Exposure, Aperture and Shutter Speed
This tutorial will give you a basic understanding of aperture and shutter speed, so that you can get the right exposure and depth-of field every time you shoot.
Introduction Exposure is the amount of light that reaches your digital camera's sensor in order to capture an image. Without enough light your photo will be underexposed (dark). With too much light, your photo will be overexposed (too bright). Exposure results from a combination of two basic camera settings - aperture and shutter speed. Both control how much light reaches your camera’s sensor. You can use different combinations of aperture and shutter speed to get a perfect exposure. However, different combinations will result in different depths-of-field. (DOF). In the interest of keeping this tutorial simple, our discussion will focus on aperture and shutter speed. We’ll touch on ISO at the end of this tutorial. Aperture Inside every camera lens is an opening called the aperture. The aperture works in exactly the same way as the pupil in a human eye works. The aperture can open to a wide diameter that will let in a lot of light, or close to a narrow diameter that lets in less light. The aperture’s diameter is measured by a number, referred to as an ‘f stop’. A constant source of confusion for beginners to photography is the fact that a smaller f-stop number represents a wider aperture diameter (which allows in more light), and a higher f-stop number indicates a narrower aperture diameter (which lets in less light). Typical aperture numbers are f2.0, f2.8, f4.0, f5.6, f8.0, f11, f16, f22, f32. Aperture diameter and f-stop number An important thing to learn is that these f-numbers are spaced one 'f-stop' apart. This means that by moving one f-stop up or down, you either double or halve the amount of light the aperture admits into the camera. In addition to controlling the amount of light that reaches your camera’s image sensor, aperture also determines the depth of field (the amount of the scene which is in focus) of your photo. A wide aperture (small f/ stop) will produce an image with a blurred background and sharp foreground, or area of focus, and a small aperture (large f/ stop) will produce an image with sharpness across more of the image. Shutter Speed Shutter speed controls the length of time that your camera’s shutter is open, allowing light to reach your camera’s sensor. Conventional shutter speeds are indicated by numbers such as 4000 (1/4000th of a second), 60 (1/60th of a second), 1” (1 second), and 30” (1/30th of a second). These indicate of the amount of time the shutter stays open to allow light in. A fast shutter speed will let in less light than a slower shutter speed. The most common shutter speeds are between 1/500 to 1/60. If you want sharp photographs while holding the camera in your hands, you cannot use shutter speeds much slower than 1/60 because it's hard to hold the camera steady. Generally, a fast shutter speed will freeze action (such as a hummingbird hovering over a flower), while a slow shutter speed will show motion blur (such as the soft blur of water in a waterfall). Getting the Perfect Exposure Light Meter To find the perfect exposure, your camera’s built-in light meter measures the amount of light that is reflected off the subject you are focusing on. Look in your camera’s viewfinder (when in any mode besides ‘automatic’), and you will see a line or bar that will look similar to one of the following: When the pointer is on the center hash mark, you have the perfect exposure (according to your camera’s calculation). Choosing the Right Combination of Aperture and Shutter Speed Aperture and Shutter Speed are Directly Connected If you open or close your aperture setting, you will have to compensate by either halving or doubling your shutter speed setting if you want to maintain a perfect exposure. Conversely, if you increase or decrease your shutter speed, you will need to compensate by changing your aperture setting correspondently. i.e. f/8 at 1/125 sec = f/5.6 at 1/250 sec = f/4 at 1/500 = f/2.8 at 1/1000 = f/2 at 1/2000. So, while there are many combinations of aperture diameter and shutter speed that will give you a well-exposed photo - which should you choose? That depends entirely upon the type of photo you are trying to capture. A close-up of a flower with the background blurred? A landscape shot with everything in focus? A shot of a bird frozen in action? A wider aperture (small f number) decreases the depth-of-field. Typically, it will throw the background out of focus. This is often used for portraits and close-up photos where you don’t want the background to interfere with your subject. A wide aperture (allowing a lot of light in) means you will have use a faster shutter speed. A narrower aperture (large f number) increases the depth-of-field. In this case, everything from the foreground to the background will be in focus. This type of aperture setting is commonly used for landscape photography, so that all of the scenery in the photo is in focus. With a narrower aperture setting, you will have to use a slower shutter speed in order to avoid an underexposed shot. A Brief Word About ISO and Noise: Your camera’s ISO setting controls how sensitive the image sensor is to the amount of light present. The higher the ISO number, the more sensitive the image sensor is to light, thereby allowing you to take photos in low-light situations. Suppose your digital camera's light meter warns you there is not enough light to correctly expose a scene. You can increase your ISO until your light meters shows that the increased sensitivity will allow you to obtain a correctly exposed photo. However, the increase in sensitivity obtained by increasing the ISO has the unwanted result of introducing noise (similar to ‘grain’ in film) into your image. Noise produced by an image sensor is undesirable and appears as distracting colored dots in areas of your image where there should be none. Tip: The best image quality is usually obtained at the lowest ISO setting on your digital camera. Usually, this means an ISO of 100 or 200. For best image quality, always shoot at the lowest ISO possible. |