Equivalent Exposure Part1 from Peter Phun on Vimeo.

You’ve probably heard photography referred to as ‘painting or writing with light’ so this lesson will borrow from this idea.

Imagine these 4 items:

• The water I have in a squirt bottle is light.
• The size of the nozzle is the aperture or opening of the lens of a camera
• The speed at which I squeeze the handle is the shutter speed.
• The piece of tissue paper is the film or digital camera’s sensor
• The absorbency of the material of the tissue is ISO or sensitivity of the camera to light

First, we all know a properly exposed picture is one that records a scene showing good details in the highlights and as well as in the shadows–the picture is neither too dark or too bright.

Then by this analogy, it follows that if I want the right ‘exposure’ I will have to spray my piece of tissue to just the right dampness, not dripping wet or not too dry.

To achieve this, I can either use a long squeeze of the handle of my squirt bottle (slow shutter speed) and small aperture or opening of the nozzle or a short brief squeeze (fast shutter speed) and a large aperture or opening of the nozzle.

So it follows that there are many, many possible combinations to achieve the same desired dampness or equivalent exposure.

Which of these combinations do I choose then? The answer lies in what you are trying to capture.

Freezing or Stopping Action.

If I’m looking to grab an action picture of someone jumping a ramp on a skateboard, then I would choose the highest shutter speed that I can for the given lighting conditions.

At this point, I need to point out that on a camera, the numbers are expressed as reciprocals so that the number you see is actually a fraction.

In the case of shutter speeds, 1000 is actually 1/1000th of a second. So to freeze our skateboarder, I would use 1/1000 sec or higher if possible.

And an aperture of f22 is an extremely tiny opening whereas f2 is actually a large physical opening of the lens.

Depth of field (zone of sharpness)

So how about if I want to get a picture which is sharp from 2 or 3 feet all the way to infinity? This is often referred to as depth-of-field.

Consider the spray of water from my squirt bottle. When I made the opening tiny, the spray traveled the furthest.

So if I want maximum depth-of-field or zone of sharpness, I should choose a big number for the f-stop. The bigger the number the smaller the opening, the larger the zone of sharpness.

Metering a scene

So, if I want to make a picture in this setting under these lighting conditions, I must first check the lighting conditions. This procedure is called metering the scene.

1. Set your camera to “Manual” exposure mode.

2. If indoors, set camera to its highest ISO (1600 or higher)

3. Point your camera at your subject.

4. Set your aperture to f4 or f5.6

5. Changing your shutter speed by turning the dial with your thumb until you see the meter is centered at zero.

6. Note what shutter speed you get.

7. Use this handy paper exposure calculator, cut out the 2 circles and  align the shutter speed you used in Step#6  to f5.6.

8. Read off all the possible combinations of shutter speeds and aperture on this dial to see all the possible equivalent exposures.