What a lens actually does A lens has one job: draw the scene as a circle on the sensor. Focal length, F-number, and image circle decide whether that drawing is usable. Lenses 12 Sept 2026
Image circle vs sensor format A lens paints a circle, not a rectangle. If that circle is smaller than the sensor, the corners are not in the picture. Lenses 12 Sept 2026
Focal length and field of view A 4 mm lens is not a field of view. FOV appears only when that focal length sits in front of a real sensor size. Lenses 12 Sept 2026
Rectilinear vs fisheye projection Rectilinear keeps straight lines straight. Fisheye trades that for a wider view. Their degree numbers are not the same unit. Lenses 12 Sept 2026
F-number, light, and diffraction F-number is how wide the lens opens. It buys light and spends depth of field. Neither extreme is free. Lenses 12 Sept 2026
Distortion, and what TV distortion actually is Distortion bends the grid the lens draws. A doorbell can live with it. A measuring camera cannot. Lenses 12 Sept 2026
MTF, and whether the lens can resolve the pixel MTF asks whether the lens still has contrast at the pitch of your pixel. A sticker that says 8 MP is not that measurement. Lenses 12 Sept 2026
Lens CRA matching Wide M12 glass often exits steeply. Small modern pixels often refuse that angle. That fight shows up as dark or magenta corners. Lenses 12 Sept 2026
Mounts and threads: M12, CS, C, and pitch A thread is a contract: diameter, pitch, and how far the glass sits from the die. Matching only the name is not enough. Lenses 12 Sept 2026
Focus type: fixed, set-once, and motors Focus type says whether the lens can move. Fixed is for a known distance. Motors are for a distance that will not sit still. Lenses 12 Sept 2026
TTL, optical BFL, and mechanical BFL TTL is the height of the lens. Back focal length is whether that lens can come to a focus on your sensor. Lenses 12 Sept 2026
Spectral band and the IR-cut filter Silicon sees infrared. A colour camera needs an IR-cut. A night camera often needs it gone. Pick them as a pair. Lenses 12 Sept 2026