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Shooting at one Fstop


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#1 Chad Stockfleth

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Posted 19 May 2006 - 10:15 AM

I've heard mention over the years and certainly on this forum about DP's who will shoot an entire film at one fstop, say 2.8. Why would someone do this? I can imagine it is to keep depth of field consistent, but are there any other reasons?
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#2 David Mullen ASC

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Posted 19 May 2006 - 10:25 AM

Not just depth of field, but the optical performance of the lens becomes more consistent.

I follow that idea to a limited extent in the sense that I try and keep the coverage of a scene consistent at one f-stop so that maintaining the same contrast ratio is easier, plus the depth of field is consistent. But I tend to use a deeper stop outdoors and for day interiors, or for scenes where the focus-pulling will be harder, or if I have to use a slower lens like a zoom, telephoto, slant-focus, etc.

Trouble is that the look of the depth of field also depends on how close you are to the subject and the focal length, so a wide-angle medium-to-distant shot at f/2.8 may look fairly deep-focus, but super shallow-focus up close with a 100mm. So you don't necessarily get more "consistency" visually, unless you limit your focal length choices and don't go a lot from extreme wide to extreme tight. This is one reason why I light inserts to a deeper stop, because macro photography tends to be shallow-focus even at medium f-stops. I always tell 2nd Unit DP's to not light an insert at f/2.8 just because I lit the master to f/2.8, because it will be too shallow-focus.

I also think one accepts visually that day exterior work will look deeper-focus than low-light night work, so even though I know all my night work will be at f/2.8, I don't shoot my day exterior work that way just to be consistent, because your eye expects to see more detail outside in daylight.

On the other hand, you don't want to be all over the map, f-stop-wise, shooting some scenes at f/8 and others immediately after that at f/2.0, because then you start having a visually inconsistent movie. The worst is when you shoot most of a scene in sunlight at f/8 but end up shooting into late dusk wide-open -- there is such a mismatch visually.
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#3 Chad Stockfleth

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Posted 19 May 2006 - 10:42 AM

Good info, that makes a lot of sense. I suppose as a follow-up question, could you describe finding and shooting in the "sweet spot" of a lens. Is this the point of the lens that has the best critical focus? Does a zoom lens react differently from a prime where this is concerned?
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#4 David Mullen ASC

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Posted 19 May 2006 - 11:01 AM

You can test each lens to find the "sweet spot" or you can just assume that in most cases, it is around 2-stops closed from wide-open, so for a T/2.0 lens, T/4.0 would be the best f-stop, optically. For a T/2.8 zoom, it would be T/5.6 therefore. This isn't always true but it's a good general rule.

I follow an even more general rule which is to just avoid the widest aperture if possible, so for a T/2.0 lens, I try and think of it as a T/2.8 lens... Sometimes that's not always possible or practical.
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#5 Matt Pacini

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Posted 19 May 2006 - 04:11 PM

That's interesting David.
Would you break that rule, if say, you were doing a very stylized film, on the level of a Tim Burton film?
I remember seeing The Crow II, and there were some deep focus shots, cut with extreme CU's with very limited depth of field (like one eye in focus, the other not), and I thought it really added to the surreal nature of the visuals.

MP
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#6 Greg Gross

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Posted 19 May 2006 - 04:46 PM

In still photography I often shoot with a 200mm prime and open it up some with a constant
f-stop. No problem with the background and I can even isolate areas of the face(in-focus and
out of focus). I like to show a little more white of the eye(below the iris). This has the effect of
bringing out the eye color more. With the 200mm I can frame and isolate and see the eyes more.
This makes for a great portrait with beautiful women. I've done this with a constant f2.8.

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#7 David Mullen ASC

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Posted 19 May 2006 - 05:44 PM

Yes, it just depends on the movie and what the director wants to do. On some movies, extreme changes in focal length, camera angle & lens height, and even depth of field are used to create a stylized look where you are supposed to notice the design of the individual shot, while on other movies, more subtlety is a good idea...

It's sort of like color -- sometimes you play with contrasting colors, other times, you want a consistent overall tone to the colors.

Anyway, one could imagine a story in which you alternate deep-focus and shallow-focus scenes, or progress from one type to the other as the story goes on, just like some films start out using mostly wide-angle lenses and change slowly to using mostly telephoto lenses, or vice-versa.

There are a couple of basic patterns in visual design. One is an overall consistent look that doesn't vary too much except when the story demands it logically. Another is an A-versus-B pattern where you contrast two different visual designs, often representing different character or story elements or worlds ("he's poor / she's rich" for example). And another is the A-to-B pattern ("he's rich but gradually becomes poor as his drug addiction overtake him".)
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