Help with converting light meter readings at DVinfo.net

Tim Veal · December 7th, 2007, 09:20 PM

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.

Carl Middleton · December 7th, 2007, 09:28 PM

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. :)

Ben Syverson · December 8th, 2007, 02:17 AM

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...

Tim Veal · December 9th, 2007, 03:11 PM

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.

John Bennett · December 11th, 2007, 05:53 PM

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.

John Bennett · December 11th, 2007, 09:04 PM

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.

Charles Papert · December 12th, 2007, 12:16 PM

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.

Tim Veal · December 19th, 2007, 09:50 PM

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.

December 7th, 2007, 09:20 PM	#1
Tim Veal Regular Crew Join Date: Apr 2007 Location: Chicago, IL Posts: 70	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.

December 7th, 2007, 09:28 PM	#2
Carl Middleton Major Player Join Date: Sep 2007 Location: Fernandina Beach, FL Posts: 562	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. :) __________________ Carl Middleton Whizkid Mediaworks

December 12th, 2007, 12:16 PM	#7
Charles Papert Wrangler Join Date: Jan 2002 Location: Los Angeles, CA Posts: 6,810	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. __________________ Charles Papert www.charlespapert.com

December 8th, 2007, 02:17 AM	#3
Ben Syverson Major Player Join Date: Mar 2004 Location: chicago Posts: 434	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...

December 9th, 2007, 03:11 PM	#4
Tim Veal Regular Crew Join Date: Apr 2007 Location: Chicago, IL Posts: 70	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.

December 11th, 2007, 05:53 PM	#5
John Bennett Regular Crew Join Date: Sep 2005 Posts: 25	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.

December 11th, 2007, 09:04 PM	#6
John Bennett Regular Crew Join Date: Sep 2005 Posts: 25	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.

December 19th, 2007, 09:50 PM	#8
Tim Veal Regular Crew Join Date: Apr 2007 Location: Chicago, IL Posts: 70	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.