Page 10 - Lighting the Un-lightable Volume 1 #3

There are many ways. Here’s an example that is rather straight forward.
On a day when the sun is intense, look at your shadow, or any shadow,
close to where it meets the ground. If the line of the shadow is sharp
and well defined, the difference between black and white will be at least
four F-stops. On a day when there are no shadows, a rainy day, for ex-
ample, the difference is one or less. That is perfect. You might not like
the gray tones of a rainy day, but as far as the sensor in your camera is
concerned, that is perfect.

However, it is rarely that simple. If you will excuse the pun, most situa-
tions are far more gray than that. Here’s how to calculate the difference
between black and white in all those gray times.

 Assign a value to white.

 Assign a value to black.

 Subtract black from white.

 Divide by 60.
The result is the number of F-stops.

 A difference of 1 or less is ideal.

 A difference of 2 is workable.

 A difference of 3 is problematic.

 A difference of 4 will be seriously difficult to resolve.

Let’s look at this in more detail. We know that absolute white has a value
of 256. Absolute black has a value of 0. Assigning a value to the “whitest
white” is a matter of asking this: If absolute white is 256, how close to
that is the whitest white in this scene?” The same is true for black - “If
absolute black is 0, how close to that are the blacks in this scene?” So,
if the whitest white is 250, and the blackest black 10, then the difference
is 240. Divide that by 60, and the answer is 4.

Can you know for sure what the value of white and black are? No. Expe-
rience and practice will teach you. In some cases, I don’t even need to
do the calculation. For example, if I am photographing an interior and I
notice a sharp black line along the floor in front of the window, I know

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