(track of a Western Coyote, on morning sand at Coral Pink Sand Dunes, Utah, USA)
The 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.
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).
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.
The straight-rule shape makes it easy to carry and stow, and helps reduce wear and tear.
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