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How to measure the diameter of a pinhole


This applies to measuring any linear dimension from a photograph. The small size of a pinhole only affects the lens requirements for the initial photograph.

  1. Take a photo which includes the object to be measured and a measuring rule, both at the same distance from the camera. In the case of the pinhole I used an extension tube on the camera to be able to get closer and take a magnified (ie, macro) view.
    Photo of lens cap with pinhole, plus ruler for calibrating

    Canon EOS 5D 24-105mm + 25mm extension @ 45mm 1/30s f/4 ISO1600

  2. Open the image in GRIP by using the Open option on GRIP's main File menu. The image will be displayed in a separate window with its own menu bar.
  3. If the image was taken in RAW mode see how to open RAW images.
  4. Calibrate the image. To do this, go to the Measurement menu in the image window and select "Calibrate distance". You will be prompted to place crosses on the image at a known distance apart, by clicking and dragging with the mouse. The crosses will appear in red, joined by a straight line in blue. On releasing the mouse from positioning the second cross, GRIP puts up a dialogue for entering the units of measurement and the separation of the crosses in those units. In this example I used two marks on the ruler which were 2cm apart:
    Calibrating with GRIP
  5. Clicking the OK button on that dialogue causes GRIP to calculate a conversion factor from pixel spacings to the required units. GRIP displays the factor as confirmation that it is now calibrated:
    Calibrated with GRIP

    NB: GRIP assumes that the camera has square pixels, so the calibration factor will be the same at any angle. The calibration is a property of the image window, not of GRIP as a whole. The Measurement menu does have an option for copying calibration from one image to another, which is suitable if all images have been taken at the same distance from the camera with the same focal length setting.

  6. Back on the Measurement menu, select "Straight line". Set two crosses just as before, but this time marking either end of the distance to be measured. On releasing the mouse at the second position, GRIP displays length and orientation measurements of the line, the length being in the calibrated units:
    Pinhole measured with GRIP

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