Photographic reference scale

(track of a Western Coyote, on morning sand at Coral Pink Sand Dunes, Utah, USA)

Make one for yourself

For instructions on how to produce three reference scales on a single, regular, 4" x 6" photographic print, as pictured below, click here or on the image below.

Click on this image for printing instructions

Make your reference scale more durable by mounting it on a piece of old ruler or similar. I use a 10cm piece cut from an aluminum ruler, and have also mounted scales on a section of a wooden paint stirring stick from hardware stores. I use a thin spread of 5-minute epoxy to mount the scale. (Avoid contact cement, I found the toluene solvent caused print dye to bleed.) To help the scale resist sliding off inclined surfaces, a piece of terry-cloth fabric glued to the back of your mount is effective. Of course it is possible to use the scale unmounted, simply cut with scissors from the photographic print.

Printing on photographic paper is a good choice because it has high contrast, resists yellowing and fading, has excellent resolution (no visible half-tone dots), and is fairly durable and somewhat water-resistant. It's also easy and inexpensive. Laser printing on paper or pressure-sensitive paper works less well; the grey patch often ends up with a half-tone printing 'grain' and paper can yellow with age and sun exposure.

Features

Scale (checkerboard pattern)

The two-dimensional checkboarder pattern of 1 cm squares provides information about a photograph's scale, in two dimensions. The high contrast black-and-white pattern is highly visible and is easily identified by viewers as a reference scale.

Colour Correction

The 2 cm neutral grey square at the right of the reference scale is intended for making colour corrections. If light illuminating the scene (including the reference scale) has a colour cast, the neutral grey will pick up that cast; for example, light from a blue sky gives a blue cast to scenes not in direct sunlight. Photo editing tools such as Photoshop can eliminate such colour casts by adjusting the colour balance of known-neutral objects back to neutral colour. In Photoshop, use the middle eyedropper of the Curves tool (see Photoshop Help on Curves for more information).

Problems sometimes occur when the reference scale is a small part of a photo: Purple fringing, chromatic aberration, and shortcomings of a camera's internal image processing can create colour fringes that bleed into the grey patch. In such cases, colour-correction based on the grey patch will be wildly and obviously off. The bleeding is usually visible when the image is highly magnification (eg., in Photoshop). In these situations it is sometimes better to colour-correct using the white sections of the reference scale (with the right-most eyedropper of the Photoshop Curves tool).

You can control how many pixels Photoshop uses when colour sampling by right-clicking when using the eyedropper.

The black and white sections of the reference scale can be used to set the high and low points of a photograph, but often the black squares will not be as dark as other parts of the photo (the black of the photographic paper is not an absolute black).

Fiducial, and Perspective Correction

Click here for a demonstration and a perspective worksheet.

The circular target with orientation and centerpoint markings can be used as a fiducial and for perspective correction -- because a circle, if photographed obliquely, appears in the image as an ellipse, perhaps tilted, providing enough information to determine the angle between the camera's view and the axis of the circle.

Futhermore, if the reference scale is aligned to horizontal, and if the camera is held horizontal (though possibly looking down or up), it's possible to deduce the angles of rotation (horizontal and vertical) of the reference scale in the real world, and the angle by which the camera was pointing down or up -- all that from just the shape of the circular target in your image! (Amazing, eh? Click the link above for a demo.)

The perspective target is most useful when the reference scale was photographed obliquely, but normally one should strive to photograph scenes perpendicularly to minimize distortion.

Compact and Rugged

The straight-rule shape makes it easy to carry and stow, and helps reduce wear and tear.

Usage Tips

  • Try to put the scale into the same plane as the object(s) of interest (so that the scale and the objects will be viewed similarly by the camera), and in the same light.
  • Use a maximum-length telephoto; zooming-in reduces variations in angle of view across the frame of the photograph.
  • Photograph perpendicular to the scene to minimize distortion.
  • If you can't photograph the scene from the perpendicular and want to be able to later recover real-world perspective information using the perspective circle, position the scale so that it is level with the horizon (though flush with the surface of interest) and shoot with the camera level to the horizon (though possibly pointing down or up).
  • Storing the reference scale in a small plastic ziplock bag reduces abrasion, eg., when carrying it in a backpack.

Comments or suggestions?

Jim Elder
Ottawa, Canada
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Creative Commons License
This work is licensed under a Creative Commons Licence.

Other Reference Scales

ABFO No 2 Bite Mark Scale. A reference scale for forensic work. Sold for about US$7. Printed on fairly stiff PVC. L-shape makes it more suspectible to damage in backpacks, etc.
IFRAO Standard Scale A colour reference scale for rock art. Distributed upon request from Australia. Printed on medium-weight paper. Protection from light delays fading.