Re: [Leica] incident metering techniques

[Godfrey DiGiorgi <ramarren@bayarea.net>]

You posit two photographic opportunities to meter with an incident light 
meter:

>[Venus de Milo]
>The statue is lit by a combination of skylight through windows on
>the left, and incandescent spotlights aimed directly at the
>statue.  The incident light where I am standing is nothing like
>the light on the statue, and I cannot stand in front of the
>statue to measure the light falling thereupon. Additionally, the
>statue has a high reflectance (much higher than neutral gray).
>I'm not sure what an incident meter would do for me in this
>instance, whereas the utility of a spot meter is patent.

Finding a correct spot to meter this with an incident meter can be a 
little challenging, agreed. I've usually been able to observe a similar 
illumination schema elsewhere that I could reach more easily. Then it's 
simply a matter of metering there and using the readings for the Venus de 
Milo. 

The reflectance of the statue will not affect the accuracy of the meter 
reading, that is the point of an incident meter. It will return a reading 
which allows that which in the scene is 18% gray to remain that same 
density on film. The value of the statue's reflectance will fall 
correctly 2 maybe 3 EV values higher than that. 

It's possible that you might want to bring the statue's value down a 
little to accomodate the exposure latitude of the film you're using, then 
you would bias the exposure reading given by the meter to suit. 
Situations like this are a perfect reason to bracket as the final outcome 
of the image will change to some degree through a 2EV exposure range and 
it's useful to be able to pick that which works the best.

>I'm standing in sunlight.  My subject is the Eiffel Tower, half a
>mile away and 1000 feet high.  It is dark chocolate in color,
>against a brilliant blue sky with fluffy white clouds that
>occasionally put parts of the tower in shade.  How can an
>incident meter tell me how to expose a photograph of this tower?
>With a spot meter, it seems that I just point to the tower itself
>and take a reading, but what can an incident meter tell me?

A spot meter, pointed at the tower itself, could give you a reading which 
is probably 1-2 EV too high since the tower could be less than an 18% 
reflectance gray target: you'll likely overexpose it if you read the 
tower itself. I'd want to check the reading against a gray card held to 
determine the reflectance of the tower. An incident meter reading at your 
present position (in the sun or under one of those fleecy cloud shadows) 
will give you a baseline exposure correct for the scene. If you're 
looking for more detail in the tower itself, I would add some exposure 
bias to that recommendation. The actual reflectance of the tower and 
surrounding scenery will fall where their reflectance places them. Since 
fleecy cloud shadows are typically 2-3EV lower than sun in the clear sky, 
to get the best results from this picture, you should wait until there 
are no fleecy cloud shadows on the tower itself and expose for the clear 
sunny sky.

Hope that helps.

Godfrey