The aperture is an opening in the lens, made by a group of thin metal blades, which controls the amount of light that enters your camera to form an image. The aperture has two main effects on exposure – it alters the amount of your image that appears in focus, and it also affects how long you will need to open the shutter for the correct exposure.
What’s an ‘f’ stop?
The word ‘stop’ comes from the very first cameras. Instead of a diaphragm of blades as used in modern lenses, early photographers had to insert a piece of metal into the lens with a hole drilled through it. This piece of metal was called a stop because it stopped some of the light getting through the lens.
Numbers marked on a lens are sometimes referred to as relative brightness and are expressed as a ratio to one. e.g. 1:2.8 would be f/2.8. The closer to zero the number is, the brighter the aperture.
Why have more blades?
The curvature of the aperture and the number of blades it’s made of has a direct effect on how smooth out-of-focus backgrounds appear. The more blades used the rounder the aperture, leading to smoother background blur, known as bokeh.
Another phenomenon affected by the aperture are diffraction stars. These occur when a strong point source of light is partially obscured in the image. The result is a bright, multi-pointed star radiating from the light source. The number of points on the star is influenced directly by the number of aperture blades. Lenses with an even number of blades create stars with an equal number of points, so a lens with a six-blade diaphragm creates a six pointed star. Apertures made from an odd number of blades create stars with double the number of points. A nine bladed diaphragm creates a beautiful 18 point star.
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