(currently geared towards Canon dslrs in manual mode)

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The Aperture is the measurement of the size of the iris within the lens. The way to record this measurement is in f-stops. (EX: f/4.0)

The higher the f-stop number, the smaller the iris in the lens becomes. This also means there will be less light coming through the lens. The smaller the f-stop number, the larger the iris, the more light is let through the lens. (the aperture, f-stop, and iris basically all stand for/describe the same thing, “the measurement or mechanical piece” that determines the amount of light that is let through the lens, and the terms can mostly be used interchangeably.)

Side note: You can refer to the aperture/iris /f-stop as so… “What / where is your aperture at?” or “What’s your f-stop?”. This is not usually referred to as “iris” other than in technical terms and explanation.

On Canon “pro” and “semi-pro” camera bodies, in manual mode, the control for the aperture on the camera is the large thumb dial on the back of the camera that encompasses the “set button”. Rotate this thumb dial clockwise and (by default) the aperture measurement will increase. Rotate the thumb dial counter clockwise and the aperture/f-stop measurement number will decrease. (You will see the change happen on the top lcd screen as well as inside the viewfinder as green lettering.)

How to Control Aperture on Canon Pro/Semi-Pro Bodies

To do this control, for the aperture/f-stop, on a “consumer” Rebel body in manual mode, hold down the a/v (aperture value) button to the right of the lcd, and rotate the index finger wheel at the the top of the camera. Rotating this wheel to the right, while holding down the a/v, by default will increase the aperture/f-stop measurement. Doing this while rotating to the left will decrease the measurement. (You will see the change happen on the rear display as well as inside the viewfinder as green lettering.)

How to Control Aperture on Canon Rebel Bodies

(If you don’t see anything changing then tap the “shutter button” half way for no more than a second, this will activate the metering on your and camera in-turn allow you to adjust settings and see the setting changing as you change them.)