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Wednesday, February 17, 2010

Depth of Field is Not Just the Number of Cowpies You Are Standing In

What the heck is depth of field? Photographers hear it all the time…shallow depth of field, greater depth of field. Now that Aperture and Shutterspeed are clearly cemented in your brain, let’s look at why they are so important.

A shallow depth of field is when the part of the image closest to the lens is in focus, and everything behind it is more out of focus. The extent to which this occurs is dependant upon how wide you open your Aperture, or how small your f-stop number is, as well as the focal length and distance the subject is from the camera.  The smaller the f-stop number (ie: f/2.8) the sharper the immediate subject and the more blurry the background therefore providing you with an image with a shallow depth of field. This is a technique popular with portrait photography where the family or individual is crisp but the background has a blur or a haze. This is because the photographer set the f-stop to provide a shallow depth of field.

My colleague Rick Rosen explains Depth of Field with the following:
Depth of Field is a factor of:
1. The focal length of the lens. The longer the lens the less the depth of field at any given aperture and focus point when compared to a lens of shorter focal length.
2. The aperture. The more the lens is closed down to a smaller aperture (larger f/number) the more the depth of field will be at any focus point.
3. The distance from the camera position to the subject. The closer the focus the less the depth of field will be at any focus point. The farther away the greater the depth of field will be at any aperture.
4. Depth of field extends 1/3 in front of and 2/3 behind the focus point. This is the relationship at infinity focus but as you focus closer that ratio changes to eventually become 1/2 in front of and 1/2 behind the subject.

From my personal experience, be careful when shooting groups. Upon photographing more than one person in a group, make sure each person is within 12 inches of each other in reference to their distance from the camera. Everyone should be no more than 12 inches behind or in front of their friend or family member is when you are choosing an extremely shallow depth of field such as 2.8 (or in some cases lenses can run as wide as a 1.2 or 1.4. Be careful when using a small f-stop like this as one person will be in focus and the rest will look like they are in a drug induced haze...) Distancing yourself from the subjects can create a greater depth of field eliminating this issue, also allowing you to shoot at a larger f-stop. Also, using a longer focal length and smaller f-stop can trip you up if you are shooting a single individual as if you are shooting at 1.4 and focus on their nose, their eyes will be out of focus.

You achieve a greater depth of field when you set a larger f-stop number such as f/3.5 up to f/22 etc and distance yourself from the subject(s). In this case, you would be more likely to be shooting a sports event or landscapes. Remember, the smaller the opening (larger the f-stop) the less light is allowed in, so you will need to consider upping your ISO and possibly slowing your Shutterspeed if you are not using a tri-pod. This is why many landscape photographers use tri-pods. They most commonly shoot at dawn or dusk, in low light situation. In order to photograph a crisp landscape with a greater depth of field, and with an ISO of 100 to allow for low noise, they need absolute stillness as they will need to set the shutter open longer, sometimes for a minute for more at a time.

So there you go! Let the barn door open and set the cattle free! OH and don't forget to wear your boots...

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