The technical goal of photography is to record the light entering the lens of the camera.
Suppose for a moment that photography was filling a five-gallon bucket with water. If you put one drop from an eye dropper in the buck the result would be almost imperceptible. You could keep putting drops in for a long time and eventually you’d have something that resembled a bucket of water. On the other had if you pointed a fire hose at the bucket for a couple of minutes – well there’d be water everywhere and you wouldn’t have a very good result. If you had the ability to turn the fire hose on for a very brief time you might also get a bucket full of water. If you could change the size of the bucket, a very small bucket would be more appropriate for the eye dropper and a very large bucket would be appropriate for the fire hose. In this analogy, you can see three things that work together to get a container of water: a) the size of the container, b) the flow rate of the water supply, c) the duration of the flow.
In your digital camera, the water is replaced by a stream of photons – light – reflected off the subject. The bucket is the image sensor. The aperture of the lens (f stop) is the size of the hose. The shutter speed controls how long the flow lasts.
So, when you hear people talk about “The Exposure Triangle” they are talking about the three things that you can set in your camera to get the desired exposure: a) shutter speed, b) f stop, c) Sensor ISO.
A small digression. The term ISO is probably an unfortunate misnomer that is widely used. ISO stands for International Standards Organization. Among other things, the ISO has established a numerical scale to indicate the sensitivity of an image sensor to light. Calling a number on that scale an “ISO setting” is like saying the numbers on your radio’s volume dial are “UL settings.” The important thing to remember is that the so-called ISO setting is akin to changing the bucket size in our water analogy. The higher the ISO number the less light it takes to get an image – think, higher number smaller buckets.
Within limits, the f stop and shutter speed are a reciprocal relationship. If you close the lens aperture one full f stop and double the shutter speed you will get the same exposure. For example, f/8.0 for 1/60 second is the same as f/11.0 for 1/30 second. Switching from f/8.0 to f/11.0 cuts the flow rate in half so we need to let it flow for twice as long. Now bring in the ISO setting. An exposure of f/8.0 for 1/60 second at ISO 100 if the same as an exposure of f/8.0 for 1/30 second at ISO 50. Dropping from ISO 100 to ISO 50 doubles the size of the buckets so we need to let it flow for twice as long.
Back in the days of film, ISO was fixed by the chemistry of the film, yes there were ways that special processing could alter the ISO by about a stop or two for some films but for the vast majority of photographers the ISO was what the manufacturer said it was. Go back even further and about the only exposure control was how long you took the lens cap off – subjects had to sit very still.
Today we have this triangle of settings at our disposal, but what is the right setting? We could argue what the meaning of right is but we’ll save that for part two. For today let’s just say that your camera is equipped with light measuring systems and it (or the engineer that built it) has an opinion of what right is.
There are two commonly used types of light meters. Those that measure reflected light and those that measure incident light. You point a reflective type meter at you subject and measure the light being reflected toward the camera. You hold an incident meter in front of your subject and measure the light striking the subject. (A flash meter is a special version of an incident light meter). Before cameras had built in light meters most serious photographers used hand-held meters and reflected meters were more common than incident meters. Today most hand-held meters are incident or capable of both incident and reflected readings with the few reflected meters are spot meters. The meter built into your camera is a reflected light meter. It’s quite obvious that for many subjects you can’t approach the subject to take incident readings.
To the meter the world is 18% gray. Meters are color blind and want to make everything look like it is reflecting 18% of the light striking it.
(Here’s an article that discusses the advantages and disadvantages of various metering devices. http://www.sekonic.com/united-states/classroom/meteringtechniques/benefitsofincident.aspx)
Why are there so many exposure modes on your camera?
Often times, a landscape photographer is shooting a still subject with the camera rigidly fixed on a tripod. Camera shake and motion blur are not a problem. In this type of shooting many photographers elect to use an automated mode called “Aperture Preferred”. The photographer sets the aperture (f stop) and the ISO. The camera selects an appropriate shutter speed. A sports photographer is more likely concerned with stopping motion and will set the camera to “Shutter Preferred” and select shutter speed and ISO and let the camera set aperture. Newer cameras have a mode where the photographer sets both aperture and shutter speed and lets the camera select ISO. Then there are many “Program Modes” where the camera will select all the setting based on the type of photography specified by the particular program mode. You will see on a lot of point and shot cameras a set of icons to depict the mode. The modes may include things like, close up, landscape, portrait, night, party, etc. Personally, I find that these program modes go a long way toward insuring bad photography! They make certain assumptions about light levels and shooting conditions that don’t always hold and they often don’t tell you what setting they selected making it difficult to know if you should override or select another mode. In manual mode, the photographer has to set everything.