Question: I’m buying a new lens. How does an aperture work and does the number of diaphragm blades matter?
Answer: The amount of light that passes through a lens and reaches the sensor is governed partly by the size of an opening in the centre of the lens barrel known as the aperture. By default, the aperture is completely closed off until you take a shot, when it opens for a certain period of time that we commonly call the shutter speed. The aperture itself is created by the movement of a set of blades that make up the diaphragm. In this illustration, the lens has five diaphragm blades, which is generally the minimum number found on mainstream lenses. So how exactly are the number of diaphragm blades significant? In terms of overall image quality, the impact is relatively minimal and not really worth worrying about.
However, the number of blades will impact on the appearance of the out-of-focus areas of your images, also known as the bokeh. It shows up most in the blurred highlights, which will take on the shape of the diaphragm opening. The difference is clear to see in the two images below. Most photographers prefer the look of more circular bokeh, which is created by lenses that have a larger number of blades. For this reason, most lens manufacturers also build their diaphragms with rounded blades (as in the illustration above), keeping the aperture opening as circular as possible. The quality of bokeh tends to be of most importance to portrait photographers, who shoot regularly at wide apertures for out-of-focus backgrounds.