Cinema lenses are precision mechanical instruments. Like every mechanical instrument they wear — slowly, often imperceptibly, until the problem becomes impossible to ignore on set.
The worst moment to discover that a lens has a problem is while you are shooting. Here are the signs to watch for before it gets that far.
Stiff or uneven rings
A focus ring should run smoothly and evenly along its whole travel. If you feel resistance at certain points, if the movement is inconsistent, or if you have to force the ring round, the lubricant in the helicoids is most likely spent or degraded. Ignore this sign and the ring will eventually seize completely.
Imprecise or unstable focus
If the lens struggles to hold focus, if the focus point does not match the marks on the ring, or if you notice focus drifting during a take, the problem is almost always collimation. It can be caused by a knock, by a loose mount, or simply by wear over time.
Dust or fungus visible inside
Internal dust visible to the naked eye, marks, haze or — worst case — traces of fungus on the optical surfaces all call for immediate attention. Fungus in particular spreads, and can damage anti-reflective coatings irreversibly if it is not dealt with in time.
Abnormal mechanical play
Moving the lens in your hands, without turning the rings, you should feel no internal play at all. A lens that rattles inside, or that produces noises or vibration, has loose or damaged mechanical components that can deteriorate quickly.
Lubricant migrated onto the glass
In some cases the helicoid lubricant, when degraded or applied in excess, migrates towards the optical surfaces and leaves a thin film that is hard to see but tells on the image. It usually shows up as a general loss of contrast, or as flare in backlit situations.
When to service preventively
Don't wait for a problem to appear. For lenses in regular use on set or in rental, a full service every 12-18 months is enough to prevent most problems. For lenses used intensively or in difficult conditions — dust, humidity, extreme temperatures — every 6-12 months is advisable.
A regularly serviced lens lasts longer, behaves predictably, and holds no surprises for the worst possible moment.
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