Camera Shutter Speeds and Apertures

So what's the difference between shutter speed and aperture?

Shutter Speed

Although, as discussed in exposure, the shutter speeds and apertures are interchangeable as far as exposure is concerned, they each have their own unique effect on the picture. Let's take a look at shutter speeds first as their effect is easily understood. We'll look at apertures further down the page.

The shorter the time that the shutter is open the sharper the photo will be.

If you are photographing fast moving objects such as cars or people running you need to select fast shutter speeds to capture the sharpest picture you can. One exception to this is when you are panning the camera with the subject, the object of the exercise here is to render the subject sharply and blur the background, so a careful selection of the right shutter speed to do both is necessary. I often find that a little blur in the right places on a picture gives a greater sense of movement than if everything is pin sharp. This blur, however, must be in the right places, normally we want to see the head and torso rendered sharply but, if the feet and hands are blurred, it can often be a good thing. Blurring the background can also get you out of trouble when there is a lot of clutter that will detract from the main subject. Getting the shutter speed right to render the correct balance of sharpness and blur on any given subject can really only be determined through trial and error. One of the great advantages of the digital camera with it's instant playback is that this learning process can be a lot shorter than it was before. If you have a zoom facility on your playback of pictures, now is the time to get familiar with it. I had my digital camera for quite a while before I realised that I could review my pictures and zoom in to check the sharpness.

Not only moving objects suffer from too slow a shutter speed. If you are holding the camera in your hand rather than having it mounted on a tripod, you will see the telltale signs of 'camera shake' (i.e. the movement of the camera) at shutter speeds longer than 1/125th of a second. A secure pair of hands will be able to get away with 1/60th or even 1/30th of a second but the camera would be better mounted on a tripod. Once again I will say at this point that the difference between a mistake and an effect is usually the degree. A small amount of blur would be considered a mistake, whereas really blurred streaks of light can be an interesting effect. It's all a question of convincing the viewer that you intended to do it.

Tip - When the shutter speed is important as with moving objects, it's a good idea to set the camera to 'Shutter Speed Priority' mode. This is where you select the shutter speed and the camera selects the appropriate aperture according to the light reading.

Of course, if you are taking photos of static objects like houses with a camera mounted on a sturdy tripod, you can leave the shutter open as long as you want without blurring. An interesting by-product of this, if you get to see really old photos taken in the first part of the 19th century, you will see that there are almost no people in the photos at all. That is because the exposure times were so long that the people had walked through the scene without being rendered. For the same reason the really early pictures, in the time of Niépce, the late 1830's, have almost no shadows because the plates took all day to expose and the sun moved across the sky illuminating the scene from both sides.

Click here for an example of using different shutter speeds.

Apertures

As well as letting more or less light into the camera the size of the aperture you choose governs the 'Depth of Field'. Depth of field means the amount of the picture, from foreground to background, that is in sharp focus. A smaller aperture will give you a greater depth of field and a larger aperture will give you a more restricted depth of field. This characteristic can be used to good effect in many ways.

If you are photographing vast landscapes on a sunny day, the chances are that everything will be in focus and you will not notice this phenomenon at all. Depth of field, or the lack of it, is much more noticeable when taking close-ups. As I mentioned in the section on moving subjects, it is often desirable to render the background of your picture out of focus. This is easy to achieve by selecting a larger aperture to restrict the depth of field.

Technical Stuff - Shutters Speeds and Apertures
What do the numbers mean?

If you look at the exposure display in your viewfinder you will see two numbers. On a normal sunny day you might see something like '125 16' or '500 5.6'. The first number is the 'shutter speed' and is simply the time that the shutter will be open for, expressed as a fraction of a second. So 125 means that the shutter will be open for 1/125th of a second, and 500 means that it will be open for 1/500th of a second.

The second number, sometimes referred to as the f-stop, tells you the size of the hole (aperture) in the lens. This number is also a fraction. The number represents the focal length of the lens divided by the diameter of the aperture. So an aperture that is 10mm in diameter in an 80mm lens will have an f number of f/8 and the setting f/16 on the same lens will be 5mm across.

From this you can see that if you change the lens to one of, say, 160mm focal length then the size of the f8 aperture will be 20mm. However, because the diaphragm is now twice the distance from the film the same amount of light will reach the film. This is a bit complex but if you have a mathematical bent and you draw it all on paper you will see why (see inverse square law). If not, just take my word for it. Now you can see that a larger 'f' number, say f/16, is actually a smaller hole and lets in less light than f/8.

	Large aperture = small f number
	Small aperture = larger f number

To make matters even more complicated, modern lenses, sophisticated beasts that they are, are not always physically the same as their focal length. So the good old f-stop acts as a nominal indicator of how much light will reach the film, rather than an accurate measurement of aperture size. This amount of light is independent of the focal length of the lens.

Also see my tutorial - ISO rating for Film Speed