Every photograph is a negotiation with light. The aperture decides how much of it gets in; the focal length decides how much of the world comes with it. Open the aperture wide and the depth of field collapses to a sliver. Stop it down and the whole scene sharpens, at the cost of light.
Distortion is the part nobody prints on the box. Wide lenses bow straight lines outward into barrel distortion; long lenses pinch them inward. Good lens design rarely eliminates distortion so much as decides where to put it — and a little of it, used on purpose, can make a picture feel closer than it is.
Then there is the iris: a ring of thin blades that pivot together to open and close the aperture. Count the blades and you can predict the shape of every out-of-focus highlight. Seven blades draw heptagons. Twelve draw a near-perfect circle of light.
Coatings matter too. Each glass surface reflects a little light back, and a lens with a dozen elements can lose a surprising amount of contrast to flare. Modern coatings trade that loss for a faint violet or green cast you can sometimes see when you tilt the front element.
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