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Pinhole Image Size Calculator

Enter the object size and distances to find the projected image dimensions of a pinhole camera.

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.

Worked example

A person 1,800 mm tall and 500 mm wide stands 3,000 mm from the pinhole. The film is 100 mm behind the pinhole. Magnification: 100 ÷ 3,000 = 0.0333. Image width: 500 × 100 ÷ 3,000 = 16.67 mm. Image height: 1,800 × 100 ÷ 3,000 = 60.00 mm. So the tool shows a magnification of 0.0333, a width of 16.67 mm and a height of 60.00 mm. The person will fit easily on a 60 × 90 mm film frame, standing upside down. Moving the film to 200 mm would double both sizes.

Try this example →

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.

About this calculator

A pinhole camera has no lens — light rays from an object pass in straight lines through a tiny aperture and land on the film or sensor plane inverted, a principle known since antiquity as the camera obscura effect. The size of that projected image is governed by simple similar-triangles geometry: image size = object size × (image distance ÷ object distance), where image distance is how far the film plane sits behind the pinhole and object distance is how far the subject is in front of it.

This calculator takes the real-world object size and both distances and returns the projected image dimensions, letting you predict framing before you shoot. Pinhole photographers building or calibrating a camera, physics teachers demonstrating optics, and darkroom hobbyists use it to figure out how large a subject will appear on the film for a given pinhole-to-film distance, since there's no viewfinder to check the frame directly.

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Comments (1)

  • Editorial team

    The sizes are only useful if they fit your film or sensor. With the defaults, a 1000 by 600 mm subject at 5 m comes out 20 by 12 mm. To make a 1000 mm wide subject fill the 36 mm width of a 35 mm frame at that distance, the depth would be 180 mm, which gives 36.00 by 21.60 mm. What film or sensor size are you building around?

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