Monday, July 9, 2012
All About Depth Of Field—07/09/12
How depth of field works, and how you can use it to your advantage
|This Article Features Photo Zoom|
- Depth of Field Defined
The depth of field in an image is defined as the area that is suitably in focus. In reality that means that when you focus on a given spot, the area in front of it and behind it will fall off gradually from sharp to not sharp. That means more than just a single point is suitably focused, it covers an area. The greater the area in focus, the greater the depth of field.
- How aperture affects depth of field
F/stop affects depth of field simply: the bigger the opening, the shallower the depth of field. The smaller the opening, the deeper the depth of field. This axiom is used regularly by photographers whether they want to isolate part of a scene and make it the only thing in focus, or whether they want to ensure a subject is tack sharp from foreground to back.
- How to check depth of field on your lens
You know all those markings on the top of your lens that indicate at what point you're focused? It's actually a distance scale that is most useful for checking depth of field. Many prime lenses, and most older ones, have hash marks indicating decreasing apertures. With the lens focused, the hash marks on top of the lens will tell you how large the depth of field will be—from what distance to what distance—at the denoted aperture. It's a great way to visualize how depth of field works right there on top of your lens.
- How to see depth of field in the camera
Most D-SLRs have a depth-of-field preview button located on the body adjacent to the lens. Pressing this button will physically stop down the lens to the selected aperture, allowing you to see through the now darker viewfinder exactly what elements will be in focus when you take the picture. Re-focusing with the depth-of-field preview engaged is a perfect way to ensure you place your field of focus specifically.
- How to verify depth of field with a flashlight
One problem with the depth of field preview button is that the more stopped down you are, the darker it is in the viewfinder. This can make it practically impossible to see when you're stopped down to f/22 or f/32. I learned back in my view camera days—when the preview was instant on a dark ground glass—the best way to check depth of field at a dim, small aperture is to use a flashlight. Have an assistant hold a flashlight in frame at the closest point you want in focus and aim the flashlight directly at the lens. Through the darkened viewfinder you'll be able to see the details of the bulb—sharp if you're in focus, fuzzy if not. Repeat the process at the back of the area you want in focus, and adjust your aperture and focal point as necessary.
- How distance affects depth of field
The closer the focal point is to the camera, the shallower the depth of field will be. Conversely, the farther away the point of focus the greater the depth of field will be. After all, the "infinity" focus point on your lens represents the distance at which everything beyond the point of focus will fall within the depth of field. This is known as the hyperfocal distance, and it is the point at which depth of field is maximized for a given aperture. In practice, this comes in handy when you've got the choice to be closer to a subject or farther away. If you need a greater area to be in sharp focus, sacrificing some enlarging power by moving back a bit could be overcome by later cropping into the perfectly focused frame.
Using a camera with a full-frame sensor will yield proportionally shallower depth of field at a given aperture and focal length. That means that if you're trying to create images that isolate the subject through the use of a shallow depth of field, this will be easier if you're using a camera with a larger sensor. The same principle applies with medium- and large-format films, which are capable of producing very shallow depth of field as well.
- How focal length affects depth of field
A lens' focal length, whether it's a short 20mm lens or a long 200mm, also affects depth of field. Shorter lenses provide greater depth of field, while longer lenses create less depth of field. In truth this is less a law of physics and more a perceived effect. Because a telephoto lens compresses the scene, the compression and magnification of a longer lens makes it appear that depth of field is shallower. In reality, you're just able to more easily see sharpness changes with the naked eye. (The reverse, of course, makes wide-angle lenses more forgiving of mistakes in focus. With them the perceived depth of field—and therefore the overall sharpness—is greater.)
- How the focal point affects depth of field
When you focus on a given point in a scene, you might think that the depth of field begins there. But in fact, depth of field begins before the point at which you're focused. When focused closer than one-third of the hyperfocal distance (i.e. fairly close to the camera) the area of suitable focus will be divided one-third in front of the point of focus, and two thirds behind the point of focus. At greater distances, the point of focus moves closer to the center of the area in usable focus. This is perhaps my favorite depth of field tool, because I regularly use it to ensure I'm placing my zone of focus very precisely by focusing one-third of the way into an area I want to be sharp.
- How to change the angle of the depth of field
The plane of focus is always parallel to the lens plane and sensor plane. Except when you're using a specialized tilt/shift lens. This is how architectural photographers compensate both for perspective distortion and to ensure their focal plane matches their subject plane. Imagine sitting at the head of a long table with your camera to your eye. With enough tilt on a lens, you can actually "lay down" the plane of focus and angle of the depth of field to be parallel with the tabletop. This way you can ensure that items on the table, from near to far, will be tack sharp, while the depth of field runs vertically up and down from the table—meaning that a particularly tall vase might be sharp close to the table but fuzzy near its top. It's a powerful way to change the fundamentals of depth of field, but it does require a specialized lens or camera with movements.