BACK TO BASICS #1 – APERTURE
The aperture is the diaphram in your lens which controls the amount of light entering the camera. (The shutter controls the time that the light falls on the film/sensor) The diagramme attached to this post illustrates the effect of closing the aperture diaphram, and how it changes the light falling on the film/sensor.
The aperture is opened and closed in a series of steps (known as ‘STOPS’) They are measured as divisions of the FOCAL LENGTH of the lens. The stops are usually f/1.8, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 (digital lenses can have part stops, usually in thirds, between the full stop measurments.)
So, if the focal length of a lens is set at 50mm, f/4 (focal length divided by 4) would be 12.5mm.
The widest apertures are the lowest f/stops, and the narrowest apertures are the higher f/stops.
What affect does the aperture have on the image? The relationship between the aperture and the speed of the shutter are determined by ‘The law of reciprocity’ as the aperture opens, the shiutter speed must reduce, abd vice versa.
A wide aperture therefore, allows a higher shutter speed. This can be helpful if you want to deepen the DEPTH OF FIELD (we’ll look at depth of field in another post), to throw the background out of focus, for example in portraiture. As you ‘stop down’ the lens making the aperture smaller, the shutter speed must slow down to allow the correct exposure on the film/sensor. This is helpful if you want to blur motion in the image, like, when you photograph a waterfall.