot an entire film.


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