Filter thread sizes are important to microscope adapters, because it is often much cheaper to make a custom adapter with a larger thread size than the camera provides. This is due to the difficulty of machining precision threads up to a shoulder, versus machining threads on the outside of a cylinder without a shoulder. The larger cylindrical adapter can then be adapted in turn to the camera’s smaller thread size by using an inexpensive, off-the-shelf step-up ring. Filter threads are not standardized other than holding to a metric form of arbitrary nominal size, but over the years camera manufacturers have come to mostly use a limited set of whole-millimeter sizes.

While it would have been economical to have a thread series such as is standardized for bolts, the industry never came to any agreement over this issue, and consequently adapting filter threads from one size to another is not always possible without a custom-machined item. The most commonly used filter threads are the 0.75mm pitch of the diameters in the following table. You are most likely to find inexpensive off-the-shelf step-up and step-down rings which directly or in combination adapt these sizes. In other words, you can make an inexpensive adapter from any one of these sizes to any of the others, usually in one step or two steps. Common Camera Filter Thread Sizes. Less expenxive and easy to find. 37mm, 43mm, 46mm, 49mm, 52mm, 55mm, 58mm, 62mm, 67mm, 72mm, 77mm, 82mm, 86mm and 95mm. Of the above sizes, 37mm is very common on consumer video camera lenses, and 58mm is very common on SLR camera lenses. The following are less common sizes for you which you may find an off-the-shelf step-up or step-down ring to one of the above sizes, from which you can get to any other common size, in one or two more steps. Not-so-common camera filter thread sizes. Adapters hard-to-find and/or expensive. 25.5mm, 27mm, 28mm, 30mm, 30.5mm, 32.5mm, 32mm, 34mm, 36mm, 39mm, 40.5mm, 44mm, 48mm, 54mm and 60mm. If you have to fit a size not listed above, chances are you will not easily find an inexpensive adapter. Since inserting a ring adds a few millimeters of axial length to the optical system, a single ring or combination of these rings may insert more than an acceptable length to the adapter. In this case there is no option but to eliminate the step rings by fabricating the target thread directly on the custom adapter, even though this may amount to an added cost. Note that the T-mount thread (42mm) and C-mount thread (1″-32tpi) not in either of the above series. Since these two threads are intended for camera lens mounts (mounting the exit end of a lens to a camera) instead of camera lens filters (mounting a filter on the entrance end of the lens), off-the-shelf adapters for them adapt things like camera bayonet rings, not filter threads