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Light Metres with Video


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#1 Daniel Smith

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Posted 28 December 2006 - 07:13 PM

Hi.

With film, the sensitivity is normally measured by either ISO, ASA or DIN.

In video, the norm is at whatever lux the capture device reads (0db), and any increase in gain is measured in 'db'.

Are there any kind of standards for db ratings?

The reason I ask if because on a film I made yonks ago the DP had a light meter, and I now regret never asking him how he got it to work with the digital camera.

What good would it be if my light meter started asking for an ISO or ASA rating?

And if the light sensitivity between cameras set on 0db differ, then what it says on the light meter could may well be wrong. Unless the lightmeter has presets for different cameras, which I highly doubt.

So, how do you get a light metre to work with a digital video camera?

Thanks.


EDIT:


ohh... dammit... this just occurred to me, do you input the lux sensitivity into the light meter? And then input the amount of gain? (i.e. 3db, 6db, 18db?)

Edited by Daniel Ashley-Smith, 28 December 2006 - 07:13 PM.

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#2 Kevin Armstrong

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Posted 29 December 2006 - 06:46 AM

99.99% of the time i rate ANY video camera's asa @ 320, unless the manufacturer states that it is otherwise, which is usually 400. but using a lightmeter can still be subjective, i.e. if you want your project to have the key -1 stop throughout the scene. rather than rating the meter/camera @ 320 and doing math all day, you can rate it @ 640 and as long as you set the cam aperture to match the meter reading you will be underexposing -1 all day. Keep in mind that the decreased latitude requires more control of your sources and their intensity. 1/3 of a stop in filmland is can be so subtle that the average DP, except the one who did the job, wont even notice. 1/3 of a stop in Video can mean all the difference. Scrims/nets/ND gel are all your best friends on any shoot, especially video.

Test your cam by getting a greycard.
1- setup the meter @ 320asa, 30fps - spotread from camera position and adjust the iris to match your reading.
2-if camera has Zebra pattern set to somewhere between 70-80 ire (some cams have silly limitations on the adjustment of zebra)
3-record the card as follows: normal, +1, +2, +3, +4, -1, -2, -3, -4, normal
4-if you do have zebra pat it should disappear @ normal aperture (@ 80ire) and barely come into view as you get to the +1, with the zebra pattern fully engulfing the greycard @ +2 exposure.
5-bring the tape to a place where you can watch the footage on a large, CALIBRATED CRT monitor (no lcd, plasma, etc)
6-voila, you can now make both an objective and subjective decision on your f-stop.

p.s.
as with film AC's be sure to open the aperture to higher stop, then close down to the proper setting. this is almost more important in video than film, because with most DV cameras the lenses are normally built-in and use "electronic" aperture and therefore will give several different views on the same f-stop reading.
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