The Triangle of Exposure

Exposure -  is the process of recording light onto your digital sensor. It is basically the amount of light (controlled by the aperture) that is captured over a specific amount of time (controlled by the shutter speed). Exposure is determined by three camera settings: aperture, ISO and shutter speed.

Aperture - is a circular opening in your lens that is adjustable from a very small circle to almost as large as the lens itself. We adjust it to let more or less light to hit the digital sensor. Aperture is denoted as fstop and controls along with shutter speed how much light hits the sensor. Big hole, more light hitting the sensor and small hole means lesser light hitting the sensor. A higher aperture number (fstop) means a lower opening and a lower aperture number (fstop) means higher the opening. Think of aperture numbers as denominators (bottom number) of a fraction and it would be easy to remember.

Aperture along with distance to your subject and lens focal length determines depth of field (DOF). The smaller the fstop (bigger number) the deeper the DOF, where everything is in focus. And the bigger the fstop (smaller number) the shallower the DOF, where only the subject is in focus. And by adjusting the fstop values you can determine how much of the image is in focus.

Shutter Speed - A shutter is a curtain in front of the camera sensor that stays closed until the camera fires. When the camera fires, the shutter opens and fully exposes the camera sensor to the light that passes through the lens aperture. After the sensor is done collecting the light, the shutter closes immediately, stopping the light from hitting the sensor. The button that fires the camera is also called “shutter release button” or “shutter button”, because it triggers the shutter to open and close. 

Shutter speed, also known as “exposure time”, stands for the length of time a camera shutter is open to expose light into the camera sensor. If the shutter speed is fast, it can help to freeze action completely. If the shutter speed is slow, it can create an effect called “motion blur”, where moving objects appear blurred along the direction of the motion. This effect is used quite a bit in advertisements of cars and motorbikes, where a sense of speed and motion is communicated to the viewer by intentionally blurring the moving wheels. Slow shutter speed can cause camera shake which in turn can produce blurred images. So it’s always a good idea to use a tripod for slow shutter speeds.

Shutter speeds are expressed in fraction of a second 1/8, 1/125, 1/1000 etc. but in most modern digital cameras you may not see the 1/ expressed (but they still are fraction). You most likely see the above expressed as 8, 125, 1000 etc.

ISO - In Digital Photography ISO measures the sensitivity of the image sensor. The lower the ISO number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) – however the cost is noisier shots. Digital cameras convert the light that falls on the image sensor into electrical signals for processing.

The slow shutter speeds needed for dark interior scenes leave photographs prone to camera blur. If you raise ISO sensitivity, you can choose faster shutter speeds and reduce camera blur. This is why people say that ISO sensitivity should be raised if lighting is poor. ISO sensitivity can be set manually by the photographer or automatically by the camera.

No-Flash Low Light Photography - When lighting is poor, you can use a flash to light portrait subjects. Flash units, however, have limited range. If you raise ISO sensitivity, you can optimally expose both the portrait subject and the background without using a flash at all.

Noise - Raising ISO sensitivity allows faster shutter speeds, reducing blur caused by subject or camera movement. You may wonder why, if that’s the case, you shouldn’t simply always shoot at the highest ISO sensitivity setting, but in fact raising ISO sensitivity can introduce a type of image artifact known as “noise” into your photographs, making them seem grainy. Raising ISO sensitivity amplifies the electronic signal, which also amplifies any noise in the signal; as a result, the higher the ISO sensitivity, the more obvious the effects of noise on your photographs. The same is true of all digital cameras. We recommend that you raise ISO sensitivity only as high as needed to avoid blur.

Sources: Nikon Website, Photo Tuts+, Digital Photography School