Controlling Aperture & Depth of Field for Portrait Blur
Aperture directly controls your Depth of Field (DOF)—which simply means how much of your photograph is in razor-sharp focus from front to back.
The physical opening inside your lens is the aperture. A wide aperture opening lets in a large amount of light very quickly, allowing for faster exposures. However, as that lens opening grows wider, less of the scene behind and in front of your focus point remains in focus. Conversely, as you close down the aperture opening, more of the background and foreground become sharp.
In photography terminology, this is referred to as "opening up" or "closing down" your lens:
- Shooting at a closed-down aperture like f/22 keeps everything from the grass at your feet to the distant trees in sharp focus.
- Shooting at a wide aperture like f/1.8 or f/2.8 gives you a shallow depth of field—isolating your subject with a soft, beautifully blurred background (often called bokeh).
Make Aperture Priority Mode Your Default Setting
In Aperture Priority mode (A or Av on your camera dial), you explicitly set the lens aperture, and the camera automatically calculates the matching shutter speed for a balanced exposure.
Aperture Priority is the workhorse mode for candid portraiture because it gives you instant control over background separation without forcing you to calculate exposure math manually in the field. You shouldn't spend your energy fiddling with dials when working with children. Keep your focus on interacting with the kids, keeping the mood light, and watching the scene unfold.
For every candid shot, ask yourself one basic technical question: How much depth of field do I need to keep my subject sharp, and is my shutter speed fast enough to freeze their movement?