The goal is to make a photo print with the 10 cm scale appearing in the print exactly 10 cm long. Unfortunately, photofinishers will project the file you provide upon an area slightly larger than the final print. In effect, they will crop a small amount from all edges of your image, because otherwise they'd have to align your image exactly with the print boundary, which would be difficult. The only way to find out how much they will crop is to try making a print and then measuring it. Usually it's not far off, and you can make it perfect by adjusting the image size and printing again.

Here's how:

• Click on this image for the full-scale file

  Click here or on the image to the right which opens the reference image file in new browser window, then save the image to your disk (right-click, 'Save as'). The image you obtain should have a width of 3141 pixels.

  The red 'arrows' are cropping indicators; they measure how much of the image was cropped by the photofinisher, just for interest. Their scale is in tens of pixels.

• Send the image file to a photofinisher for printing as a 4x6 inch print. Matte finish reduces glare and fingerprints.
• When the print is received, use a ruler to measure the 10cm scale on the print as accurately as possible (you should be able to estimate fractions of a millimeter and thus get centimeters to two decimal places, eg., 10.13 cm). If the printed scale is as close as you'd like to 10 cm, you're done! Just use scissors to cut out the scales. Otherwise, you can adjust the image size and reprint it, as explained below.

• Measured length of scale (cm), eg., "9.98" Width of image (pixels) New width (pixels) New height (pixels)

  Use the form at the right to calculate new dimensions for the image file, at which it should print at your photofinisher at 'actual size'.

  To set the image file to the new desired dimensions, ideally you should use an image editing tool that can change the dimensions of the file without resampling the image. Here's how to do that using Photoshop:

  1. Open the image file.
  2. Edit->Canvas Size...
  3. In the Anchor section, click on any corner.
  4. Enter the new dimensions (remember to enter both the new width and height, to maintain the correct proportions).
  5. Press 'OK'. If you are reducing the size, say 'OK' if a warning comes up asking about cropping.

  This will alter some of the red cropping arrows, but that's OK; just ignore them from now on. By adjusting the canvas size rather than adjusting the image size, we preserve the pixels of the image (if instead we just resize the image, the pixels would be resampled, likely degrading the image). If you'd don't have Photoshop or equivalent, you'll have to adjust the image dimensions by resizing.

• Print the re-dimensioned image. The print should come back with a scale much closer to 10 cm. If not, make further adjustments, remembering that reducing the image file's pixel width causes the printed scale to become longer. Photofinisher printing machines are usually stable, so once you get it sized, it will probably reprint at that photofinisher without need for further adjustment.
• Use scissors to cut the reference scales from the print along the guide lines (and at the base of the millimeter scale for the bottom edge).

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