Stop, Just Stop!

Our eyes and ears have logarithmic responses. That is to say that we respond naturally to twice the light intensity, or twice the pitch of a sound, or twice the volume.

This explains the octave in music. The same goes for the eye.

A stop in photography represents a doubling of the light.

Memorize this.

Now recall the basic equation:

Luminous Energy ∝ L × r² × t

As a general rule, L is what it is. Whether you have a brightly lit garden, or a candlelit dinner, you have no control over it. What you can control however are r and t. So what would it take to double the light?

Well, you can either double the time, or increase the radius by √2.

If you understand this, you will understand the f-stop system which goes as 1.0, 1.4, 2, 2.8, 4, 5.6, etc. They are each increasing by exactly √2. (If you don't understand this statement, we will go into the f-stop stuff in gory detail later.)

Many many photographers do not grasp this. How a stop represents both increasing time, or dialing "down" the aperture. Is it time, or is it aperture?

The answer is both! You increase total light energy by letting more in.