In my last article, I explained what the basic of exposure is. In this article, we are going to go beyond that to understand further. We are going to go through the three elements namely Aperture, Shutter speed and ISO that affect the exposure.
What is aperture? An aperture is a hole or an opening of your lens that can be adjusted to make the opening larger or small to control the amount of light reaching the film or image sensor.
We are not going to the details of the F number or how it is calculated. However, the important thing to remember is the smaller the F number like F1, F1.4, F2, F2.8, the larger the aperture opening, the larger the number like F16, F22, F32, the smaller the aperture opening.
At the back of the camera between the lens and the film/sensor, there is a shutter curtain which opens when the photograph is taken. Shutter speed or exposure time is the effective length of time a camera’s shutter is open.
1/8000, 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1, 2, 4, 8, 15, 30
These are typical shutter speed a camera supports ranging from 1/8000 second to 30 seconds.
Below is the cross section view of a Digital Single Lens Reflex camera. You can see the opening of the lens which is the aperture and the shutter which open and close when taking photo.
The ISO in photography is the sensitivity of the film or image sensor to the light
Higher ISO numbers like ISO 1600, IO 3200 mean the sensor is more sensitive to the light. Lower ISO number like ISO 100 or ISO 200 mean the sensor is less sensitive to the light.
These three elements work together to determine the exposure and there are many combinations of these three elements that will produce “correct” exposure, meaning producing image close to 18 % gray.
For example, under certain condition and giving a fixed ISO number of say ISO 400, an aperture setting of F8 with shutter speed of 1/125 will give us correct exposure. If we change the aperture setting to say F5.6, the shutter speed that will give us correct exposure is 1/250.
In another word, the amount of light doubles going from F8 to F5.6 and from F5.6 to F4. The amount of light also double from shutter speed of 1/500 to 1/250 and to 1/125. We can complete this calculation with the following:
The amount of light reaching sensor doubles following the arrow direction:
Aperture: F22, F16, F11, F8, F5.6, F4, F2.8
Shutter speed: 1/8000, 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1, 2, 4, 8, 15, 30
ISO: 100, 200, 400, 800, 1600, 3200, 6400
Fortunately all the modern camera have auto mode feature to auto set the ISO, aperture and speed for us. If we would like more control, there are manual mode or semi manual mode like aperture priority and shutter speed priority where we set one of the value and the camera will automatically decide the other setting for us. We will talk more about that in next article.