# A question about characteristic curve

Discussion in 'Exposure Discussion' started by jernejk, Sep 18, 2010.

1. ### jernejkMember

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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

2. ### keithwmsMember

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

3. ### jernejkMember

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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?

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5. ### Stephen BenskinMember

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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.

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Last edited by a moderator: Sep 20, 2010