While this is certainly bound to confuse more than a few readers, the important thing to understand is that aperture diameter changes at different focal lengths in order to maintain the same aperture value (and thus the same effective quantity of light). Think of it as an equation with f representing your focal length in millimeters, over a set aperture value (ie 2.8), which will give you a set aperture diameter in millimeters, which correlates to light projection through an object. This sort of conceptual mathematics is also what should help you to understand that the lower the aperture value number (ie. f/2.8), the MORE light will be allowed through the lens, while a higher aperture value number (f/22) will let LESS light through the lens. This is due to the inverse nature of dividing the focal length by the size of the opening. As the opening shrinks (referred to as a narrower aperture) the resulting number gets larger.
If you’re thoroughly confused, talk to your high school math teacher, or just know that with aperture, smaller numbers equate to more light and wider openings, while higher numbers mean less light due to smaller openings. It’s backwards. And it’s math. But most importantly, know that it’s backwards. As far as I know, this is the only place in photography that this happens, and as such can be tough to wrap your head around, but once you get it, hopefully it will stick.
What is an F Stop?
While studying aperture and aperture values, you may have heard the term F stop thrown around from time to time, and may be wondering what that’s all about. While a “stop” change amounts to twice or half the amount of light, regardless of the mechanism used to make that change, the term “F stop” is referring to a stop of light as it relates to aperture, or effectively the difference in quantity of light when doubling the aperture value. As such, F stop and aperture value are often used interchangeably.
The term stop can be used when adjusting ISO, shutter speed or aperture, and as such can be a little misleading when only referring to a camera’s aperture value or “F stop.” It can get even more confusing because most modern cameras allow the user to change aperture values in increments of ⅓ stop values. Therefore, while f/2.8 to f/4 is considered one full stop (as the size of the aperture effectively doubles, thus doubling the amount of light), to change your F stop from f/4 to f/4.5 is not a full stop.
This can be a little misleading for folks when they start trying to change their F stop one stop. As a result, I typically use the term aperture value, and avoid using the term “stop” unless I mean to have someone actually double their exposure.