Exposure is the amount of light that hits the image sensor or film. Correct exposure is obtained when an image looks right to the viewer, neither too light nor too dark.
There are 3 related variables that together determine ‘correct’ exposure: ISO, f stop (aka aperture ) and shutter speed. We will be addressing each of these in great detail as the class moves forward. Here in the simplest terms, is a introduction:
To obtain the ‘correct’ exposure we have three variables to juggle. As we adjust each of these, we are allowing either more or less light to be captured by our sensor. The light meter in our cameras give us a tool to measure light levels and obtain the desired exposure.
ISO is how sensitive your sensor (film) is to light. Just like in the old days with film, 100 is often good for bright outside shooting, while 400 is often good for inside shooting. ISO settings start anywhere from 50 to 200 and go up to 6400 or higher, depending on the manufacturer and model of the camera. e.g. 50, 100, 200, 400, 800, 1600, 3200, 6400 The higher the number, the more sensitive the sensor will be to light, allowing one to shoot in darker situations, at faster shutter speeds and / or smaller apertures.
Like shutter speed and ISO, the f stop setting is a factor in how much light hits the sensor when you make an exposure. Just as the iris in our eyes can open and close in response to different light levels, how the f stop is set will allow more or less light to pass through the lens and strike the sensor. e.g. f1, 1.4, 2. 2.8, 4, 5.6, 8. 11, 16, 22, 32……. The smaller the f stop number, the wider the lens opening and the more light will be let in during an exposure. These aperture settings also control depth of field, how much is in focus before and behind the focus point. The bigger the aperture number, the greater the depth of field.
Shutter speed controls how long the shutter is open while making an exposure. e.g. 4 seconds, 2 seconds, 1 second, 1/2 second, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000 etc. To freeze action a higher speed is needed, while slower speeds allow blurring through subject movement and/or camera movement blur.
Modern digital cameras allow for automatic and manual exposure settings. For the purposes of learning how exposure works, we are asking our students in level 1 to set the dial on top of your camera to ‘M’ for manual at first. Since dealing the 3 variables of exposure can be a lot to learn for a beginner, we are asking our students to set your ISO to 200 for all assignments, and we will focus our attention of f stop and shutter speed.
On top of your camera you’ll find a an LED readout. To obtain the correct exposure, you will need to ‘zero out’ your LED reading. Adjusting your f stop or shutter speed, move the line to the middle ‘0’ between + and –