Lesson Five: Reciprocity

In going from our Mark I to our Mark IV camera we've retraced a hundred years of the history of camera engineering. The Mark IV has all the essential pieces to do photography. There's only one problem: we still don't really know how to use it.

Digital: Again, everything on this page applies equally to film and digital cameras.

To use a camera we need one extremely simple but critical concept. To get a handle on it let's go back to the key fact from Lesson One:

Key fact: It takes a doubling of the actual brightness of light for us to perceive a single step in apparent brightness. Each step going from left to right in Fig. 3 reflects twice the number of photons as the one before it.

The modern camera has three main dials for controlling exposure: ISO, shutter, and aperture (f/stops). The markings on each of these dials seems arbitrary:

The ISO control on modern cameras has some sub-set of the following settings:

6 - 12 - 25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400

Each choice represents a doubling/halving of light sensitivity.

Then there is the shutter speed dial. Here are the typical settings again:

2' - 1' - 1/2' - 1/4' - 1/8' - 1/15' - 1/30' - 1/60' - 1/125' - 1/250' - 1/500' - 1/1000' - 1/2000'

And, of course, each choice represents a doubling/halving of exposure time.

Finally, the aperture dial:

f/1 - f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - f/64

Each marking represents a doubling/halving of the amount of light that can reach the film or the sensor of a digital camera from the lens.

Originally, the term "stop" was used as we used it in Lesson Three, to refer to the amount of light stopped by the diaphragm. Since then "stop" has taken on a generalized meaning in photography: any doubling/halving of brightness is called a one stop change in exposure value. Going from ISO 400 to ISO 800 – one stop. Going from 1/4 sec. to 1/2 sec. shutter – one stop. Going from f/8 to f/11 aperture – one stop.

To put this principle to use, let's first look at the normal way of operating. Your first job is to decide which ISO rating you are going to use for your shots and load that film into your film camera or choose that setting on your digital camera. That leaves you with shutter and aperture to control your exposure. (In Program or other fully automatic mode your camera may well automatically change the ISO setting for you based on input from the light meter.)

Key facts:

To increase your exposure by X stops you can either increase your shutter speed by X stops or decrease your aperture by X stops.

To decrease your exposure do the opposite.

To keep your exposure constant but vary shutter or aperture, change them by an equal but opposite number of stops.

Let's say the meter indicates the correct exposure is 1/30th sec at f/11. You decide to cut your exposure time to 1/500th second (perhaps to freeze the action of a ball game), which means going from 1/30th to 1/60th to 1/125th to 1/250th to 1/500th, which is four stops. To keep your exposure good, you have to compensate for this change by getting more light on to the film or sensor. So you go to a wider aperture: from f/11 to f/8 to f/5.6 to f/4 to f/2.8 – again, four stops.

1/30th at f/11 is equal to 1/500th at f/2.8. It's also equal to 1/60th at f/8 and 1/125th at f/5.6, etc. They're equal in that each lets in the same amount of light. Choose whichever combination gives you the best compromise between the depth of field you want and the shutter speed you want.

Your turn: You'll need manual mode (M on the mode dial) for this exercise. If your camera has a menu choice for switching from 1/3 stop settings to full stop settings, do that first. Now find a fairly bright scene to work with, find a good exposure that records the scene neither too brightly nor too darkly, Now vary the aperture and shutter inversely by (preferrably) one stop intervals and observe the lack of change in exposure.