The depth information in a stereo image must not exceed certain limits.
Why should we observe the shooting rule?
Since our brain can process only a limited amount of depth information, the depth contained in a stereo image must be limited so that the entire stereo picture can be observed as a uniform spatial image. A violation of the shooting rule may lead to a decay of the spatial image into individual image sections. If we want a stereo photo that is pleasant to view, adherence to the shooting rule is a condition which should be observed under all circumstances.
Which values and tolerances are dependant upon the shooting rule?
While taking a stereo picture, the spatial depth produces a lateral displacement of the two corresponding points in the left and right stereo images. For close objects this displacement is larger - and it is smaller for distant objects. The difference between the largest (produced by the "closest object") and the smallest (produced by the "most distant object") displacement is called deviation or maximum parallax. The deviation should not exceed 1/30th of the entire image width, which can be detected without twisting the head (for the experts: this value follows from the 70'-condition).
Note 1: If the subject allows it, also the space in front of the stereo window can be used. For these cases the deviation limit is increased with the exact value depending on the subject. For the parts of the image that are in front of the stereo window, an additional deviation up to 1/50 of the image width may be generally well accepted.
Note 2: The restriction on the deviation to 1/30 of the image width always refers to the same detectable image area without twisting the head. For example, if the projected stereo image is so large that it can no longer be perceived at the same time, the maximum deviation value should be reduced accordingly. For cinema projection with very large screens, therefore, a reduced deviation of 1/40 of image width is recommended.
Note 3: For presentations of varying aspect ratios every individual maximum deviation values must be related to the image width of the widest image. For portrait images, this leads usually to significantly higher maximum deviation values.
Note 4: The maximum lateral offset of 1/30 of the image width is to be understood as an upper limit and can, in individual cases even be too large, especially when the nearest point and the farthest point in a stereo image are precisely cutting.
A simple distance rule to comply the shooting rule is:
Shooting rule: Beware of insufficient distance!
The near point distance is the distance between the camera and the closest motif point.The stereo base (distance between the lenses) is about 65mm at true stereo cameras. We frequently find - especially in older literature - instead of the focal length of a fixed value, for example, 35 or 50 mm. Since most currently used stereo cameras have zoom lenses with adjustable focal length, these simple rules of thumb are valid only for the corresponding fixed focal lengths. The above mentioned distance condition applies to all motives which includes infinity(e.g. the horizon) as a figurative element.
How do I adhere to the shooting rule?
When taking pictures, you only need to ensure that no object is closer to the camera than the allowable distance to the closest point, calculated using the above rule, for example 2.3m with 35mm focal length, 3.3m with 50mm focal length or 4.5m with 70mm focal length (assuming a stereo base of 65mm). If the far point is not at infinity, the minimum allowed near point distance may be somewhat reduced. In these cases you may evaluate the exact value by applying the DGS Stereo Base Calculator. (All focal length figures are equivalents based on the 35mm picture format.)
Want to know more? Visit one of our regular meetings throughout Germany and discuss with us. Become a member of the DGS!
Photo credits: Edited by Gerhard P. Herbig after Chaval’s Fotoschule, Diogenes Verlag, Zürich.