Doug Kerr
Well-known member
Hi, Ted,
You say:
As to the constant 'C' that Sekonic states as 340 for the dome, I have read a lot that the common 'C' for a flat receptor is 250 ... haven't found what they say about that - you?
I had not been aware of that.
But I will say that perhaps 340 is typical for the dome (cardioid) collector (as we often see for Sekonic), and if the intimation of the ISO standard is followed (that C for a cosine collector should be 0.75 that for a cardioid collector), that would call for 255 for a cosine (flat) collector. So that fits what you have seen.
The mystery is that it looks as if for the flat receptor (used for duplex metering) and the dome receptor (used for "Norwood" metering) to give comparable results for common key:fill lighting setups (as we would hope), the value of C would have to be the same for both configurations.
So if the standard means to imply that the value of C should substantially differ between cosine and cardiod configuration, I do not understand why.
******
If we reverse engineer the exposure calculator for the Sekonic L-398A meter, and take the "illuminance" indication shown on the meter movement as "correct", that wouild essentially say that the value of C is very nearly 340, the "marked" value.
If this is a fact for the dome collector (cardiod directivity) configuration, then for the meter to exhibit a C of 250 in the flat collector (cosine directivity) configuration, with the flat collector in place, the illuminance indication on the meter movement would have be about 4/3 the actual value.
Doug
You say:
As to the constant 'C' that Sekonic states as 340 for the dome, I have read a lot that the common 'C' for a flat receptor is 250 ... haven't found what they say about that - you?
I had not been aware of that.
But I will say that perhaps 340 is typical for the dome (cardioid) collector (as we often see for Sekonic), and if the intimation of the ISO standard is followed (that C for a cosine collector should be 0.75 that for a cardioid collector), that would call for 255 for a cosine (flat) collector. So that fits what you have seen.
The mystery is that it looks as if for the flat receptor (used for duplex metering) and the dome receptor (used for "Norwood" metering) to give comparable results for common key:fill lighting setups (as we would hope), the value of C would have to be the same for both configurations.
So if the standard means to imply that the value of C should substantially differ between cosine and cardiod configuration, I do not understand why.
******
If we reverse engineer the exposure calculator for the Sekonic L-398A meter, and take the "illuminance" indication shown on the meter movement as "correct", that wouild essentially say that the value of C is very nearly 340, the "marked" value.
If this is a fact for the dome collector (cardiod directivity) configuration, then for the meter to exhibit a C of 250 in the flat collector (cosine directivity) configuration, with the flat collector in place, the illuminance indication on the meter movement would have be about 4/3 the actual value.
Doug