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Old December 7th, 2007, 09:20 PM   #1
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Help with converting light meter readings

My light meter (Sekonic L-408) can only be set to speeds of 3o or 60 - not 48. I'm sure there is a mathamatical way to use one of these settings and correct the resulting f stop for shooting at 24p. Can anyone tell me how? It would beat having to buy a new meter.
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Old December 7th, 2007, 09:28 PM   #2
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You might have better luck with this question on the Photon Management board, possibly a moderator could move your post?

Wish I could help.... Though I would think if it takes 30 watts of light to light for 1/30 exposure, it would take 24 watts for 1/24 exposure... but I am probably very wrong. :)
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Old December 8th, 2007, 02:17 AM   #3
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Are you sure your meter doesn't have any steps between 30 and 60? It would be really bizarre for a meter to only give you information in full stops...
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Old December 9th, 2007, 03:11 PM   #4
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In shutter priority mode I can only set 30 or 60 for the shutter speed. I need 1/48. I was suprised too when I tried it. I know the version above mine is called a Cine meter, so maybe they reserved the film based shutter speeds for that model. I'll try posting in the Photon board too.
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Old December 11th, 2007, 05:53 PM   #5
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A shutter speed of 48 lets in 1/3 more f-stop of light than a speed of 60.

Here are the numbers:

30 means 1/30 sec = 3.3 milliseconds
48 means 1/48 sec = 2.1 milliseconds
60 means 1/60 sec = 1.7 milliseconds

To find number of f-stops: log(2.1/1.7)/log 2 = 0.32 stops, about 1/3 f-stop

So, meter for 60 and open up the iris by an additional 1/3 f-stop.
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Old December 11th, 2007, 09:04 PM   #6
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Correction: Meter for 60 and close the iris 1/3 of an f-stop.

A shutter speed of 48 lets in 1/3 f-stop more than 60. So, you have to close down the iris 1/3 of a stop to compensate.
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Old December 12th, 2007, 12:16 PM   #7
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I have a Sekonic spot meter that has a 1/50 setting (effectively the same as 1/48) all the way at one end of the shutter speed range--sorry, forgot which end offhand.
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Old December 19th, 2007, 09:50 PM   #8
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Awesome. I knew there had to be a mathimatical way to figure it out. I just couldn't find it, and math is not my best subject. Thanks for the help John. Then I checked my meter, and like Charles said, after you reached the high end of shutter speeds there are some options listed as f/s. I can get a 24 or 64. I'll have to check out what that is. One way or another it will work.
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