Editing Pinhole camera (section)

=== Calculating the f-number and required exposure ===
The [[f-number]] of the camera may be calculated by dividing the distance from the pinhole to the imaging plane (the [[focal length]]) by the diameter of the pinhole. For example, a camera with a 0.5&nbsp;mm diameter pinhole, and a 50&nbsp;mm focal length would have an f-number of 50/0.5, or 100 (''f''/100 in conventional notation).

Due to the large f-number of a pinhole camera, exposures will often encounter [[reciprocity (photography)|reciprocity]] failure.<ref>{{cite web|url=http://www.nancybreslin.com/pinholetech.html|title=Nancy Breslin's pinhole camera exposure tips|first=nancy a.|last=breslin|website=www.nancybreslin.com|access-date=29 January 2018}}</ref> Once exposure time has exceeded about 1 second for film or 30 seconds for paper, one must compensate for the breakdown in linear response of the film/paper to intensity of illumination by using longer exposures.

Exposures projected on to modern light-sensitive photographic film can typically range from five seconds up to as much as several hours, with smaller pinholes requiring longer exposures to produce the same size image. Because a pinhole camera requires a lengthy exposure, its [[shutter (photography)|shutter]] may be manually operated, as with a flap made of opaque material to cover and uncover the pinhole.