Peoples often searches what mm diameter of filter, macro lens will be perfect. It actually depends only on the particular lens. Here is Easy Guide to Find What Size Filter Your Lens Need. Older lenses has not many ways than finding old manual or Google search or finding experienced peoples. Newer lenses intended for digital cameras usually have the parameter printed on the lens plus box packaging, specification also declares the parameter.
Find What Size Filter Your Lens Need
Different lenses usually have different diameter of screw mounts. The lenses can be used on the same camera but may have different diameter of screw mount. For long term, it is likely that one will have ND filters of different diameters for different lenses.
Notice that front of your lens has a 0 with a strike through like symbol – the Ø symbol. That is used as universal symbol on camera lenses to indicate the needed diameter of the screw mount on the front of the lens. The below Sony 50mm f/1.4 Planar T* lens has thread diameter of 72mm :
In some old lenses, the Ø numerical may be present on the reverse side of lens cap or closer to adjustments. In old days, the symbol and numerical values usually engraved. As these days they are printed, they are located where the user unlikely to regularly touch to make it disappear.