Aperture and Aperture Priority Mode – Part I

Photo Perspectives participating photographer, ZB, captured the stunning image shown above.  Not only is it a great image, but an excellent starting point for a discussion on aperture.

Aperture seems to be one of those concepts that people sometimes see as a barrier to furthering their photographic knowledge because it is a fancy word that involves math.  To make matters worse, additional terms like f-stop, bokeh, and depth of field get thrown around with it.  Using appropriate terms is important, but not as important as understanding how simple but powerful aperture really is.

Aperture is the opening inside the lens that a photographer can adjust to let more or less light through to the camera sensor or film .

The aperture is an adjustable opening inside the lens that the photographer can open and close to let more or less light get to the camera sensor of film.  When a scene is dark, the photographer can make the opening for light to pass through wider (open up the aperture).  If the scene is bright, the photographer can make the opening smaller so not too much light gets through to the sensor.

The aperture is similar to the pupil of the human eye – the pupil gets really big when it is dark out to let as much light in as possible.  When it is bright, the pupil shrinks down into a very small opening to control how much light gets through.

©2022 HG


Unlike the human pupil, the aperture cannot be set anywhere between all the way open and nearly closed.  The aperture has specific points, or stops, between open and nearly closed where it is allowed to be set.  The setting or label that defines a specific opening size is the f-stop.

Each of the allowable aperture settings for the given lens are identified by a letter f, followed by a slash division sign “/,” followed by a number.  Some common f-stop settings are shown below.  The f and the numbers hold deeper meanings, but, as an introduction, one need only understand that lower numbers reflect larger opening sizes.  The aperture (size of the opening in the lens) gets smaller with increasing f stop number.

So, low f-stop numbers, like f/2.8, reflect wide openings that let a lot of light through the lens quickly; whereas higher f-stop settings, like f/16,  have smaller openings that allow less light to pass through the lens in the same amount of time.

Confused?  Perhaps the best analogy is considering a hose being used to wash down a wall.  If the hose has a big opening, more water can flow through more quickly than if the hose has a small opening.  A hose with a small opening will work – it will just take a lot longer to get the same result as the hose with the big opening.   If one thinks of the hose opening as the aperture, the water as light, and the wall as the camera sensor, a larger opening (low f-stop number) allows more light to reach the camera sensor quicker.

Some common aperture values are found below.

To recap, an f-stop like f/5.6 is simply a setting for how wide of an opening (aperture) in the lens the photographer chooses.  If the photographer ends up with a picture that is too dark at f/5.6, the photographer may want to open up the aperture to a larger setting like f/4.  If f/5.6 leaves the photographer with too bright of an exposure, the photographer may wish to close down the aperture to a setting of f/8.


The picture of glassware above was shot at f/11 with a 1/4 second shutter speed, and an ISO of 100.  If the ISO and shutter speed are held constant, the chart below demonstrates how changing the aperture in both directions impacts the final image.

The difference in light admitted by each of the apertures above is a factor of 2. f/2.8 lets in twice as much light as f/4 which lets in twice as much light as f/5.6 and so on.  Conversely, f/16 lets in half as much light as f/11 which lets in half as much light as f/8, et cetera.  You can see these differences in the images above each f-stop.

The most important things to know from all of this:

  • The adjustable opening in the lens that controls how much light gets through is called the aperture.
  • Photographers can set specific lens opening sizes by choosing an f-stop value.
  • More light is let through by f-stops with lower numbers.

Bonus things to know:

  • Making the aperture opening wider is called opening up the aperture or lens.  Making the aperture opening smaller is sometimes called closing down or stopping down the lens.
  • When a photographer uses the widest aperture for the lens, the photographer is said to be “shooting wide open” or “having the lens wide open.”
  • Lenses with f-stops of f/2.8, f/2.0, f/1.4, or wider are often called fast lenses because of how quickly such an f-stop gathers light.
  • A stop of light refers to the doubling or halving of the amount of light reaching the camera sensor.  Photographers will talk of “adding or reducing exposure by a stop” when referring to how much to increase or decrease exposure. 
  • Many DSLR cameras, like the Canon EOS series, can set lens apertures in 1/3 stops.  In other words, the doubling or halving of the light between full stops can be broken down into thirds to allow for even finer adjustments in how much light is coming through a lens.

Please be constructive and appropriate when posting comments below.  Many of our readers and contributors fall within the 5th through 12th grade age range.


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