Your lens’s filter size is printed as a diameter symbol followed by a number in millimeters, usually somewhere on the front of the lens barrel, on the inside of the front cap, or in the manufacturer’s official specification sheet under a filter size or filter diameter listing. That number, not the lens’s focal length, is what determines which screw-in filter will physically fit.

Locate the Diameter Marking

Look for the diameter symbol, a small circle with a diagonal line through it, followed directly by a number and “mm,” such as a marking near the front rim of the lens barrel. This is the standard way manufacturers indicate filter thread size, and it’s usually engraved or printed right around the filter threads themselves.

If the marking on the lens is worn, hard to read, or the lens has no visible thread at all, check inside the front lens cap, since caps are frequently molded with the size printed on the inner surface, or check the manufacturer’s official specification page for that exact lens model, which will list the filter diameter as one of its stated specifications.

Separate Three Different Measurements

Three different numbers on a lens are all expressed in millimeters, and confusing them is one of the most common filter-buying mistakes. Focal length describes the lens’s angle of view and magnification, and has nothing to do with what filter fits it. Filter thread diameter is the actual screw thread size at the front of the lens. Lens barrel width is the overall physical diameter of the lens housing, which is typically larger than the filter thread since the housing includes focus rings, zoom rings and external controls.

  • Focal length: describes framing and magnification, unrelated to filter fit.
  • Filter thread diameter: the actual screw size a round filter needs to match.
  • Lens barrel width: the outer housing size, generally larger than the filter thread.

As an illustration, a lens with a focal length in the 50mm range might use a filter thread that’s a completely different number, such as in the high-40s or low-50s millimeter range depending on the specific model, purely by coincidence rather than because the two numbers are related. Always check the actual printed filter diameter rather than assuming it matches the focal length number.

Check Unusual Front Elements

Not every lens accepts a standard screw-in filter. Some lenses, particularly certain wide primes and compact designs, have a fixed hood built into the barrel with no filter thread at all. Ultra-wide lenses with a curved or bulbous front element also can’t accept a flat screw-in filter directly, and instead require a dedicated bayonet-style holder system sized specifically for that lens model.

Some super-telephoto lenses and select wide-angle designs place a small filter slot near the rear mount instead of at the front, sized for gel or thin drop-in filters that have nothing to do with the front element’s dimensions. Professional telephoto lenses sometimes use a drop-in filter cartridge system as well, which is a completely separate accessory from any round front filter.

Confirm Fit Before Ordering

Before ordering a filter, confirm the exact lens generation or version you own, since manufacturers occasionally change the filter thread size between versions of what’s otherwise marketed as the same lens name. A camera body’s brand has no bearing on filter size either; two lenses of the same focal length, even from the same brand, can use entirely different thread diameters, so never infer filter size from the camera or lens brand alone.

  • Check the printed diameter symbol on the lens or cap.
  • Cross-reference the manufacturer’s specification sheet for your exact lens model and version.
  • For holder-based systems, confirm both the adapter ring size and the holder’s overall width compatibility, not just the round thread number.