Sizing & Fit Guide
How to measure correctly and choose the right solar filter size.
Measure the attachment point. Solar filters are sized by how they attach — not by aperture.
Slip-on filters slide over the outside of the front end (outer diameter, OD).
Threaded filters screw directly into lens threads (Ø).
What “Size” Means
- Slip-on filters: sized by the outer diameter (OD) of the surface they slide over.
- Threaded camera filters: sized by the Ø marking on the lens (exact match).
- Binocular filters: both barrels must have the same OD.
- Solar film sheets: sized to the cell you build.
Measuring Outer Diameter (OD)
- Identify the front (sun-facing) end of your optic.
- Measure the surface the filter will slide over (barrel or hood).
- Measure the widest point the filter must pass over.
- Record the measurement in mm or inches — either is fine.
This example measures slightly over 4" (about 4 1/16"). A 4" filter would be too small — always round up if you’re between sizes.
Slip-on filters are retained with thumb screws. Some movement is normal — the filter should remain in place without forcing.
Device-Specific Notes
Telescopes & Spotting Scopes
Most use slip-on filters sized by OD. Measure the rigid front housing where the filter will seat.
Binoculars
- Filters are sold as matched pairs.
- Both barrels must have the same OD.
Camera Lenses (Threaded)
Threaded filters screw directly into the lens threads. Use the Ø marking on the lens — this applies only to threaded filters, not slip-on sizing.
Choose Your Filter Size
Manual method
- Find the filter whose fit range includes your measurement.
- If unsure or between sizes, choose the larger size.
Final Safety Check
- Inspect for pinholes, cracks, tears, or loose elements before use.
- Confirm the filter remains in place when installed.
- Never look at the Sun through unfiltered optics.
Correct sizing is a critical part of safe solar observation.