When you shoot in Aperture Priority mode you set your aperture and your camera sets your aperture.
Aperture controls light reaching your camera’ s sensor. Aperture size is measured in f-stops. The best way to think of them is as fractions. With f/4 being f/8, f/8 twice as large as f/16 and so on and so on. Your aperture also controls depth of field (DOF). DOF is a measurement of how much of your photo is in focus, in front and and behind your point of focus. Your aperture setting determines weather your background is in or out of focus. A low aperture setting of say 5.6 and below will produce an image with a shallow depth of field. (ie will give you that beautiful blur). A narrow DOF is better for shooting multiple subjects because you want more in focus. DOF will also vary depending on the focal/lens length and the distance you are from your subject
Each lens has a unique set of apertures. Some lenses have a constant aperture and some have a variable. Lenses with a aperture of F1.4, or F2.8 are considered “fast lenses”. (fast lenses let more light in)
I use aperture priority mode 80 % of the time. I like to control my DOF. Shooting in aperture mode allows me to guide the viewer to a specific focal point of the image. Sometimes my camera is setting the shutter speed at 1/25th of a second and I have to decide if I need to open up the aperture more or bump up the ISO.
Because I know how my camera makes these decisions I’m able to manipulate it starting with one point of the equation triangle (ISO) then adjusting another point for artistic purpose (aperture) while letting my camera tackle that last point in the equation (shutter speed). It’s part art part science.
Have you heard the word “Bokeh” used in photography? In photographic terms, bokeh is the blur, or aesthetic quality of the blur, an out-of-focus areas of an image, or the way your lens renders out-of-focus points of light.
Great portraits should be free from distracting backgrounds. Choosing the correct entire, focal distance and shooting distance will help to create a beautiful “bokeh” and if you choose the right settings you won’t need to buy that f2.8 lens we all have on our wish lists! Use a longer focal length – preferable longer than 135mm and reduce your shooting distance between you and your subject.
Depth-of -Field preview button
This button /feature is not on all cameras. The purpose of this button allows you to preview the overall DOF of your image. On many DLSRs the depth of field button is located on the front of your camera, right where your left hand will rest when holding your camera, next to the lens. Pressing this button will tell the camera to set the aperture at the selected level, allowing you to see what the shot will look like in the view finder particularly the depth of field. This enables you to see how much of your image is in focus sharply.