Sincerely, Timur I.

To understand depth of field you need to understand what the circle of confusion is.

Basically the circle of confusion is what you get when a point isn’t focused properly. When this happens you get a circle rather than a point. As long as this circle is smaller than the resolving ability of the recording media (in our case a digital sensor) then the point will look sharp.

The normal measurement for the circle of confusion limit in photography, or the circle of confusion diameter limit, is the largest blur circle that will still be perceived by the human eye as a point when viewed at a distance of 25 cm.

The focal plane is the distance from the lens at which a point will resolve on the sensor as a point. It’s in focus. This only happens at one distance, there’s no range of distances where everything is perfectly focused.

If you imagine a point on a piece of paper in front of the camera, at exactly the focal plane of the lens. This point is perfectly in focus. If we move it even a tiny amount towards or away from the lens it will be out of focus, albeit very slightly. If we move it steadily toward the camera we’ll see the point will form a circle rather than a point, and this circle will get bigger and bigger the further it moves from the focal plane. Once the circle reaches the size of the circle of confusion diameter limit we have reached the near end of the depth of field for the lens at this focal distance and aperture. If we return the paper to the focal plane and then move it away from the lens until the point becomes a circle the size of the circle of confusion diameter limit we have found the far end of the depth of field.

The maths behind all this is fairly simple trigonometry and can be looked up on wikipedia. Some interesting points to make though:

1. The depth of field calculators are working to a definition for the circle of confusion which is subjective. As I’ve said above, the normal measurement is the largest blur circle that will still be perceived by the human eye as a point when viewed at a distance of 25 cm. If you’re viewing from nearer or further then the whole calculation is shot, as you’ve changed one of the most important variables.

2. The resolving power of the sensor is very important. The more pixels, the higher the resolving power and the lower the threshold for the circle of confusion limit. A point will spread over more than one pixel sooner making the image look blurred. Another good reason not to get sucked into the megapixel myth.

3. The size of the sensor changes things too. The larger the sensor, the smaller the depth of field. For this reason it’s virtually impossible to get a narrow depth of field with a point and shoot camera. Also, it’s hard to get everything in focus with a medium/large format camera.

4. Depth of field can be changed by altering the aperture or changing the focal length and moving your point of view. Zoom in and walk back to isolate your subject against a blurry background.

5. As the focal plane moves away from the camera (as you focus on things further and further away) a point is reached where everything behind the focal plane falls within the depth of field. This is known as the hyperfocal length and is very useful for landscape photography.

Wikipedia is a great place to look if you’re not sure about any of the terms used here or fancy looking up more detail.