There are two problems:  one is that the Minolta meter doesn't match the
sensitivity of the human eye (at least it doesn't say so in the specs. for
the Model IVf), so you would need to have a calibration for each type of
light source that you expect to use. Combinations of light sources
(incandescent and fluorescent, for example will drive you nuts.)  Second, is
that the field of view for the Minolta meter is not "cosine".   There are
various acceptance angles.  That is, the exposure meter is not designed to
"look" at all of the light reaching it (180 degrees worth) as an illuminance
meter does.  There are attachments -- diffuse plastic domes, typically, for
the Minolta that can help with that problem, but you would have to calibrate
the Minolta against an illuminance meter under lighting systems with
different types of distributions to check.

Unfortunately, measuring illuminance (lux or footcandles) using a meter
designed for photography has traditionally been difficult.  But, using an
illuminance meter for photography does work fairly well.  Some illuminance
meters, in fact, come with a little slide rule device which does the
conversion.

Terry McGowan


Royce Bair wrote in message <[log in to unmask]>...
>I have a nice, $400 Minolta IV exposure meter (for photography) that
>will take incident light readings (both for ambient and flash).  The
>readings it gives are super accurate for film/camera exposure settings.
>However, I have a need to convert the f-stop and shutter settings to
>either "footcandles" or "LUX" levels.  I need this conversion for some
>scientific testing I'm doing.
>
>Let say, I set my meter to an ASA or ISO of 100, and my meter gives me
>an ambient light reading of for a camera exposure setting of F8 @ 1/30
>second -- what would be the level of LUX or footcandles of light that
>has fallen on my subject?
>
>Does anyone know how to get this information?  I don't want to buy a
>light meter that gives these reading in LUX or footcandles, when I
>already have a very sensitive meter.
>
>BTW, I know how to convert between LUX and footcandles.
>
>Royce Bair
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