The ø number on the front of your lens is the filter size. It is not the focal length.

If the ring says ø49, you buy a 49mm filter. That is the whole rule. Most wrong-size orders I see start with someone reading the “35” or “50” on the barrel and treating that as the thread.

Where the ø actually sits

Look at the front element ring, right around the glass. Manufacturers usually print something like ø49 or 49mm next to the filter symbol. Flip the lens hood off if it is in the way. Some older barrels hide the mark under the hood mount, so you have to twist the hood off before the number shows.

On my current street kit—a Lumix GH7 with an OM SYSTEM 12–40mm f/2.8 PRO II, the same ~1.2kg handheld setup I have walked Hong Kong with since March 2025—I still check that front ring before I order glass. Muscle memory lies. The printed ø does not.

When I had my Pen-F and a 45/1.8, the same habit applied: find ø first, then shop. Different bodies, same thread logic.

What that number is not

Focal length is not filter size. A 35mm lens can take a 49mm, 52mm, or 58mm ring depending on the design. Sensor format does not set the thread either. M43, APS-C, and full frame all use the same millimetre labels once you are buying a screw-in filter.

The diameter of the front glass can look bigger or smaller than the thread. Trust the printed ø, not how wide the optic looks in a product photo.

If the mark is gone or never printed

Some lenses skip the front print. Others lose it after years of hood swaps and cleaning. Then:

  1. Check the manufacturer’s lens page or the PDF chart for “filter diameter.”
  2. Look at the rear of the lens cap; a few brands stamp the size there.
  3. Email me the exact lens model (and any adapter you keep mounted). I will match the millimetre from our size list rather than guess from a blurry phone photo.

Do not force a near-miss thread. A 52mm ring on a 49mm barrel either will not start or will cross-thread. Adapters exist for stepping up, not for “close enough.”

Buy the millimetre, then pick the glass

Once you know the ø, shop that size across the filter type you actually need—clear protection, CPL, or black mist—not a random “kit” that happens to be in stock.

Our screw-in line runs roughly 39mm through 105mm depending on the filter. Everyday clear protection such as the XT996S ($44.99–$79.99) covers 39–82mm. A value CPL like the XT003 ($39.99–$69.99) sits in the same 39–82mm band. Daily black mist on the XT888 Bottom 1/4 ($49.99–$119.99) stretches 39–105mm, including common Leica/Fuji 39mm threads and the 49mm sizes used on Q3 43 and adapted X100 setups.

I keep an XT996S on the 12–40 for seaside evenings as front-element insurance; on that lens I have not seen a visible IQ hit from leaving it mounted. That is protection use, not a reason to skip reading your own ø.

Find the ø. Then shop that millimetre. If the mark is missing, email me the lens model and we will match it.

— Leo Liang