(track of a Western Coyote, on morning sand at Coral Pink Sand Dunes, Utah, USA)
Click here or on the image below to obtain an image of three reference scales to send to a photofinisher to produce a regular 4" x 6" photographic print, as pictured below (or print on your own photo-printer).
|Click on this image for printing instructions|
Photofinishing 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.
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.)
A compact 10cm reference scale with a checkerboard for 2D scale, a circular fiducial mark for perspective measurement, and a grey patch for colour correction that is printable on a photo-printer or as a regular 4"x6" photographic print.
The checkerboard 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.
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).
Caveats: When the reference scale is a small part of a photo, shortcomings of a camera's lens and internal image processing can cause purple fringing, chromatic aberration, and other defects that bleed into the grey patch. In such cases, colour-correction based on the grey patch may be wildly and obviously off. The bleeding is usually visible when the image is highly magnified (eg., using 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).
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.
Furthermore, 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?
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.
The straight-rule shape makes it easy to carry and stow, and helps reduce wear and tear.
Photographic reference scale
Jim Elder, Ottawa, Canada