Aperture Basics

Apertures, irises and f-stops. What do they all have in common? For starters, they’re all terms for the same thing—the size of the opening inside a lens that determines the amount of light that is able to pass through during a given exposure. As shutter speed dictates time, the aperture dictates amount.

Since apertures can be a bit tricky to understand for beginning photographers (after all, small numbers represent large openings and there’s a mystifying correlation between the size of an opening and “speed”—here’s a handy printable cheat sheet full of aperture information that can be carried around in a camera bag for quick consultation at a moment’s notice.

Before I send you off to interpret the graphic on your own, though, here’s a quick rundown of the principles illustrated herein.

– Apertures are also called f/stops or the camera’s iris. (Video folks are especially fond of the term “iris.”)
– Small number, big opening. Big number, small opening. It might seem backwards, but it’s true.
– Large apertures allow more light into the camera, which means a faster shutter speed can be used. Thus, a “fast” lens has a wider maximum aperture.
– The phrase “wide open” means a lens set to its maximum aperture to allow the most light into the camera. A “stopped down” lens is set to a smaller aperture.
– The sharpest aperture on a given lens is found two to three stops from wide open. This is partially due to diffraction, which is the bending of light that occurs at small apertures.
– Apertures are measured in full stops, and most digital cameras allow users to set the fractional stops between the full stops in increments of one-half or one-third stops.
– A one-stop change in aperture represents a doubling or halving of the amount of light. This is useful in making a corresponding shutter speed changes to create a correct exposure while adjusting for depth of field, motion blur, etc.
– Depth of field—the area in a scene that is usably sharp—increases as the size of the aperture opening decreases. Focusing distance and focal length also impact depth of field as well.

For something that seems so simple, there sure are a lot of aperture nuances. Print out this quick reference guide, though, and after a bit of practice you’ll be well on your way to building a fundamental understanding of f/stops. Download The Aperture Guide here.

If you’d like to help other new photographers with some aperture advice of your own, please share it in the comments below.

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