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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.
Common mistake: a telescope’s aperture size is not the same as the outer diameter used for slip-on filters.

Measuring Outer Diameter (OD)

  1. Identify the front (sun-facing) end of your optic.
  2. Measure the surface the filter will slide over (barrel or hood).
  3. Measure the widest point the filter must pass over.
  4. Record the measurement in mm or inches — either is fine.
Tape measure across telescope front showing outer diameter measurement

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.

Camera lens showing filter thread size marking

Choose Your Filter Size

Find my slip-on filter size
Shop slip-on filters Shop binocular filters

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.