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Notes on Lens Aperture, Shutter Speed and ISO

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Notes on Lens Aperture, Shutter Speed and ISO Notes on Lens Aperture, Shutter Speed and ISO Many of us use the automatic settings on our cameras and most of the time this produces good, or at least acceptable, results. However, if you want to become more creative when taking photographs it is important understand the manual settings on a camera and in particular lens aperture, shutter speed and ISO and the relationship between these. In summary lens aperture is the size of the diaphragm or hole in the lens that the light goes through to reach the light sensitive screen. Shutter speed is how long the hole is open for and ISO measures the sensitivity of the image sensor (in digital photography), or, in traditional film photography, it measures how sensitive a film is to light. Lens Aperture Lens aperture is measured as an f/number or f-stop. The smaller the f/number the bigger the “hole” in the lens. Technically it is a ratio of the focal length of the lens and the diameter of the “hole”. For example, a lens of 50mm focal length and an aperture of 25mm gives an f/number of f/2. An aperture of 3.125mm gives f/16. The main intervals in the aperture settings are f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Every lens has a limit on how large or how small the aperture can get. The maximum aperture of the lens is much more important than the minimum because it indicates the “speed” of the lens. A lens that has an aperture of, say, f/1.2 or f/1.4 as the maximum aperture is considered to be a fast lens, because it can pass through more light than, for example, a lens with a maximum aperture of f/4.0. That is why lenses with large apertures are better suited to low light photography. Why is the selected aperture important? The chosen aperture will determine how much of the scene is in focus. A small aperture (big number e.g. f/16) will have a greater ‘Depth- of-Field’ (DOF). DOF is the distance in front of and behind your subject that will also be in focus. For example, if you focus on a flower 8 feet away with a small aperture, everything from about 5 feet to 15 feet will be in focus. If you take the same photo from the same place with a small f/number (big hole) the flower (or most of it!) will be sharp but everything in front of the flower or behind will be out of focus. This may well be what you require to make the flower “stand out” from the background. With the camera set to automatic the resulting picture may well have everything in focus, the flower and the foreground, and, perhaps more importantly, the background It should be remembered that altering the aperture requires alteration of the other two variables – shutter speed and ISO setting, to get the right amount of light onto the light sensitive surface. Digital Single-Lens Reflex (DSLR) cameras and some compact cameras usually have two settings where you, the user, can control the size of the aperture: 1. Manual mode. With this setting the user has control of aperture, shutter speed and ISO. 2. Aperture Priority mode (“A” on most cameras, and for some unknown reason “Av” on Canon). With this setting the user sets the aperture and the shutter speed will be automatically set so that the right amount of light reaches the image sensor. With this mode the user also has control over the ISO. More on ISO later. Many pro photographers use the aperture priority mode most of the time. To illustrate the use of aperture priority mode have a look at the two pictures below. f/4 (shutter speed 1/2000th of a second, ISO 250). Note background out of focus. f/22 (shutter speed 1/50th of a second, ISO 250). Note background more in focus. Compare these with the picture below which was taken with the camera set to “Automatic”. Automatic setting. f/10 at 1/500th of a second, ISO 400 Notice that the automatic setting produced a picture with settings that I would call “middle of the road”. The picture is “acceptable” but you may require the background to be “more out of focus” which the aperture priority mode can give you. Notice also in aperture priority mode how the shutter speed changes to compensate for the change in aperture so that the same amount of light reaches the image sensor. When setting the aperture you will need to keep an eye on the shutter speed. Too low a speed could give you a blurred image due to camera shake. A golden rule is that shutter speed should be as long (or as short) as the focal length of the lens. If the lens has a focal length of 100mm the shutter speed should be 1/100th of a second or faster. Finally, for the mathematically minded consider the f/ values f/4, f/5.6, and f/8. Note that f/8 is double f/4 and this means that the hole in the lens is twice the diameter at f/4 compared to f/8. You will remember from schooldays that doubling the diameter of a circle increases the area by 4 times. Thus going, for example, from f/5.6 to f/4 doubles the amount of light getting to the image sensor. To compensate for this the shutter speed will have to change from. Say, 1/50th of a second to 1/100th of a second when changing the aperture from f/5.6 to f/4. I hope you are not too confused at this stage! All you really have to remember is the smaller the aperture (large f/number) the greater the depth of field. If you are not too exhausted let us go on to shutter speed. Shutter Speed Shutter speed can create effects such as freezing action or blurring motion. In simple terms shutter speed is the length of time a camera shutter is open to expose light onto the image sensor. Apart from Manual mode mentioned earlier most DSLR’s and some compact cameras have a setting called “Shutter Priority mode” (“S” on most cameras and again for some unknown reason “Tv” on Canon). This mode allows you, the user, to set the shutter speed and also ISO. When using a camera set to shutter priority mode the aperture will be set automatically, within the limits of the lens of course. Shutter speeds are measured in fractions of a second, for example, 1/250th of a second. Selecting the “correct” shutter speed will also help to avoid images being blurred due to camera shake. There is a rough rule-of-thumb that is easy to remember. A standard lens is 50mm, minimum speed to avoid shake is 1/50th of a second. A 150mm lens needs 1/150th of a second; a 500mm lens needs 1/500th of a second. It is important to choose the right speed to show movement but to retain sufficient detail to show the subject. Having said all the above the use of shutter priority mode is not commonly used. For example, nearly all professional sports photographers use aperture priority mode. Why is this? Because it is important to control depth-of-field in sports pictures and just keep an eye on the shutter speed to make sure the camera isn't picking one that is too low. If it does, then increase the ISO so that the camera will chose a faster shutter speed. Aperture/shutter speed relationship I have already touched on this at the end of the Aperture section. On a reasonably bright day if we point the camera at something whilst the camera is set to ‘automatic’ it will think the best aperture for most things is f8 and a reasonable shutter speed is 1/125th of a second, for example. If however we want to reduce the depth of field so that the subject is pin-sharp we alter the aperture to f4 which makes everything except the subject go blurry. However f4 is a lot bigger hole than f8 so more light will reach the image sensor and the picture will be too bright so the shutter speed will need to be increased, say to 1/500th of a second. When using a camera in aperture priority mode the shutter speed will be changed automatically as you change the f/number. Most DSLR’s and some compact cameras have a fully manual mode where you choose aperture, shutter speed and ISO. If the combination you chose will cause too much or too little light reaching the image sensor then the camera will advise you via what is referred to as an exposure value meter. Without going into more detail exposure value (EV) is a number that represents a combination of a camera's shutter speed and f/number, such that all combinations that yield the same exposure have the same EV (for any fixed scene luminance). Are you keeping up? If so I’ll go onto ISO. ISO In the good old days of film, anyone remember? This was referred to as ASA. ISO are numbers that try to relate the sensitivity of the image sensor to the sensitivity of films we used to use so that old photographers can grasp what is happening. Who said “old”!!! ISO is the level of sensitivity of your camera to available light.
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