Camera Exposure Basics: Shutter Speeds, Aperture, F Stops, and ISO In photography, exposure is a reference to how much light, and how long that light is exposed to the cameras' image sensor when the camera shutter release is pressed. In simple terms, the camera's exposure value settings determine if an image will be too light, too dark, or correctly exposed according to normal standards. The correct or desired exposure is obtained by using a combination of the camera lens Aperture, the Shutter, and the ISO rating functions. Those three functions are commonly known as the "Exposure Triangle". The cameras' exposure settings also determine the depth of field in an image. That is, how much of the picture, including the background and foreground appear to be in or out of focus. Additionally, the camera exposure settings determine if motion in a picture causes the subject to appear blurred or "frozen". Using your camera in the Automatic Mode works well most of the time. However, there may be times that the cameras light meter can be fooled by the lighting in a scene. When that happens, the automatic exposure settings might produce an under or over exposed picture. When you are using a Digital SLR camera or a Bridge camera, you can choose specific settings to adjust the exposure. However, if you are using a basic compact camera, the exposure settings are automatically set and cannot be manually adjusted. Try using the Exposure Compensation EV feature or try using one of the Scene Modes of your basic compact camera if your pictures are not coming out the way you would like. What are the functions of the Aperture, Shutter, and ISO rating? The camera lens aperture, or more specifically, the size of the lens aperture opening determines the amount of light that reaches the camera's image sensor. (see the tutorial, camera lens aperture to view a diagram of the various sizes of the aperture opening.) The various aperture settings are called "F Stops". The aperture settings with the smaller F Stop numbers allow more light to reach the image sensor than the settings with the larger F Stop numbers. For example, the F8 setting (or F Stop ) will allow more light to reach the image sensor than the F11 setting. The shutter speed determines the length of time that same light will be exposed to the camera' image sensor. Shutter speeds are listed in seconds or fractions of a second. Those times reflect how long the shutter will be open and how long the image sensor will be exposed to the light. A shutter speed of 1/60th of a second keeps the shutter open longer than a shutter speed of 1/125th of a second. The ISO setting determines how sensitive the camera image sensor is in relation to the available light. A camera set to ISO 400 will need less light to produce a correctly exposed image than when it is set to ISO 100. You can use various combinations of the shutter speed, aperture, and ISO settings to arrive at the same "correct" or desired exposure. Listed below are some standard aperture settings, shutter speeds, and ISO settings. (many cameras will have settings in increments between those listed here. But we will just use these to keep things simple) Changes in the setting of any of the above functions are referred to as "Steps". For instance, changing the camera lens aperture setting from F4 to F8 would be called two steps up. Changing the setting from F11 to F8 would be considered as one step down. The same terms would be used in reference to changes of the shutter speed or ISO setting. Changing exposure settings Lets say you are taking a picture of a scene and the camera's light meter suggests an exposure value of F8 aperture, 1/125 shutter speed at ISO 100. However, the image appears slightly too dark in your viewfinder or LCD screen. Thats not much of a problem. You just need to allow a little more light to reach the image sensor. Changing the aperture setting from F8 to F5.6 or F4 will make the aperture opening wider. That in turn will allow more light to reach the sensor. And hopefully, one of those settings will give you the desired effect. If you prefer to change the shutter speed instead of the aperture setting, change the shutter speed from 1/125 to 1/60 or 1/30. This will allow the light to reach the image sensor for a longer period of time. It will also lighten the picture the same as when the aperture setting was changed. The point is, changing the shutter speed one or two steps up or down will have the same effect as changing the aperture setting one or two steps up or down. (or however many steps you may want to use). Here are a some more examples of when you might want to adjust the suggested exposure settings. Changing the aperture setting: Lets say you are taking an outdoor daytime portrait. The camera's