A question about characteristic curve

[jernejk]

Dear All,

I'm looking at different film's characteristic curves and I don't understand what exactly does 0 stand for on the X axis.

For example, bw400cn http://www.kodak.com/global/en/professional/support/techPubs/f4036/f4036.pdf has 0 quite on the right side, only 3 f-stops from the shoulder.

Is 0 set at the point where 18% gray at film's ISO is? Or is this point arbitrary?

Thanks

 

[keithwms]

log (1) =0, so the zero corresponds to 1 lux-second exposure.

 

[jernejk]

Ah, ok... so that's log exposure in lux-seconds at film plane, right? How do I translate 1 lux second to anything more familiar? Is there a rule of thumb, eg: a gray card under bright sun, f5.6, 1/100, ISO 100 is approximately 1 lux second on film plane?

 

[ic-racer]

Negative characteristics are used quite often in sensitometry. Note that in this book the unit for exposure is millilux-seconds. The former standard unit was lux-seconds. Five hundred millilux-seconds equals 0.5 lux-seconds. The log of 500 millilux-seconds is 2.7 but the log of 0.5 lux-seconds is a negative number (-1.7). All characteristic curves that used lux-seconds as the unit for exposure use negative characteristics.
Be prepared for this when looking at most characteristic curves because the change to using millilux-seconds is fairly recent and most published curves use lux-seconds as the unit for exposure. Page 17

Required reading http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

 

[Stephen Benskin]

Ah, ok... so that's log exposure in lux-seconds at film plane, right? How do I translate 1 lux second to anything more familiar? Is there a rule of thumb, eg: a gray card under bright sun, f5.6, 1/100, ISO 100 is approximately 1 lux second on film plane?

I've uploaded an attachment that might help. The equation for Hg (exposure at the metered exposure point) is 8 meter candles * shutter speed or 8/ISO. B&W film speed is calculated using 0.80/Hm which falls 10x or 1.0 logs below Hg. 0.80/ISO will give you the exposure at the speed point. Example, for a 125 speed film Hg = 0.064 mcs and at the speed point it would measure 0.0064 mcs. 18% gray falls 1/2 stop above the metered exposure point. The exposure at the film plane for 18% gray with a 125 speed film would be half again 0.064 or 0.096 mcs.

I have a digital copy of the paper, "Calibration Levels of Film and Exposure Devices" which I'll email if you're interested in delving deeper into the topic.