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Focus and Exposure help need


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#1 Derick Thomas

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Posted 02 September 2006 - 01:37 PM

Hi,
I am a digital shooter moving over to the film world. I just bought a cp16. Its been tuned up and ready for work. I've shot a sample music video to familairize myself with the film process and immediately I realized that I need help in a few areas: Focusing (meaning, keeping a moving subject in focus with a telephoto lens 12-75), Exposure Knowledge(At which F-Stop the camera should be when using a certain speed film when lit with a certain kind of light, I.E sunny, night time, or indoor artificial light).

Of course, the first thing I did was buy some books, but you guys can explain it in a way any man can understand it.

On film sets I've seen people measuring the talent's distance from the camera lens. Is this to ensure an accurate focus?

I've also noticed during filming that, it was hard to tell if I had a sharp focus in certain low light levels. Is this why they measure?

Can anyone put a film speed beside these lighting situations
indoor dance club bright color lights:
indoor room regular artifical light:
outoor: afternoon 12pm.
outdoor 4pm:
outdoor 7pm golden hour:
nightime bright street ligths carlight:

Much help is needed.
Thanx.
dt.
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#2 Mike Rizos

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Posted 02 September 2006 - 09:31 PM

Yes, the reason for measuring the distance is for focusing. Because most filming is done at wide f-stops, accurate focusing is extremely important. If the subject moves, focus is adjusted accordingly. For this to work you need to make sure the backfocus is set properly. On a prime lens if the lens focuses correctly at infinity, this usually means the focusing will correspond to the distance marks on the lens. On a zoom, you need to make sure the lens holds focus throughtout the zooming range, when set at a particular distance. If it doesn't hold focus, it means it needs servicing and the distance marks won't correspond to the actual focusing distances.

Yes some viewfinders get really dim when the lens is stoped down, rendering focusing difficult. You can open the lens completely to focus, or you can take a measurement. In dim light you're probably wide open anyway, so you have to measure.

I don't completely understand your last question. If you're asking what will the exposure be under those conditions, it's very difficult to say. You need to invest in a good light meter and go from there. Film is too expensive not to use a light meter. Outdoors you can probably use the sunny 16 rule, but there is no reason to play guessgames.

So get a light meter, figure out the camera's shutter angle, input that and the film speed in to it, and that will give you the aperure. Check the archives for meter reccomendations, and read all those books you got.
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#3 Daniel Stigler

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Posted 02 September 2006 - 09:35 PM

You need to get an exposure meter to get a reading to what stop to set the lens. Meters and metering tecniques are discussed here. A search should give you a fair amount of results.
The guys you saw measuring distances were the 1st ACs/Focuspullers. It's our job, within other things, to keep the chosen subject in focus by turning the lens barrel. Have a look at your lens. It should show distance marks.

A usual filmspeed used outside during daytime would be 50ASA, during nighttime 500ASA. Indoors it depends heavily how you light. Get a lightmeter and take readings in everyday situations to get a feeling for that.
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#4 David Mullen ASC

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Posted 03 September 2006 - 12:15 AM

When you're talking about the lower levels encountered in night interiors, generally you're looking at using a 500T stock. When you have higher levels, you have more options -- there are no right or wrong choices, it just depends on the grain level you want and whether you want to use a daylight-balanced stock in daylight conditions, indoors or outdoors, or just filter a tungsten stock.

If you are using a zoom lens, it is possible to zoom all the way in and get eye-focus marks for the different positions that the actor will be moving, but often with shorter primes or when you are just measuring the empty space where an actor will end up standing, you need to use a tape measure -- especially if you have a hard time judging focus in the viewfinder.
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#5 ryan_bennett

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Posted 03 September 2006 - 09:44 PM

Seriously, there's sometimes no right or wrong filmstock, just think about what you would need in terms of daylight/tungsten, grain/no grain, all of that. The best thing to do is to do a lot, and I mean a lot, of tests and copy down all of what you do, the lens, focus, f-stop, filmstock etc. so you can redo the results if needed. Get a good light meter too. Also, focus with a telephoto lens is usually a shallower depth of field, that is harder to keep in focus/less is in focus at once, whereas a wide angle has a greater depth of field. Your f-stop also has an effect on your depth of field.
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#6 Derick Thomas

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Posted 04 September 2006 - 10:42 AM

thanks guys. your experience and information has been a great help. i think you're right about the test and light meter. Checking prices for those right now.
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