Aperture Demystified (If Only Just Slightly)

Aperture (and to some extent, shutter speed) have long confused budding photographers. I’m sure we have all said, “You mean to tell me that the larger the aperture, the smaller the number?”

What is Aperture?

In photography, the aperture defines the size of the opening in the lens, which in advanced cameras can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of the shutter speed, and variation in film speed (ISO), this will regulate the photograph’s degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure a sufficient exposure to light, just as a slow shutter speed will typically require a smaller aperture to prevent excessive exposure to light. (Definition courtesy of the Wikipedia.)

The Larger the Number the Smaller the Opening?

Each f-stop has twice the area of the next full f-stop. So, f/8 lets in twice the light of f/11 and half the light of f/5.6. The number is actually a mathematical statement that says how large the aperture is in comparison to the lens. I.e., if you have a lens with a diameter of 40 mm, then f/2 would mean that the diameter of the aperture is 20 mm and f/4 would mean that the diameter of the aperture is 10 mm.

 
Enough with the Math and Definitions: Tell Me Why This is Important

Three simple reasons: depth of field, light, and motion. Apertures settings affect these items in multiple ways.

Depth of field is the zone of acceptably sharp focus in a photograph, as exampled by the image to the right. A F2.8 aperture will have a small depth of field relative to a F16 aperture (which will have a much larger depth of field).

The combination of light and aperture determine shutter speed, which affects how motion is displayed in photographs. An aperture of F2.8 will allow a lot more light into your camera than an aperture of F16. So in order to get a correct exposure, the shutter will be open less time at an aperture of F2.8 than if the aperture is F16. This means that motion is more likely to be displayed at an aperture of F16 than at an aperture of F2.8.

The following table to displays the differences between a small aperture and a large aperture:

Aperture Demystified
Aperture	F2.8	F16
Depth of Field	Shallow	Wide
Size of Opening	Larger	Smaller
Light	More	Less
Shutter Speed	Faster	Slower
Motion	Less Visible	More Visible

Posted by Matt Harris on Sunday June 19, 2005