Digital SLR Tip #2 - Understanding Aperture and Depth of Field

Understanding aperture values is one of the most important aspects of photography. The aperture will determine how much light your lens will let in during the exposure (taking a picture) and also plays a major role in depth of field. Depth of field essentially means how much of the picture is in sharp focus from the closest item to your camera, to the furthest away item from your camera. Depth of field is very important because sometimes you may want deeper depth of field; for example, taking a picture of someone in front of a building where you want both the person and the building to be in focus. Other times you may want a shallow depth of field; for example, when you want to take a picture of a flower and have everything in the background out of focus. In order to understand how you can manipulate your depth of field, you must first understand aperture values.

What is aperture? Aperture is defined as 1. an adjustable opening in an optical instrument, such as a camera, that limits the amount of light passing through a lens. The diameter of such an opening is often expressed as an f-number. You’ve probably seen values like f/3.5 – f/5.6 on your lens, or heard people talking about f-stops. Well hang in there and I’ll explain what all those numbers mean…once you wrap your head around the concept, its really not that complicated. First you need to see what the f-stop values are for you to better understand the numbers.

Each one of these numbers represents a full-stop. So moving from an f/1.4 to an f/1.6 is a full-stop, and moving from an f/8 to an f/9 is also a full stop. Each time you move a full stop the light let in changes significantly. The smaller the f-number the more light is let in, the larger the f-number the less light is let in. In addition to the amount of light being let in, don’t forget about depth of field. The smaller the f-number, the shallower the depth of field, the larger the f-number the deeper the depth of field. You can always stop down the aperture to let in less light, so any lens can be stopped down to f/22. However, going the other direction in order to let more light in is a different story. Most consumer level lenses will have a maximum (widest open) aperture of f/3.5. Professional level lenses typically have a maximum aperture value of f/2.8 or lower and will be much more expensive than consumer lenses. This is why professional photographers can shoot indoors with no flash and still have enough light for a proper exposure. With most consumer level SLR cameras and lenses you'll need a flash to give you enough light in a moderate to dimly lit room.

While you continue to learn, try shooting in aperture priority and experimenting with depth of field. While in aperture priority your camera will select the shutter speed in order to (or try to) properly expose the picture…so all you have to worry about is adjusting your aperture values. In the next tip we’ll talk about shutter speed!

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-M&L