How does the distance from the pinhole to the image plane affect image size, sharpness, and exposure?
Asked 12/15/2017
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In a pinhole camera, what changes when the image plane (paper/film/sensor) is moved closer to or farther from the pinhole while keeping the pinhole size the same? I’m especially trying to understand three things: how the image size changes, how sharpness changes, and how exposure time is affected. Is there an optimal distance for a given pinhole size, and can longer exposure compensate for any loss of sharpness?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
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The image size increases linearly with respect to distance between the pinhole and the film plane. The image gets larger in all dimensions.
The image gets sharper as you increase the distance between the pinhole and the film plane, but after a point, the effects of diffractions become important, so here is an optimum pinhole size (alternatively, for a given pinhole, there is an optimum imaging distance).
Light intensity at the film plane decreases with the square of the distance between the pinhole and the film plane, so you need to increase exposure. This is equivalent to normal photography, however, as the amount of gathered light is proportional with the square of the pinhole. The usual concept of aperture (f/D) as a factor in exposure is the same, just that the iris is replaced by the pinhole and the focal length is replaced by the distance between the pinhole and the film plane.
Since exposures will probably be long, you probably need to take into account reciprocity failure.
Originally by user49699. Source · Licensed CC BY-SA 4.0
user49699
8y ago
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Moving the image plane farther from the pinhole makes the projected image larger; moving it closer makes the image smaller. The size change is approximately linear with pinhole-to-image distance.
Exposure also changes: as the image plane gets farther away, the light reaching it becomes dimmer, so you need a longer exposure. In general, illumination falls roughly with the square of the distance, similar to how a higher f-number needs more exposure in a lens camera.
Sharpness is a tradeoff. With a real pinhole (not infinitely small), each scene point forms a small blur circle on the image plane because the hole has diameter. Increasing distance can improve the image scale and, up to a point, perceived sharpness. But if the pinhole is made too small, diffraction softens the image. So for any given pinhole size, there is an approximate optimal image distance; equivalently, for a given distance, there is an optimal pinhole size.
Longer exposure can compensate for lost brightness, but not for lost sharpness. If the image is blurred by geometry or diffraction, extra exposure only makes a brighter blurred image.
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