It is one of the most used terms in the industry and one of the least understood. Plenty of DPs know collimation matters, but few know exactly what it means, or what an uncollimated lens actually does to you on set.
What collimation is
Collimating a lens means aligning the internal optical groups so that the lens's focal plane falls exactly on the camera's sensor or film plane. In practical terms it means that when the focus ring reads "1 metre", focus lands at exactly 1 metre — not at 95 centimetres, not at 1.1 metres.
It also means focus is even across the whole frame — that the centre and the edges of the image are equally sharp with the ring in the same position.
When a lens loses collimation
Collimation can be thrown off by a knock — even a light one — that shifts one of the internal optical groups out of position. It can deteriorate over time as the internal locking mechanisms wear. It can be compromised by earlier work carried out without the right instruments.
In some cases collimation was never right to begin with — unserviced vintage lenses can carry deviations accumulated over decades that nobody ever corrected.
How a collimation problem shows up
The symptoms of an uncollimated lens are not always obvious on a monitor — especially on fast sets where there is no time for careful checks. The most common signs are focus that "doesn't quite convince" even though it is technically correct, differences in sharpness between the centre and the edges of the frame, or focus that seems to land slightly off the mark on the ring.
In critical situations — a close-up in a car, shallow depth of field, fast glass shot wide open — an uncollimated lens can cost you whole days of shooting.
How a lens is collimated
Collimation calls for dedicated instruments — an optical bench or a collimator — and specific knowledge of the lens being worked on. It is not something to improvise.
The process involves partially disassembling the lens, checking the positions of the optical groups against the original specifications, adjusting the locking elements, and a final check on instruments before reassembly.
Collimation and mount changes
One case where collimation becomes essential is when a lens's mount is replaced or adapted. Every mount has a specific flange focal distance — the distance between the mount flange and the sensor plane. If that distance is not right to a tenth of a millimetre, the lens cannot be collimated correctly on the new camera body.
It is one of the most common mistakes in DIY conversions: the lens seems to work, but it never reaches the focus accuracy it should have.
How often to collimate
Collimation is part of every full service. If the lens has not been knocked and is serviced regularly, it needs no extra work. If you notice one of the symptoms above — or if the lens has taken an impact — a collimation check is the first step.
A properly collimated lens is a lens you can rely on. It is the difference between an instrument and a precise instrument.
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