I reckon that it is a good idea to compile a set of terms and concepts related video and video editing; being my passion and all. First I will start with a few terms concerning the Digital Video Camera and then move onto editing terms and concepts. Once I have enough, I will add a separate page with all this information that can be accessed independently.
So onto the Iris.
The Iris is a device in the lens of the camera that has an opening allowing light to fall onto the sensor or film. That opening is called the aperture and is adjustable in increments or stops. Each stop or f-number allows half the light in as the previous one. The term f-number or f-stop is used to define the ratio of the focal length to the diameter of the aperture. Where focal length in a digital video camera is the distance from the sensor to the rear of the lens. But why the funny numbers? f/2, f/2.8, etc?
f-stop# = ƒ/D
Each f- stop allows half the light of the previous, so to let in all the light through then the aperture would be fully open with a diameter almost equal to the diameter of the lens. Therefor a camera with a 72mm lens diameter would have the aperture open to 72mm for a f-stop of f/1. For half of the light to be let in then the area in the aperture opening would have to be halved. So using πr² to determine the area of the fully open aperture we get 4071.5 and half of that is 2035.75 which is a diameter of 50.9. This then is 1/1.4 of the full diameter of f/1.4, and so on. As you see the D in the equation is a ratio of the Aperture to the Lens Diameter and each stop is a relationship each halved area of the aperture to the full area.
Ok the truth is that you don’t need to know all this, but it is certainly nice to understand it. In the end it is really about how you use the iris as a tool to set the exposure correctly, that is to control the amount of light that hits the sensor. Too much light or too little and you will loose detail.
One would either use the eye to gauge the best setting or a light meter, in either case you would want to expose for the subject or object of interest – not the background.
The setting you choose can be influenced by the shutter speed as well. A slower shutter will allow the light to hit the sensor for longer and therefor cause more exposure – meaning that you may have to close the aperture a bit. However this sort of balance is required for more professional camera; video and still.
The f stop does play a role in another area of the camera; the Depth of Field which is the the area that remains in focus, more on that in another article.