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does shooting at 2.8 give better contrast than 8


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#1 thomas-english

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Posted 17 April 2006 - 04:04 PM

I have been very much confused by what I read today in AmericanCinemtographer

pg 84 April 2006 - Marked Man

To create a good look, dense negative for the DI, Sova strove to maintain a stop of 2.8 thoughout the shoot. "If a shot done at T8 is cut in with a shot at 2.8 there will be a big difference in contrast and colour that you will have to even out in DI"

Can someone explain the principles behind this please? Does this mean a shot at T8 would look funny cut in with a shot with ND.9 and at T2.8 this is something I have never concerned myself with.

Thanks
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#2 David Mullen ASC

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Posted 18 April 2006 - 04:52 AM

Assuming (a big assumption) that your lighting contrast ratio is the same, the contrast for a f/2.8 shot shouldn't be visibly different than one shot at f/8. A few lenses get a little more flarey at wide apertures, but that's not common.

However, a shallow focus shot seems more low contrast than a deep focus shot. This is because when a large area of the frame, like the background, goes out of focus, the highlights and shadows in that area will merge into a sort of midtone. You can see this on a waveform monitor when you throw an image out of focus. However, the contrast of the object in focus remains the same.

In terms of lighting, one can tend to not use enough fill light when working at f/8, whereas at f/2.8, natural ambience alone may start to fill in the shadows.
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#3 Mike Panczenko

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Posted 19 April 2006 - 07:17 AM

It's the same principle as, theoretically, one can light a night scene to a stop of f64 and just stop it down enough, but it won't look right because the light spread and fall off will not be the same as actually using low light levels from the start. Even though the scene may be exposed correctly, the qualities of the light differs.
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#4 David W Scott

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Posted 19 April 2006 - 09:00 AM

I wondered the same thing when I read that article.

David, I understand what you are saying about shallow/deep focus -- that makes a lot of sense.

But the article suggests a difference in the behaviour of the neg. The way it is phrased, it claims that density is affected by F-stop, regardless of correct exposure. I believe that is true for exposure length (i.e. shutter angle), as dictated by the film's reciprocity characteristics. Is there anything about F-stop (beyond the DOF issues David mentioned) that would affect density/colour of a negative?

Edited by David W Scott, 19 April 2006 - 09:01 AM.

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#5 Dan Goulder

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Posted 19 April 2006 - 09:54 AM

To create a good look, dense negative for the DI, Sova strove to maintain a stop of 2.8 thoughout the shoot. "If a shot done at T8 is cut in with a shot at 2.8 there will be a big difference in contrast and colour that you will have to even out in DI"

It's hard to tell from the context whether he's referring to cutting between different scenes at shifted points in time, or cutting within the same scene. If the cut is within the same scene, you do want to maintain a consistent stop between the master and the close ups, otherwise the scene is made more difficult to edit without the appearance of a 'glitch'.
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#6 David Mullen ASC

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Posted 19 April 2006 - 01:34 PM

Yes, I believe that's partially it -- you need to maintain the same f-stop to retain the character and contrast of the lighting through the scene. Suddenly lighting one of the shots at f/8 after using f/2.8 means that you are increasing the key, which will overpower a lot of the natural ambience even if you raise the fill level. It's often hard to light scenes to high f-stops because you are working with such bright key lights that it is hard to judge the lighting contrast by eye and you tend to not use enough fill light.

But in terms of how a negative behaves, no, it makes no difference if you expose correctly. If you flat-lit a color chart / gray scale and shot it at different f-stops, adjusting the light on the card to match the f-stop, you won't see a change in the behavior of the negative (its density or contrast.) However, the lens can behave differently at the extreme f-stop positions.
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