How to Choose the Right Lens Cap
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Caps are the first thing to go missing on a vintage lens, and the first thing people order the wrong size of. The reason is that "lens cap size" doesn't mean one thing — it depends on which kind of cap the lens takes.
Short answer
- Front cap that clips inside the filter thread — the size is your filter size. ⌀52 lens, 52mm cap.
- Front cap that slips over the outside of the barrel — the size is the outside diameter of the front ring. Measure it; it is not the same number as your filter size.
- Rear cap and body cap — these have nothing to do with diameter. They're matched to the lens mount.
If you don't know your filter size yet, start with how to find the filter size of any vintage lens.
Front caps: two different systems
Pick up the lens and look at how the cap sits — or, if the cap is long gone, look at the front of the lens itself.
Inside type (pinch or snap caps). These sit within the filter thread and hold themselves there with small sprung tabs. You squeeze the centre to fit and remove them. Because they locate against the thread, they are sold in filter sizes: 49mm, 52mm, 55mm and so on. If your lens has a filter thread, this is usually the easiest cap to find.
Outside type (slip-on or push-on caps). These go over the outside of the front ring and grip by friction, sometimes with a felt or velvet lining. Older lenses — especially ones with no filter thread at all — use these. They're sized by the outside diameter of the ring they sit on.
The important part: those two numbers are not interchangeable. A lens can take a 52mm inside cap and a slip-on cap of a completely different number, because one measures a thread and the other measures the outside of the barrel. Always know which type you're buying.
There is also a third, less common kind: caps that screw into the filter thread. Those are sized by filter size, like the inside type.
How to measure for a slip-on cap
Measure straight across the outside of the front ring — the metal edge the cap will sit over — with a ruler or, better, calipers.
Slip-on caps are usually made to fit snugly rather than exactly, and old ones vary, so a cap that is a fraction over or under can still hold. Even so, buy to the measurement you took, not to your filter size.
Rear caps and body caps
These are matched to the mount, not to a diameter. A rear cap for one mount will not go onto another, no matter how similar the sizes look.
So the question to answer isn't "what size" but "which mount" — a screw thread mount, or one of the many bayonet mounts. If you're not sure which your lens has, that's worth identifying before ordering anything that attaches to the back of it.
A body cap does the same job at the camera end, and follows the same rule.
An adapter ring changes your cap size
If you keep a step-up ring on a lens, the front of the lens is now the ring's size, not the lens's. A 49–52 ring means you now want a 52mm cap, even though the lens itself is 49mm.
It's a small thing that catches people out — the original cap suddenly doesn't fit anything. More on rings here: how to choose a step-up or step-down ring.
Does the rear cap matter more than the front one?
Both protect glass, but they're not doing quite the same job.
The front element is large, tough, and the part most likely to be knocked — that's what a front cap is for, and it also keeps dust off between shots.
The rear element sits much closer to the film or sensor. Marks and dust there tend to show up in an image more readily than the same marks on the front, simply because of where it is in the optical path. A lens stored without a rear cap collects dust exactly where you'd least like it.
If you can only find one, most people would put the rear cap first for storage, and the front cap first for carrying.
What to check when buying used caps
- The sprung tabs on pinch caps. They lose their spring, and a cap that doesn't grip will fall off in a bag.
- Cracks around the rim of plastic caps — the usual failure point.
- The lining on slip-on caps. Felt or velvet inside can be worn away or shed, which loosens the grip and leaves fibres on your glass.
- Deformation. A cap that's been squashed oval won't seat evenly.