How deep is your field?

Professional photographers often talk about depth of field, but what do we mean and why does it matter?

Simplistically, depth of field (DoF) is distance between the closest and farthest objects from the camera which appear sharply focussed. It's usually said that the things which affect DoF include the focal length of the lens in use, the aperture which is set on the lens and the closeness of the nearest object. You can use it to control the sharpness and thereby the visual impact of objects in different parts of your pictures.

Use a lens with a wide aperture, you get shallow DoF and can throw items in the foreground or background out of focus at will. Use a small aperture and you can maintain sharpness over a huge range of distances - look up 'the f/64 club' sometime for one extreme. On many older lenses, in the days before auto-focus and ultra wide angle zooms, the manufacturer used to helpfully mark a depth of field scale on the lens body. The idea was that you could gauge the depth of field by comparing the marked lines against the aperture marks. You could turn the focus ring until it matched the distance for the closest object you wanted sharp against one line and the furthest against the other, adjusting the aperture to the value indicated as necessary. In theory, that meant you could quite accurately control the way people would view your images.

But that's only half the story...

More fundamentally, DoF depends on the capabilities of your own eyes. Your eyes, like any optical system, have limited resolving power. There's a very simple experiment you can do to test this:

Using a ruler and a wide-nibbed pencil or pen, draw a pair of thick parallel lines on a sheet of paper with a gap between them of exactly 1mm. Pin the paper to a wall or door and walk away a few paces. Check to see if you can still see the gap between the lines. Yes? Then try again from a bit further away. Repeat until you are just too far away to see two lines, instead seeing just one, thick line. Measure the distance you've walked from the paper and you have a fairly accurate guide to your own optical resolving power. You can easily convert this to an angle if you treat the distance between the two lines as the short side of a triangle and the distance from you to the paper as the long sides.

Now, think about a photographic print. It's most likely made up of points rather than line, but the same principle applies - if the angle between two dots on the print is smaller than the angle your eyes can resolve, then you will just see them as one dot, not two (or more).

So, putting the concepts of the DoF of the camera and the resolving power of your eyes together, what you get is an understanding that it's not actually necessary (or possible) for any part of the image to be absolutely sharp. In fact, anything which is rendered sufficiently sharp on the print that your eyes can't see any unsharpness  will look sharp to you anyway. Sounds obvious, really, but that leads to several perhaps surprising conclusions that photographic reference books seldom mention.

- How sharp an image appears to you depends on both it's scale and your proximity to the print. Just like the lines on the paper.
- The DoF marked or suggested on the lens assumes a 'standard' size print viewed at a 'standard' distance. Any other viewing arrangements make it a nonsense.
- In general terms, any picture viewed smaller or from further away will look sharper overall, and the apparent depth of field will be greater as a result.

So, by all means use DoF as a way of controlling the relative sharpness of objects in a composition, but don't be slave to the concept, Depth of field is not absolute.