Principally, the depth of focus is the range of distance before and after the subject in a scene that appears to be in focus in the final print or viewing medium. A landscape photographer may prefer that most of the scene appears in focus, while someone else may want nice bokeh for the background. Choosing the lens, film size and aperture affect the depth of focus achieved.
In general, larger formats (8x10, 4x5, medium format) have less depth of field for the same viewing angle than smaller formats (medium format, 35mm, 16mm subminiature). As one commenter on The Online Photographer put it, "...in practice, big lens/big hole = small DoF; small lens/small hole = big DoF." Since the f/stop is a ratio, the iris opening itself is smaller for the same f/stop on a shorter (wider angle) lens than a longer lens. Conversely, the larger the film format, the larger the actual aperture is for the same f/stop than for a smaller film format for a lens of the same viewing angle.
With that introduction, take a look at what TOP has to say on Depth of Field. Followed up later by noted photographer, Ctein, Depth of Field, the Sequel. While there is enough material there for good coverage of the topic for a weekend read, FilmMaker IQ has an excellent video that shows just how depth of focus works in under 20 minutes. It is physics, with a good dollop of art.
For more in depth reading, try Jeff Conrad's Depth of Field in Depth.
Hyperfocal Focus
Hyperfocal focus is a technique that uses depth of field to give the impression that most of a scene is in sharp focus. By selecting an aperture and distance for the lens and film format chosen, the depth of field extends from half the distance set to as far as the camera can see. This technique is popular among landscape photographers and works well with normal to wide angle lenses typically used. Simply put, a distance is calculated and set for the lens and aperture, and voila, a sharp photo is had.For older cameras with depth of field marks (these may appear as lines with f/stops marked at the end or as colour coded lines around the focus ring). In this case, simply match the infinity mark on the focus ring with the outer mark for the aperture used. For landscape photography, f/11, f/16, f/22 or smaller are preferred. The inner mark will then typically point to the minimum distance of focus.
Take a look at DoF Master Hyperfocal Distance for more details. That site also has hyperfocal distance charts for various lens and format pairings. The software there has not not been used nor endorsed by this blogger, use at your own risk.
That is it for now, stay tuned for the next article!
