Carl Zeiss Distagon T* 21mm f/2.8

Point it at a brick wall and the bricks stay straight. For a 21mm that is the entire trick, and this Distagon pulls it off with less rectilinear distortion than almost anything in its class. Run a building's vertical edge right down the side of the frame and it refuses to bow. That low distortion is exactly why people reach for it on cityscapes and interiors, where a curved horizon or a leaning wall ruins the shot.

Distagon is Zeiss's name for a retrofocus wide. The optical block sits pushed forward so the rear element clears the reflex mirror, because that is the only way to land 21mm on a Contax body without locking the mirror up. All that glass is why the filter thread runs out to 82mm. The T* multicoating earns its asterisk against flare; aim it into a low sun and you get a tidy ghost or two instead of a washed-out frame.

Wide open at f/2.8 the center already bites, which is not how fast 21mm lenses usually behave. Stop down to f/5.6 and the whole frame locks in. The Zeiss fingerprint lives in the microcontrast, the way a near edge stays cleanly separated from the tones behind it, and in dense, faintly cool color. The honest cost is vignetting. Open it up and the corners drop a stop or more; by f/8 on a landscape it is gone.

It earned a reputation in its day for low distortion and clean corners, and most shooters who have lived with it still talk about it that way. It has a second life now, adapted onto Sony and other mirrorless bodies, because the rendering holds up under a high-resolution sensor and the low distortion means you are not leaning on software to straighten every horizon. This is the original C/Y Distagon, made in Germany, which is part of why clean copies still command real money. Zeiss later built a ZE and ZF Distagon 21 for DSLRs, but that is a different computation.

Landscape work with this lens means filters, and an 82mm grad is a real commitment of money and front weight. Stack a two-stop graduated ND for a bright sky, then meter the foreground and dial the filter factor into Zone Light Meter so the reading already accounts for the glass in front. Get the exposure under the filter right and the negative comes back needing almost nothing.