# digital light meter

### #1 Jim Nelson

Jim Nelson
• Basic Members
• 166 posts
• Student

Posted 16 August 2010 - 10:32 PM

Hi,

when I use a digital light meter to shoot a movie, I know that I have to key in the ISO. But what about the frame rate and the shutter speed.

Do I have to key in the frame rate or the shutter speed or both?

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• Sustaining Members
• 7373 posts
• Cinematographer
• Los Angeles, Ca

Posted 16 August 2010 - 10:43 PM

Keying in the frame rate will key in the shutter speed, or vice versa.

Film cameras, for the most part, work on a spinning mirror shutter. So the whole thing is 360 degrees, of which, normally, 180 degrees is a mirror and 180 is open.. so 1/2 of the circle lets light hit the film and 1/2 doesn't. Now, there is a formula for shutter speed based on your FPS and this given assumption,

shutter speed = (frames per second * 360)/shutter angle

in your typical film camera this would be

Shutter speed = 24*360/180
Shutter speed = 8640/180
Shutter speed = 48

That 48 is really 1/48th (put a 1 over it). so on a digital light meter which has an fps setting, by keying in 24fps the meter figures the rest.

Or, if you have a meter that just has shutter speed you can key in 1/48s (or 1/50th which is close enough) and both will give you the same reading.

Generally, meters made for cine use have a "cine" mode, meaning you input a FPS and an ISO and it does the math for you for a 180 degree shutter. Of course, if you change the shutter angle (to say 90) you need to adjust your stop, but such things are always the same, for example, changing a shutter from 180 to 90 lets in 1/2 as much light, and on a lens in order to double your light you open up 1 F stop (meaning if you were filming 24fps with a 180 degree shutter, and your meter read F4 then 24fps @ 90 degrees would be an F2.8).

I like working with FPS, as it's easier than working on all this math, so try to find a meter that lets you work in FPS
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