[Prev][Next][Index]
Re: Sticking my neck out - again
>TO MEASURE LIGHT LEVELS CHEAPLY:
>Do you have a 35mm camera with a built-in light meter? One of the
>"older" kind, in which you manually set the shutter speed, ASA or DIN
>film speed, and hand-adjust the f-stops?
I found an approximation for converting readings from a camera's light
meter into foot-candles (Source: Greenhouses--planning, installing
and using greenhouses. Ortho Books, 1991).
Set your camera to ASA 25 and 1/60 shutter speed. Aim and focus the
camera so the object you want to measure fills the viewer. Adjust the
f-stop until the needle indicates the correct exposure.
If the object you are metering is WHITE, then:
f-stop foot-candles
2 100
2.8 200
4 400
5.6 800
8 1,600
11 3,200
If the object you are metering is GREEN (like a plant leaf), then you
must increase the f-stop by two settings to get the correct amount of
light (e.g., if it reads f5.6, add two f-stops, resulting in f11 and
3,200 foot-candles).
A camera's light meter only reads reflected light, so if you measure
from something white (e.g., 90%+ reflectivity), you are getting a
pretty good measure of the amount of incident light. Green plants
reflect much less light (about 20%), so you must adjust your f-stop
accordingly.