Graphics Reference
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Fig. 5.1 The interior of the same cube as viewed by different focal length lenses
the camera. With fi lm cameras, the focal plane is the fi lm itself. Digital cameras use
a light-sensitive plate. In both types of cameras, the distance between the center of
the lens and the focal plane will equal the focal length of the lens. The focal length
affects the image in two ways. The longer the lens is, the closer the subject seems to
be to the camera and the less distorted the image is (Fig. 5.1 ). This is why short lenses
are called fi sheye lenses and long lenses are telephoto. A fi sheye lens causes consid-
erable spherical distortion, but telephoto lenses have very little distortion. For exam-
ple, a straight length of tracks would appear as a perfect half circle to a short fi sheye
lens, but would be almost perfectly straight in a telephoto lens.
The human eye has an approximate focal length of 17 mm (Shlaer 1972 ).
A 17 mm lens is only slightly longer than a typical fi sheye lens. Default lens lengths
in CG applications tend to be either 35 or 50 mm. Both cause less distortion than a
human eye. The range of virtual lenses available in CG can be any number you care
to type, whether or not it corresponds to a real-world lens. Each of these will draw
the scene differently, so it is important to select the lens that is best for your project.
Default lenses are usually acceptable for general use, such as model viewing, but
this isn't always true because the lens can affect modeling decisions. If you need to
optimize your scene by not building objects that aren't visible to the camera or if the
render lens affects composition signifi cantly (as in the alignment illusion project
example in Sect. 6.5 ) , you will need to select the exact focal length you will render
with before you model anything. For the same reason, it is good to set your render
resolution in advance, so that you can easily see how your objects are cropped by
the camera.
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