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Peanut Butter Spreads Light on the Science of Photography

and...

The Amazing Little Red Jacket

Frank J. Dispensa, PPA Certified, M.Photog.Cr., API, APM

What happens to the light intensity when you double the distance from the subject to the light..?

That has been the question from the beginning of time. The Pharos of Egypt, all the great scholars of the universe and

yes… even Adam and Eve pondered this very same question. Some say the light intensity is cut to ½, a loss of 1-stop.

Others proclaim the intensity is indeed cut to ¼, a loss of 2-stops.

So let’s demystify the technical.

First we must understand that light is unique and does not follow our day-to-day logic. Most all of our day-to-day measuring sequences are linear in nature.

Linear Example...

Let’s take a cannon with a 30-degree angle of trajectory, load one bag of gunpowder, load one 10-pound cannonball

and light the fuse. Now let’s say the cannonball traveled a distance of one mile.

One would assume that if we loaded two bags of gunpowder, keeping all other parameters equal, we would expect

the cannonball to travel approximately two miles. And give or take, it would. Now this is linear and logical…

Double the powder (power), it travels twice as far.

Now let’s move from our linear and logical cannons and cannonballs to an experiment that will explain why light is nonlinear, but still logical. Hang on you’ll see.

Now let’s set up another experiment and see why light does not follow our every day linear logic. We have a slide projector placed 10 feet from a screen. We adapted the lens to project a square light pattern making things easier to understand. Please note that, in our example, the light pattern projected from a distance of 10 feet is 2 feet high and

2 feet wide (4 square feet). And if we meter the intensity we get f/8.0 at 1/500.

I now introduce to you Mario the mason.

Notice Mario has a wheelbarrow, which contains peanut butter. Yes! Peanut butter. Mario will now use his extraordinary talent to cover the 2-foot square pattern of light with peanut butter. He has been instructed to make

sure the peanut butter is exactly 1-inch thick. This precision is the most important part of the experiment and

that is why we went to the great expense to bring you Mario.

Let’s move the projector back to 20 feet (doubling the distance). Now for one million-dollars. Did the pattern grow vertically or horizontally. Be very careful now…. Again, the red jacket. Oh! You say the light pattern expanded in

both directions... Yes! Yes! Congratulations to the little red jacket.

Now we ask Mario to do something that takes all of his skill. Mario will evenly spread the 1-inch of peanut butter

over the expanded (16 square foot) light pattern.

“¼ of an inch” came loud and clear from the little red jacket in the last row. Okay little girl. Let’s find out. Mario please measure the depth of the peanut butter. A drum roll please. Mario carefully measures the depth of the peanut butter. And the answer is... Wow! ¼ of an inch. Congratulations little girl you are correct!

Let's Review...

When we went from 10 to 20 feet the light pattern doubled both vertically and horizontally. It went from 4 to 16

square feet. The original light intensity of f/8.0 now had to cover 4 times the original area causing it to drop to 25%

of the original f/8.0. When we loose 1 stop we cut the intensity to 50%. When we loose 2 stops we cut the intensity

to 25%. So our new intensity at 20 feet is f/4.0 a loss of 2 stops.

The little red jacket is once again correct.

I must talk to the amazing young lady. Hello young lady, I thank you for attending my seminar and giving us all of those correct answers. Wow! I’m impressed. What is your name? “Ah… they call me Red”. So, Red, you came, to

my seminar because you want to be a professional photographer? “Well ah… like…yes!” How old are you. "Ten”

she she replied. And what grade are you in? “The 6th grade,” answered...

Can you imagine only ten years old and in the 6th grade. Brava, Red, brava.

Frank Dispensa Seminars, 30 Brothers Road, Wappingers Falls, NY 12590 ~ dispensa@optonline.net ~ 845.297.5541.