Letter to a Complete Beginner

People seem to have a gift for making things more confusing by the simple act of trying to explain them.  I think that this has to do with the way figurative language can take on a life of its own. Although the tail may be said to wag the dog, it is a figure of speech; there is no point speculating how often or how far the dog's feet leave the ground. There is no dog.

Here's my attempt to confuse people about exposure, in a helpful way, which I wrote some years ago.  It has been used by photographic educators in several contexts, with reportedly good results.  The current snowy conditions in Britain make it seem timely.


Dear Complete Beginner,

A camera is a very simple device: a light-tight box with a hole in the front that can be open or shut (but not both) and a light sensitive medium held flat inside against the back will do it. If you want to get fancy, you can put a lens in front of the hole to improve the optical qualities of the captured image. And a means to make the hole bigger and smaller and another to open it faster or slower are definitely an advantage.

I know some modern cameras give an impression of scary complexity, but that's an illusion. The only meaningful choices are (a) how long the hole is open, (b) how big the hole is, and (c) how sensitive to light the chosen medium is. That's it. Everything else is packaging.

So, setting aside the business of sensitivity*, the technique of photography, if not the art, boils down to "exposure" i.e. choosing combinations of (a) and (b) above.

From a technical point of view, there is no difference between a "good" and a "bad" exposure. Both happen in the same way. The "correct" exposure is simply a matter of judgment: the one you prefer. However, most people tend to agree that a picture that is completely black is "underexposed", and one that is completely white is "overexposed". Pitching it somewhere in the middle, so that only the darkest dark is black and only the brightest bright is white, with a good spread of tones/colours in between, is generally considered the way to go. Achieving this is quite simple, but infuriatingly difficult to explain.

The root of the problem is that light meters are very stupid. It is well said that "To a man with a hammer, everything looks like a nail." A light meter is like a man with a hammer, except that to a light meter, everything looks like it is made out of grey wool. Imagine a world knitted out of grey wool, and you're thinking like a light meter. A certain quantity of light enters the light meter, and it thinks: "Today, this much light is being reflected from all that grey wool".  It then interprets that into camera terms, as an "exposure" that will perfectly render the grey-wool world outside.  Say, "At ASA 100, use shutter speed 1/125 at aperture f/16".

Of course, as well as grey wool, the world is made of black tarmac, white snow, faces of various colours and reflectivities, and so on. But the meter is a one-trick pony; it simply interprets a quantity of light as if it were reflected from grey wool. That's what it does.

So, when it receives light reflected off the proverbial black cat in front of a black wall, it thinks, "If a grey wool cat against a grey wool wall is reflecting so little light it must be very dark out there! Let in more light! Make the hole bigger! Or shut it more slowly!" But it's not as dark as that, it's just that our subject is very dark and reflects much less light than grey wool, and so we close the hole (aperture) down a click or two smaller than the meter says we should (or choose a shutter speed a click or two faster).  Because it's brighter than the meter thinks.

Similarly, when the incoming light is reflected off a white dog playing in perfect white snow the meter thinks "Well, if a grey wool dog playing in perfect grey wool snow is reflecting so much light, it must be very bright out there! Let in less light!  Make the hole smaller! Or shut it more quickly!" But it's not really that bright out there, it's just that our subject is very light and reflects more light than grey wool would, and so we open the aperture up a couple of clicks bigger (or choose a shutter speed a couple of clicks slower).  Because it's not as bright as the meter thinks.

This is very counter-intuitive: it takes a real effort to remember that bright white snow needs MORE exposure, not less.  Generally two to three whole stops more.  Every instinct screams, "It's too bright!  Shut it down!".  But not making this "exposure compensation" means that the snow ends up grey, not white, in your picture, and anything darker than the snow ends up black (the image is "underexposed").

Now here's the clever part: all cameras are made so that each stop ("step" or "click") of faster/slower shutter speed lets in or shuts out exactly the same amount of light as each stop of smaller/bigger aperture. So you can use either: two stops of slower shutter speed is the same as two stops of bigger aperture. Or both: one stop of slower shutter speed plus one stop of bigger aperture has the same result as the previous two: two stops more exposure. You have choices!

But, of course, you only really have choices if you stop using your camera in fully-automatic mode.  The next step is to become aware of the pictorial consequences of your available choices of aperture and shutter speed, but that's another subject.

A lot of words to explain something so very simple...  In summary, always try to remember (a) there is no dog, and (b) a snow scene requires 2-3 stops more exposure.


* Sensitivity:   this operates on the "no free lunch" principle.  A "slow" (i.e. less sensitive) film or sensor setting will give higher image quality, a "fast" (i.e. more sensitive) one will give lower image quality.  This assumes your definition of "quality" implies that "less grainy/noisy" is better, which it may not do if you are photographing fast moving objects like footballers on a dull winter's afternoon.  These days, digital cameras give much better results at "faster" speeds than film ever did.  400 ASA film was regarded as "fast" (with a marked increase in graininess over 100 ASA).  A modern digital camera will go up to at least 800 ASA with little decrease in quality.  To an ex-film photographer like me, this is as difficult to understand as Twitter.