What size thread is a 1.25 filter?
Most 1.25-inch filters used in astronomy and photography use a standard screw thread around 28.5 mm in diameter with a 0.75 mm pitch (commonly listed as M28.5×0.75). The 1.25-inch label refers to the filter’s outer diameter, not the thread, which sits inside the ring and threads into a holder. Some brands use 28.6×0.75 or near-equivalents. This article explains the standard sizes and how to verify compatibility with your gear.
Understanding the standard sizes
The term "1.25-inch" describes the filter’s external size, not a universal thread size. The actual male thread you’ll encounter on most screw-in filters is a metric thread around 28 mm in diameter, with a fine pitch of about 0.75 mm. Here are the typical sizes you’re likely to see:
- M28.5×0.75 — the most commonly cited standard for 1.25" filters.
- M28.6×0.75 — a close variant used by some manufacturers; fitting can be compatible with standard holders but check tolerances.
- M28×0.75 — an older or budget option that is near-equivalent to the above.
- Note: the outer diameter of the filter assembly (the part that sits in the holder) is usually around 31.75 mm (1.25 inches).
These figures refer to the screw-thread portion that secures the filter in a threaded holder. If you’re unsure of the exact size for your kit, measure with calipers or consult the manufacturer’s specs.
How to verify your specific filter’s thread size
Because variation exists across brands and eras, it pays to confirm before buying replacements or adapters. Use these steps:
- Check the product details for the thread spec (look for M28.5×0.75 or M28.6×0.75).
- Measure the thread with calipers to determine the major diameter (around 28.5–28.6 mm) and pitch (about 0.75 mm).
- Compare with your filter holder’s required threading or bring the component to a shop for a hands-on check.
Having the exact thread specification ensures a snug, secure fit and prevents cross-threading during use.
Summary
For a typical 1.25-inch filter, expect an M28.5×0.75 thread size as the standard, with some brands using M28.6×0.75 or close variants. The filter’s outer diameter remains about 31.75 mm, while the thread itself is around 28.5 mm in diameter. Always verify the precise spec from the manufacturer or measure the thread to ensure compatibility with your filter holder or adapters.
Is m10 1.25 fine thread?
For instance, an M10x1. 5 (coarse) thread boasts a 1.5mm pitch, whereas an M10x1. 25 (fine) thread has a 1.25mm pitch. This distinction is critical in applications that require high precision and strength.
What size thread is a 1.25 inch filter?
Very common in astronomy on barrels of 1.25" eyepieces and 1.25" filters. While 1.25"=31.7mm, the actual thread diameter is 28.5mm.
What does 1.25 thread pitch mean?
For metric fasteners, you will see a M8 x 1.25 or an M8 x 1. For thread pitch, the distance between two points is the second number meaning the higher the number the fewer threads there are. This means the M8 x 1.25 is the coarse threading and the M8 x 1 is the fine thread.
How to find filter thread size?
If it's not there or you've lost your cap. You can also find it on your lens. Sometimes it looks like a circle with a line through it followed by a.
