Depth of Field

The Depth of Field tab lets you calculate the depth of the field of acceptable sharpness in a print. This depends on the subject distance, focal length, and aperture. It also depends on the "circle of confusion," which is a measure of "acceptable sharpness." The program has CoC values built into it for all common film formats, but you can put your own value in, if you like.

About Depth of Field

When you focus your camera's lens at a certain subject distance, objects at exactly that distance from the optical center of the lens are exactly in focus. Technically, everything else is out of focus. In practice, however, there is a "zone of acceptable focus" surrounding the focus plane, so that objects within that zone are considered "in focus" and those outside that zone are "out of focus". The size of this zone is called the "depth of field."

About the Circle of Confusion

To understand depth of field, you have to understand the "circle of confusion" concept.

Imagine a perfect white point in an empty black room. The point has no height, and no width. If you focus a lens on that point, it forms a perfect point on the film as well, if you ignore lens imperfections. If, however, you focus slightly before or after the point, the point will image on the film as a small disc, or circle. If that circle is small enough, it will look like a point, not a circle. The "circle of confusion" is typically calculated as the largest on-film circle that you see as a point when you make an 8×12 print and view it from 10 feet away. Anything larger is seen as a small circle, and is therefore perceived as out of focus.

f/Calc comes with common CoC values built into it for common film formats, but they are plainly only a generalization. If you plan on making prints larger than 8 × 12, or viewing them from closer than 10 feet, you will need to use a smaller circle of confusion to get the same degree of apparent sharpness. The CoC value also takes into account imperfections in the camera and enlarging lenses used, and the resolution of the film and paper used to make the image. If you change any of these for better or for worse, you will need to use a different CoC value.

f/Calc uses the commonly-accepted CoC value of 0.033mm for 35mm film, but some companies like Zeiss use a more demanding value of 0.025mm when making the depth of field marks on their lens barrels. That number is calculated as 1/1730 of the diagonal of the frame. You can use the same formula for other film formats. For 6 × 4.5 film, for example, you could use (sqrt(60mm² + 45mm²) ÷ 1730) = 0.043mm instead of 0.05mm.

General Rules About Depth of Field

1. Smaller apertures make for deeper in-focus fields. However, keep in mind that extremely small apertures (f/22 and smaller) can actually degrade images due to diffraction, which happens because lens apertures are not perfectly round. (This is why better lenses have more "blades" making up their aperture — more blades makes for "rounder" apertures.)
2. Wide angle lenses have deeper field depths at a given aperture than telephoto lenses do.
3. Larger format films, like 6cm×6cm, provide "sharper" images than 35mm film. This is because a 0.033 mm circle imaged on 6×6 film is comparatively smaller than the same point imaged on 35mm film.

Formulæ Used

First, we must find the hyperfocal distance:

where f is the focal length of the lens, A is the aperture value and c is the circle of confusion value.

The near edge of the zone of acceptable focus is found with:

and the far edge is found with:

where h is the hyperfocal distance calculated above, and s is the subject distance. (Technically, s is the distance from the optical center of the lens to the focal plane, but this distinction usually only matters in close-up photography.)

All lengths f, c and s must be in the same unit, such as millimeters.