Depth of Field and Hyperfocal Distance Calculator for Film Cameras
Depth-of-field is how much of a photograph is in acceptable sharp focus from in front of the focus point to behind it. The depth of field increases with f-number which means that photographs taken with a low f-number (large aperture) will tend to have subjects at one distance in focus, with the rest of the image (nearer and farther elements) out of focus. This is frequently used for nature photography and portraiture because background blur can be aesthetically pleasing and puts the viewer's focus on the main subject in the foreground. Higher f-numbers (small aperture) are used more in landscape photography.
Hyperfocal distance is the distance at which you should focus to produce the greatest depth of field. If you focus at infinity, the in-focus field will stretch from the hyperfocal distance to infinity. When you focus at the hyperfocal distance the in-focus field will stretch from half the hyperfocal distance to infinity. This is useful for landscape photography to get as much foreground in focus as possible and still keep elements at infinity in focus.
This calculator is designed specifically for roll films that were used in Art Deco and vintage cameras. The most common formats are included.
Assumptions for the calculations of DOF are subjective. The calculator above assumes that the resulting photograph will be viewed as an image with a diagonal measurement of about 30cm (12 inches) viewed from a distance of 25cm (10 inches). Viewing a 10" x 8" (25cm x 20cm) photograph at this distance would fulfil these requirements. This is the standard method of calculation.
When your camera is focussed at the Hyperfocal distance, the nearest point in focus is at half that distance and the furthest point in focus is at infinity. This is the maximum depth of field you can get.
For example: Using a Rolleicord and 120 (8 exposure) film, with a 75mm focal length lens with aperture set to f/8, the hyperfocal distance is 32.2ft. This means if you focus on a point at 32.2ft then everything will be in focus between 16.1m and infinity. If you change the aperture to f/11 then the hyperfocal distance is 22.9ft. This means if you focus on a point at 22.9ft then everything will be in focus between 11.5ft and infinity.
Circle of Confusion
A lens can precisely focus objects at only one distance. Objects at other distances are defocused. Defocused object points are imaged as blur spots rather than points. The size of these blur spots is called the circle of confusion (CoC). The common criterion for 'acceptable sharpness' in the final image is that the blur spot be indistinguishable from a point. For most people, the closest comfortable viewing distance for a photograph is approximately 25 cm. At this distance, a person with good vision can usually distinguish an image resolution of 5 line pairs per millimeter (0.2mm). If the blur spot in the final photograph is greater than 0.2mm then the image points are perceived as 'out of focus'. This is the criterion used to determine depth of field.
The maximum size of the blur spot on the film surface needs to be much smaller than 0.2mm for good focus because it is going to be enlarged to the final photograph size. The smaller the image on the film, the smaller the circle of confusion has to be. For instance, the circle of confusion on a 35mm film must be less than 0.029mm to be considered as 'in focus'.
Changes in photograph size or viewing distance will change the perceived depth of field. If the viewing distance is increased then the perceived depth of field is increased. If the photograph is smaller than 10" x 8" then the perceived depth of field is increased. For whole-image viewing, if a final image larger than 8" x 10" is viewed at a correspondingly greater distance, the perceived depth of field is the same.
Cropping the image before enlarging will reduce the perceived depth of field.