In a correctly exposed image just enough light has reached the sensor of the camera. Modern digital cameras are superb at getting the exposure right. I can't even remember the last time I saw an image online that was grossly overexposed or underexposed. In overexposed images too much light has reached the sensor. They look washed out with large areas of pure white. In underexposed images not enough light has reached the sensor. They look dark with large areas of pure black. However, if a wrong exposure is subsequently corrected with an image processing software, both types of images tend to look grainy and blurry, and they lack contrast. That's why it's sometimes difficult to tell if an image was underexposed or overexposed, but if the exposure is off, it will be very obvious that something is wrong with the image, even for somebody who knows nothing about photography.
However, if digital cameras are so good at getting the exposure right, why not let the automatic do it's job and forget about the whole process? For each photo there are countless ways to achieve a correct exposure, and for the end result of the image and its sharpness, it matters a great deal not only that you get the right exposure, but also how you get it. It is impossible to take control of this when you shoot in fully automatic mode. Modes like macro, sports, portrait, and so on are a crutch to give people some control, but save them from having to understand the basics. As it is, I have never seen a camera that had a mode for 'fish' or 'aquarium', and understanding the basics about exposure is not hard. Once you do, you can put your camera into your own personal fish and aquarium mode!
There are three factors that determine exposure: shutter speed, aperture, and sensitivity of the sensor to light (ISO). Raising and lowering each of these values has certain pros and cons:
Shutter speed: This is about how long the shutter stays open, and is measured is 1/s. Short shutter speeds help to avoid camera shake, and they also freeze motion. If you want to have a sharp picture of a fast fish, you want to use a fast shutter speed, that is one where the shutter stays open for a very short time. To give an example , for freezing motion 1/1000s is better than 1/60s. Unfortunately a fast shutter speed also means you get little light on the sensor. You need to choose the other values so that you still get enough light to achieve a correct exposure.
Aperture: This is about how large the opening in your lens is, and it is measured in f numbers. A large f number like 16 means a small opening in the lens, and a small f number like 2.8 means a large opening in the lens. The smaller the opening in the lens is, the more depth of field (DOF) you get, meaning a wider area of the subject will be in focus. Only the area in focus will be shown as sharp, not blurry. With a small DOF of f2.8, only the eye of one fish might be in focus and everything else looks blurry. With f5.6 you might get the whole fish in focus, but the other fish in the frame is still blurry. Maybe with f8 both will appear sharp. Obviously you want to use a large f number to get as much as possible DOF, but a large f number means a small aperture, that is a small opening in the lens that lets only a small amount of light to the sensor. Again, you will need to choose the other values so that you still get enough light on the sensor to achieve a correct exposure.
Sensor sensitivity: This is about how sensitive your sensor reacts to incoming light, and it is measured in ISO. At ISO100 the sensor is a lot less sensitive to light than at ISO1600. At ISO1600 you need a lot less light to reach the film to achieve a correct exposure than at ISO100. Unfortunately a high sensitivity means you will get a grainier image. At ISO1600 an image will be grainier than at ISO100. If you want to avoid graininess, you must choose a low ISO, but once again you will need to choose the other values so that you achieve a correct exposure.
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