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Shooting at varying apertures


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#1 Yaron Y. Dahan

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Posted 21 November 2006 - 12:53 PM

I remember that about a year ago I went to some Digital Cinema Society talk at a film festival, and one of the filmmakers who shot a feaure in HD said that if possible one should always shoot at the widest aperture possible.. he mentioned something to do with either with the way the compression is done or something with the pixels or artefacts.. I can't remember anymore. It was so long ago.
So my question is, does this have any basis? Is this true only for HD, what about DV?

I know for example that in still photography, all else being equal, and disregarding issues of depth of field, usually three f-stops closed down from the widest is considered the "optical center" of the lens. Is this true for video too?
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#2 Chris Durham

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Posted 22 November 2006 - 12:21 PM

One reason to shoot as wide as possible is to limit depth of field, which aids in the "film look" by minimizing the sharpness of background objects and more closely approximate the DoF of film.
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#3 Michael Nash

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Posted 22 November 2006 - 04:52 PM

The "optical center" of lens depends on the lens. Many newer high-end video/HD lenses are optimized to perform well at wide apertures.

You often want to shoot with wide apertures on 2/3" and 1/3" chip cameras to minimize depth of field. But I can't think of any way that aperture would affect compression artifacts, assuming you have a proper exposure and you don't have too many optical artifacts from the lens (like chromatic distortion, flare, etc.)
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#4 Michael Collier

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Posted 22 November 2006 - 10:28 PM

A wide open appeture will soften background and foreground elements, as was mentioned before, and when the camera is compressing the image, high-frequency detail is a killer (like anything with texture thats in focus) if its out of focus its much easier to compress without showing artifacts (esp. with cosine compression, such as JPEG and DV and most modern compressors.) HDV applies to this, however HDV, as with anything MPEG, will suffer from quickly moving or changing images.

I would recomend shooting at a greater than normal focal length rather than shoot wide open. Very few lenses for video I have found that work well wide open. Most go very noticably soft after an F2.8 or F2 depending on the lens, and most will have sever chromatic aberation when you shoot at longer focal lenghts with the lens wide open (or with high contrast scenes and highlights). This applies to higher end proffessional lenses as well as prosumer cameras (but not neccisarily the top of the line 2/3" lenses, some of those do OK)
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#5 David Mullen ASC

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Posted 22 November 2006 - 11:49 PM

Sometimes when shooting a wide, wide-angle master shot in HD where everything will look in focus anyway (even if I shot wide-open) mainly because nothing is very close to the lens, most of the objects are at a distance, I will light the scene to an f/4, let's say, and shoot the master at that f-stop so that the lens is at its sharpest. Then when I go for tighter coverage, I switch in the ND.60 internal filter and shoot at an f/2.0 to reduce the depth of field. The slight loss of sharpness is fine because now I'm shooting faces, not wide shots of rooms where I want more detail. So I get the shallow focus look for the medium and close shots, but the wide shots are sharp.
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#6 Yaron Y. Dahan

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Posted 18 December 2006 - 11:37 AM

Sometimes when shooting a wide, wide-angle master shot in HD where everything will look in focus anyway (even if I shot wide-open) mainly because nothing is very close to the lens, most of the objects are at a distance, I will light the scene to an f/4, let's say, and shoot the master at that f-stop so that the lens is at its sharpest. Then when I go for tighter coverage, I switch in the ND.60 internal filter and shoot at an f/2.0 to reduce the depth of field. The slight loss of sharpness is fine because now I'm shooting faces, not wide shots of rooms where I want more detail. So I get the shallow focus look for the medium and close shots, but the wide shots are sharp.


Excellent advice. But, and maybe this is a fool's question, does the 2 stop change make that much of a difference? I haven't noticed that in video, well at least not on the level of a 12 inch monitor, its not noticable like it is in film. And correct me if I'm wrong, but doesn't using an extreme Tele give the most filmlike depth of field, because then stuff starts to go out of focus???
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#7 David Mullen ASC

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Posted 18 December 2006 - 01:45 PM

Excellent advice. But, and maybe this is a fool's question, does the 2 stop change make that much of a difference? I haven't noticed that in video, well at least not on the level of a 12 inch monitor, its not noticable like it is in film. And correct me if I'm wrong, but doesn't using an extreme Tele give the most filmlike depth of field, because then stuff starts to go out of focus???


Certainly 2-stops makes a difference in depth of field, especially on medium to longer lenses when shooting tighter converage. Ask a focus puller if it makes a difference whether a close-up is lit to an f/2.0 or f/4.0...

On a smaller monitor, it would be harder to see, yes, but that would be true in any format.

Telephoto lenses give the illusion of less depth of field because they make a background that is farther away (and thus more out of focus naturally) look bigger in the frame, more enlarged, so you can tell it was out of focus.

If you shot a wider shot on a wider-angle lens and then cropped and enlarged it in post to match the view of a telephoto lens, you'd have the same effect -- the depth of field hasn't really changed, but now you can see just how soft the background was behind the subject because you've enlarged that portion of the image.

Conversely, wide-angle lenses can give the illusion of greater depth of field because the background recedes in size more rapidly, making it harder to see how out of focus the details back there are.

Note that I'm talking about lenses that give a wider-angle or more telephoto effect, which is related to the format -- I'm not talking about the focal length of the lens.
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