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"How To Take Better Pictures By Knowing Exactly What You Are Doing"
... or at least
"How To Know Exactly What You Are Doing"
Most of what will follow in this document can best be experienced
using a camera that can be operated in full manual
Don't get me wrong: I'm not saying that you cannot take perfectly-exposed pictures with everything on "Full AUTO", just don't expect to learn much about photography in the process :-)
When you take a picture of your favourite subject (let's suppose it is a simple one, for the moment: an evenly-lit meadow will do just fine), how light or dark it will eventually appear in the final image depends on THREE variables: Film (or Digital Sensor) SENSITIVITY (ISO), Diaphragm APERTURE and Shutter SPEED.
25 32 40 50 64 80 100 125 160 200 250 320 400 500 640 800 100 1250 1600
Each doubling in ISO corresponds to a doubling in the film (or sensor) sensitivity to light.
In photographic terms, the word STOP is used to indicate a 2-fold change in a given quantity. So, for example, it can be said that ISO 800 is 1 STOP "faster" than ISO 400, meaning that it is twice as sensitive to light. On the contrary, ISO 100 is 2 STOPs "slower" (i.e. its sensitivity to light is only a quarter of that of ISO 400).
The use of a higher ISO enables you to shoot using faster shutter speeds, but it also inevitably entails larger film grain (or "noise" in digital cameras), and therefore somewhat lower ultimate resolution - much as if your photos were mosaics made using larger tiles.
Once you have chosen the ISO you are going to use, you are left with only two variables for determining the TONE that your beloved meadow will exhibit on the final picture (i.e. how light or how dark it will result).
1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32
Each one of these seemingly bizarre numbers stands for a particular ratio of the focal length of the lens in use (f) to the diameter of the circular hole through which the light passes before exposing the film, and is generally indicated as f/n (e.g. f/4, f/8, ...).
This very ratio is what determines the amount of light that passes through a given lens in a given lapse of time.
Let us take into consideration a "normal" lens for a moment (f=50mm). If the diaphragm is set so that the diameter of its circular hole is 12.5mm, that particular setting will be indicated as f/n, where n = 50/12.5 = 4, i.e., f/4. If we now take this lens off and mount a 100mm telephoto (f=100mm), the longer optical path inside the lens will require a larger hole in order for the same amount of light to pass through to the film. To be precise, the diameter of the hole will have to be doubled. We then have: 100/25 =4, or f/4 again. This explains the convenience of using ratios, and not absolute numbers (such as 12.5mm, 25mm, etc.) to measure diaphragm apertures: we no longer need to perform any mental calculations in order to know how much light will reach the film when we change lenses.
What remains to be explained is the "strange" sequence of the f/numbers (1, 1.4, 2 ...). It can be easily verified that each of these numbers equals the product of the preceding one times the square root of 2. Since the intensity of the light that passes through a circular hole varies according to the AREA of the hole (which is proportional to the square of its radius), there is exactly 1 STOP of difference between each two consecutive aperture values. It is important to remember, however, that aperture values are actually ratios, and so, for example, f/4 will let in twice the amount of light as f/5.6, and not vice versa (1/4 is greater than 1/5.6 !).
One last observation: a real lens will never have the whole range of possible apertures, but will be limited to a more restricted range (e.g. f/2.8 to f/22). Moreover, many common lenses have a widest aperture setting that is intermediate between two standard values, such as f/1.7 (mid-way between f/1.4 and f/2) or f/3.4 (mid-way between f/2.8 and f/4). Finally, it is also always possible to use intermediate diaphragm apertures in between the standard values (many modern cameral allow for 1/3 or 1/2 STOP adjustments).
1 - 1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000
It is self-explainatory that, since each successive value is half of the preceding one, it will only let half as much light reach the film. In other words, we can say that, for example, a speed setting of 1/60 is 1 STOP slower than 1/125.
Most modern cameras sport a wider range of shutter speeds, sometimes reaching as fast as 1/8000 s; however, you will find that such high speeds are seldom, if ever, useful in the real world (unless you are using ultra-high ISO in very bright conditions - which you shouldn't anyway). On the contrary, the availability of slower settings, such as 2, 4 and 8 seconds is a bonus in many situations, such as night photography.
We have now come to the point where we should start to try and explain WHAT ON EARTH ALL THESE AVAILABLE SETTINGS ARE FOR.
I believe it is clear to everybody that to fill a bucket with water, you can either open the tap completely for a few seconds, or do it drop by drop in a much longer time.
In a very similar way, you can get your subject to show quite the same TONE of colour either by using a fast shutter speed coupled to a wide diaphragm aperture, or a slower speed coupled to a PROPORTIONALLY narrower aperture.
Let us suppose, for example, that your exposimeter suggests a shutter speed and aperture of 1/1000 @ f/2.8. Shooting with these settings, you will get a correctly-exposed meadow (neither too light nor too dark). If on the other hand you wanted to use a 1/60 speed, which is 4 STOPs slower, you could then set the diaphragm to f/11 (4 STOPs from f/2.8), and end up with yet another perfectly-exposed photograph.
Even though the TONE of the meadow will be the same, however, the two pictures will differ in a number of ways.
1/1000 @ f/2.8 - The fast shutter speed allowed the camera to "freeze" the motion of the moving subjects (e.g. a dog running in the foreground), but the wide aperture has thrown the trees in the background completely out of focus (they are blurred on the final picture).
1/60 @ f/11 - The slower shutter speed was no longer able to "freeze" the dog, which appears blurred, but the narrower aperture has rendered the trees sharp.
From this simple example, it is clear that there is never a single correct shutter speed / diaphragm aperture pair, but rather several equally acceptable choices, each one leading to a different effect.
In general, faster shutter speeds allow the photographer to "stop" the action when there are moving subjects. They are also used to minimise the risk of getting a blurred image because of camera shake when hand-holding the camera.
As a rule of thumb, you should always use a tripod when shooting with a shutter speed that is slower than the reciprocal of the focal length of the lens (e.g. 1/60 s with a 50mm lens, 1/125 s with a 100mm lens). It should be noted that this rule applies to 35mm-equivalent focal lengths, so when using a reduced-frame camera (e.g. DX, with a 1.5x crop factor), correspondingly faster shutter speeds should be employed (e.g. 50mm x 1.5 = 75mm, hence 1/80 s minimum; choose 1/125 if 1/80 is not available).
Slower shutter speeds are instead useful to stress subject motion - a particularly pleasing effect with streams and waterfalls. In these cases, a sturdy tripod is a must.
Diaphragm aperture determines the extension of DEPTH OF FIELD in front of and behind the main subject - that is to say, what will be sharp in focus and what will be blurred. Wider apertures (e.g. f/2.8, f/4) are useful to isolate the main subject against a uniformly blurred background - they are frequently used for portraits. On the contrary, narrower apertures (e.g. f/11, f/16) make for a greater depth of field, meaning that almost everything will be sharp - useful for landcapes, for example.