Learning Photography Course — 5. Lenses, and How Focus Works

Module 5 of the Learning Photography Course. What a lens is actually doing, why it has fifteen pieces of glass in it, and how a camera works out that something is in focus.

A pinhole has to throw away almost all the light to be sharp — that is the first trade in photography and it cannot escape it. A lens escapes it by bending the rays back together instead of discarding them, and everything else in this module follows from that one idea. The image circle, and why a rectangle is cut out of a circle. Focal length defined properly — from the rear principal point, at infinity — which is why a 300mm telephoto is not 300 millimetres long, and why a lens does not become a different lens on a different camera.

Focal length against field of view, and why 'normal' is roughly the diagonal of the format rather than anything about the human eye. Prime against zoom, and why a constant-aperture zoom is so heavy and so expensive — it is geometry, not greed. What a stop of maximum aperture really costs, and the T-stop, which is measured transmission where the f-number is only a calculation. Elements, groups, aspherical and low-dispersion glass, and coatings, which are invisible and matter enormously. The aberrations, one at a time, and an honest account of which of them software can fix and which it cannot. Stabilisation, and the one sentence that matters: it corrects the camera moving and can do nothing whatever about the world moving. Macro magnification, the Scheimpflug condition, and exactly what a teleconverter costs. Contrast detection against phase detection — why one hunts and the other does not. And depth of field, where the circle of confusion turns out to be a judgement about viewing rather than a measurement.

The module closes on the demonstration it has been promising: photograph the same subject at the same size from two distances with two lenses and the backgrounds differ completely; photograph from one spot with both and crop to match, and the two are identical. Perspective is made by where you stand. 'Compression' is the look of standing far away.

No stabilisation figure, no lens test score and no price is quoted anywhere, because each belongs to one body with one lens on one day. Where a quantity matters, the module states the relationship instead.

Educational documentary. Not financial or investment advice.