How to Set F-Stop on Canon EOS 40D
If you want to create more arresting photographs, take advantage of an advanced digital SLR like the Canon 40D, which has multiple control dials to make changing settings on the fly much easier. Advanced photographers prefer to shoot in manual modes to control things like depth of field (how much of the image is in focus), which is controlled by the aperture (f-stop), or to stop motion, which is controlled by the shutter speed. Shallow depth-of-field shots have just the subject in focus with the background blurred out, which can look dramatic, while large depth-of-field shots have both the subject and background in focus.
Rotate the Mode Dial on the left side of top of the camera to either "A" or "M." "A" stands for aperture (f-stop), which is used to adjust the f-stop only. "M" stands for manual and means you are changing both aperture and shutter speed. The process for changing f-stop is different in the two.
Look through the viewfinder and rotate the Main Control Dial on the right side of the top of the camera (located just in front of the LCD screen). The aperture numbers will show in the viewfinder (they will also show on the LCD screen in front of the Main Control Dial). Select lower numbered f-stops for shallow depth of field and higher f-stops for greater depth of field.
Rotate the Quick Command Dial on the back of the camera when shooting in manual mode to change the aperture. Rotating the Main Control Dial on the top of the camera will change the shutter speed, not the f-stop. You can view the settings through the viewfinder or the top LCD screen.
Press the Depth of Field button on the front of the camera underneath the Lens Release button to stop down the aperture setting and preview how the image will look by viewing it in the viewfinder.