Saturday, September 22, 2007

Understanding Depth of Field

What Is Depth of Field (DOF)?

Depth of field (DOF) is the distance wherein objects are in focus and the background are out of focus.

This is the pragmatic explanation.
To be technically correct, DOF is the zone of acceptable sharpness, the area in front of, and behind, a focused subject that appears in focus.

Circle of Confusion (COF)

Technically, only the subject in focus -- and all other objects at the same distance -- are in focus; everything else in front and behind are out of focus. How much out of focus depends on a term called the "Circle of Confusion (COF, or COC)."

Imagine we are photographing three (3) dots. They are the tiniest dots the human eye can clearly make out, and of course, we are assuming perfect 20-20 vision and ideal light condition.

So, here we have these 3 dots, arranged one behind the other (with the closest dot to the right of the middle dot, and the farthest dot to the left of the middle dot, so a camera can take a picture of all 3 dots).

Now, we focus our lens on the middle dot, which comes out in perfect focus. The two other dots also appear in focus, but peering closely at the resultant photograph, we notice, however, that the dot in front of and behind the middle dot appears as circles instead of perfect dots. I.e., technically, they are out of focus, but to our naked eyes (at a "normal" distance), they "appear" in focus.

It is this circle that we call the "Circle of Confusion." So the COF is the diameter of a dot such that when we view it with the naked eye, it appears in focus. If this circle gets past this diameter, our eyes tell us it's out of focus.

Lens manufacturers have to decide what that diameter is going to be and design their lenses accordingly to be able to resolve a dot within that COF so that it appears sharp to us.

What Affects DOF

What we really want to know as photographers is what affects DOF so we can control DOF in our pictures. For a long, long time, photographers have gone with the following three criteria:

  • lens aperture
  • distance from subject
  • focal length

While the first two are technically correct, the third one has raised somewhat of a storm of controversy among certain circles. Why exactly, we will make clear later. Let's look at each of the three criteria in more detail.

Lens Aperture

The aperture is simply the size of the opening that allows light to go through the lens. It is expressed in f/stops (also referred to as f/value or aperture value), and a typical aperture range is f/2.8 - f/8, giving the range from maximum (large at f/2.8) to minimum (small at f/8) aperture.

A small f/value (e.g. f/2.8) indicates a large aperture.

A large f/value (e.g. f/8) indicates a small aperture.

So, f/2.8 is a larger aperture than f/8.

Generally, a large aperture gives a shallow DOF, and a small aperture gives great DOF.

Putting Aperture into practice:

If you want only the subject the lens focuses on to be sharp, and everything else to be out of focus -- such as a portrait with the background nicely blurred -- then you would "open up the aperture," i.e. use a large aperture.
If you need most of your picture to be in sharp focus -- such as a landscape scene -- then you would "stop down the aperture," i.e. use a small aperture.

So go out and practice I think you can make a perfect on doing it since we are all using digital from which we can evaluate our photos instantly at no cost...

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