"It's interesting to note how the lens design affects the nodal point. I had
read that the nodal point would be equal to the length of the lens, offset from
the film plane -- nothing could be further from the truth! The 50/3.5 was the
only one even close to that rule-of-thumb! It's interesting that the 18/3.5
nodal point is actually IN FRONT of the front element!"
Telephoto designs (in which a negative group sits behind a positive group) have
nodal points that are _closer_ to the film than the lens's focal length.
Wide-angle (inverted telephoto) lenses (which have a positive group behind a
negative group) have nodal points that are _farther_ from the film than the
lens's focal length. This type of design makes SLR wide-angle lenses possible,
by keeping the rear elements away from the mirror. It also has a nice
side-effect -- it reduces cosine^4 effects, so you get more-even illumination.
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