Pinhole Image Size Calculator

How it is calculated

A pinhole forms an image by straight rays, so the image and the object are similar triangles. The tool first finds the magnification m = depth ÷ distance, where depth is the distance from the pinhole to the screen or film and distance is the distance from the pinhole to the object. Then image width = object width × m and image height = object height × m. All four inputs are in millimetres and the image sizes come out in mm. Because m is below 1 for ordinary scenes, the image is smaller than the object; it is also upside down and mirrored, which the sizes do not show. The distance must be above zero. The formula does not account for the size of the hole, diffraction or sharpness.

Common mistakes

• Mixing units. All four fields are mm: a subject distance of 3 m must be typed as 3000, otherwise the image is 1,000 times too large.
• Swapping the two distances. Object distance is pinhole to subject, pinhole-to-screen is the short distance behind the pinhole; swapped, the magnification becomes more than 1 and the image looks larger than the object.
• Assuming the result shows whether the subject fits the frame. It tells you how big the object appears; compare it with the film width and height yourself.
• Forgetting that the image is inverted. The numbers are sizes only; the picture is turned upside down and mirrored.
• Expecting a sharper picture from a bigger result. Image size comes only from the two distances; sharpness depends on the pinhole diameter, which this tool does not use.