light meter indicates that a correct exposure setting is an aperture setting of F8 and a shutter speed of 1/60. However, while you are looking through the camera viewfinder to compose the picture, you realize that the background is too busy and is drawing attention away from the subject. No problem. Your aperture setting determines depth of field. That is the amount of background and foreground that will be somewhat in or out focus) Simply change your aperture to a wider setting to try to throw the background out of focus. Lets say you choose the aperture setting of F2.8. With that setting the background should become pretty much out of focus and less distracting. You can now concentrate on your subject being the center of attention instead of background objects. But Wait!! When you changed the aperture setting from F8 to F2.8, the aperture opening size was increased by three steps and a whole lot more light will reach the sensor. If you take the picture now, it will be way overexposed. Changing the shutter speed: To get back to the proper exposure, you will need to decrease the amount of time the light is being exposed to the sensor. Just increase the shutter speed three steps by changing the speed from 1/60 to 1/500. This will cut back the amount of time the light is exposed to the sensor by 3 steps. You are now back to having the ideal exposure setting again. Sometimes you might be more concerned about the shutter speed than the aperture setting. For instance, you might be trying to photograph a moving car using the settings from the previous example. (F8 aperture and 1/60 shutter speed) These settings might give you a blurred picture of the car because the shutter speed of 1/60 might not be fast enough to "freeze" a very fast moving subject. To avoid a blur in the photo, Simply increase the shutter speed 3 steps to 1/500. (or to the faster shutter speed of your choice) Next, increase the aperture opening 3 steps from F8 to F2.8 You will then have a properly exposed picture of the car without the blur. Remember this: Once the correct exposure has been determined, the aperture and shutter speed settings can be changed in direct relation to each other. This exposure chart shows a few examples: Suggested Setting for Correct Exposure: Aperture F8 Shutter Speed 1/60 Alternate Settings to obtain Same Correct Exposure Aperture F11 F5.6 F4 F2.8 Shutter Speed 1/30 1/125 1/250 1/500 Changing ISO setting Lets not forget about the ISO function we mentioned earlier. The ISO is a measurement of the digital camera sensor sensitivity to light. A camera set to ISO 400 is more sensitive to light than when it is set to ISO 100. What that means is that at ISO 400, the camera image sensor needs less light to produce a properly exposed image than when the image sensor is set to ISO 100. Check the tutorial on ISO for more information. Let us once again use the example of an F8 aperture and 1/60 shutter speed as being the correct exposure setting suggested by the camera's light meter. Lets also assume that the ISO setting was 100 when the reading was made. If we change the camera ISO setting from ISO 100 to ISO 200, less light will be required to make the proper exposure. Therefore we can either decrease the aperture size to F11 (one step) and leave the shutter speed at 1/60, or we can increase the shutter speed to 1/125 (one step) and leave the aperture setting at F8. Here are a couple of examples showing how your exposure settings will change when you change the ISO setting. (assuming a suggested exposure of aperture F8 and shutter speed 1/60 at ISO 100) Changing Aperture Changing Shutter Speed ISO Aperture Shutter Speed ISO Aperture Shutter Speed 100 F8 1/60 100 F8 1/60 200 F11 1/60 200 F8 1/125 400 F16 1/60 400 F8 1/250 800 F22 1/60 800 F8 1/500 Always remember that aperture, shutter speed, and ISO are inter-related. It might seem a little tricky at first, but after you change the aperture/shutter/ISO combinations a few times, it will become like second nature to you. Check the Camera Aperture tutorial or the Shutter Speeds tutorial if you need more details about each function. The camera's aperture and shutter speed settings can be manually changed when you are using a Digital SLR camera, an Interchangeable Lens camera, or certain Bridge Compact Cameras. If you are using a basic Point and Shoot Compact camera, the exposure settings are automatically adjusted and cannot be manually changed. However, the principles listed below are still at work in your camera every time you take a picture with a Point and Shoot Compact camera. Check out the Automatic Shooting Modes tutorial to see how your camera applies these principles when a camera is used in the automatic mode. |
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