The key thing to remember is that the central concept to making a photograph is controlling the amount of light that gets through the shutter and to the sensor. There are other considerations, but this one, which is called “exposure” is key. And there are only two things you can do to affect this: change the size of the shutter and change the amount of time it is left open.
Changing either of these settings has another effect on the resulting image aside from controlling exposure. The first setting, called aperture or f-stop, affects focus. Changing the size of the hole made by the shutter also changes the amount of your picture which is in focus (called “depth of field” – the less of the shot that can be in focus the “shallower” the depth of field). Large holes lead to shallow depth of field, small holes increase depth of field, at the expense of light, which can lead to underexposure.
The second setting, called shutter speed, will affect how moving objects appear in your image. Fast shutter speeds can “freeze” motion – a spinning ceiling fan, for instance, will have its blades appear sharp and motionless. As the shutter speed is slowed the motion of the blades will blur, until the blades disappear (having not stayed in one place long enough to register on the image). Note that as you leave the shutter open longer you lose the ability to hold the camera still enough to produce non-blurry pictures. For me, on a non-caffeinated day, this occurs around 1/60 of a second. As my friend Chris points out – the effect of camera shake is more pronounced the longer your focal length is. For a somewhat-wide-angle focal length of 17mm the length of the arc that a small shake makes in the final image is very short, so hardly noticeable. But at 300mm (telephoto) a small shake of the camera translates into a very noticeable blurring of the image. As Chris points out – “The rule of thumb is the reciprocal value of the focal length in secs – you can hold a 50 mm for 1/50th of a second, 300 mm for 1/300th. If you have a stabilizer, you can halve the shutter speed once per stage. So a three-stage stabilizer should get you up to about 1/40 sec for a 300 mm tele.” You can also brace the camera on something, or use a tripod. Of course, as the shutter is left open longer the amount of light allowed to hit the sensor goes up, eventually leading to overexposure. So if you want to have a long shutter speed to blur movement, you might have too much light to get the correct exposure even with the smallest aperture.
So as you see, even though there are only two things that affect exposure, with one other “side effect”, things still get complicated.
The simplest situation is where you have uniform light and a static subject. Then you can simply select the aperture that gives you the correct depth of field and the shutter speed to give you the correct exposure for that aperture and you’re good.
At the other end of the spectrum, imagine a scene where there is a small spray of water from a waterfall, which is in some sunlight that is filtered through the trees. You would like to blur the motion of the water (to illustrate the motion of the water) and you would like to blur the background to call attention to your subject. When you get the aperture you want for the shallow depth of field, you realize that the shutter speed required to have the correct exposure is too fast to blur the water. You then realize that even if you could expose correctly for the water the rest of the scene would be underexposed, likely just black. But of course if you expose for the woods around then the water spray would be overexposed, likely just white. What to do?
That is a situation I like to call “tricky”. For now about the best thing you can do is be aware of your light and when you see a “tricky” situation shoot at many different f-stops and shutter speed combinations and cross your fingers that something works out. This is called “exposure bracketing“. When it does work out, try to remember it for next time.
Shutter Speed is measured in seconds and fractions of seconds. Aperture (or f-stop) is measured in “f numbers” or “f-stops”. A small f-stop means a large hole and a large f-stop means a small hole. So, f/1.8 is a large hole. f/32 is a small hole. f/1.8 gives you a shallow depth of field, f/32 a large depth of field.
Ofttimes your camera allows you to set its controls to:
As a beginner it’s easiest to set your camera to full-auto and see if you like what it did. It’s educational to see what settings the camera selected. Be aware that your camera will look at a scene and attempt to set these numbers such that the predominant light or dark color appears as 18% grey. If either there is no predominant light or dark, or you don’t want the predominant light or dark part of your scene to appear 18% grey, such as a snowy field or a cave interior, this is a tricky situation. You will need to add exposure to the snow scene to get white, or take it away from the cave scene to get the black you want.
At times you might want to control the depth of field. For such times you can select the “Aperture Priority” setting and let the camera select the shutter speed.
At times you might want to control the motion-blur. For such times you can select the “Shutter Priority” setting and let the camera select the aperture.
Me, I tend to leave it on manual.
Addendum: Per this post there’s a bit more to deal with when it comes to aperture. To sum it up: your DoF depends on the aperture, the focal length you’re using and the distance to the subject.
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