"Peter Irwin" <pirwin@[EMAIL PROTECTED]
> wrote in message
news:fg8cpp$qeq$1@[EMAIL PROTECTED]
> Richard Knoppow <dickburk@[EMAIL PROTECTED]
> wrote:
>>
>>
>> I think you are refering to log exposure. I am going to
>> have to research this because I don't remember if the
>> ASA/Jones method used the same units (lux seconds) as are
>> currently used.
>
> Thanks. I think a metre-candle-second is the same as
> a lux second.
1 lumen (lm) = 1/683 Watt at 555nm = 1 (non-existent) green candle (cd)
1 lumen / m^2 = 1 lux
1 lux-second = 1 cd-second/m^2
Or 1 lux-second is exposing the film to the light of a standard
candle at a distance of 1 meter for one second.
Sensitometry is usually performed at a color temperature
of 5,500K - a blue filter and a dim bulb (2,250K (?))
is the usual source. The bulb is calibrated to a
'standard' bulb.
Lumens/Lux are defined photoptically, using the spectral
sensitivity of the eye. Strictly speaking using lux for
film response is horse poop unless you use 555nm light.
The green light 'luminous efficiency' of 683 lm/watt
incident radiation is also used for the 5,500K source
as doing it properly opens a huge can of worms that
wouldn't really add anything useful.
There is no legitimate ISO speed for
film that has a spectral sensitivity that differs
significantly from that of the human eye.
> I've been trying to get a handle on what the X axis
> on the H&D curves means in practical terms.
ISO/ASA = 0.8 lux-sec / (lux-sec required for 0.1 OD on the film)
> -2.1 - ISO B&W speed point (0.1 above base + fog)
Correct: 100 = 0.8 / lux-sec
lux-sec = 0.008
log (lux-sec) = log (0.008) = -2.097
> -1.05 - where usual light meter tries to put the average
Here it gets messy ... but it's a good enough number -
~3.5 stops more exposure than that required for 0.1 OD.
Or, ~1/8 second at 1 meter from a candle - no lens
on the camera - gives an exposure creating a negative
that should print to 18% grey.
--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/index.htm
n o lindan at ix dot netcom dot com
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