Sizing & Fit Guide: How to Measure for Solar Filters

Correct sizing is essential for a safe, secure fit. Measure first, then confirm your size before purchasing.

Measure first. Solar filters must fit securely on the front (sun-facing) end of your optical device. Sizing depends on how the filter attaches: slip-on (OD) or threaded (Ø).

What “Size” Means for Solar Filters

Slip-on filters are sized by outer diameter (OD).
Threaded camera filters are sized by the lens’s Ø thread diameter.
Binocular filters must fit both barrels.
Solar film sheets are sized to the cell you build.

Measuring Outer Diameter

How to measure OD correctly

Identify the front (sky-facing) end of your optic.
Remove any lens hood, cap, or cover.
Check the first 1 inch (25 mm) of the tube.
Measure the widest point the filter must pass over.
Measure in millimeters (mm) when possible.

Example: measuring OD at the front of a telescope

Tape measure across the front opening of a telescope showing the outside diameter measurement (OD) — Measure the **outside** width at the sun-facing end. If there’s a flare or trim ring, measure the widest point the filter must pass over.

Choosing a slip-on size

Select a size just larger than your OD.
If between sizes, choose the next size up.
Never force a filter onto an instrument.

Important: Do not use a camera lens Ø thread size as a slip-on reference.

Device-Specific Sizing Notes

Telescopes & Spotting Scopes

Measure the rigid tube or objective housing where the filter will seat.

Binoculars

Filters are sold as matched pairs.
Both barrels must have the same OD.

Camera Lenses

Use the Ø marking on the lens (e.g., Ø77 mm).

You’re Ready to Confirm Your Size

Next step: Enter your measurement to see the best-fit options.

Solar Film Sheets

Film sheets are trimmed to fit a custom cell and are not sized by OD.

Final Safety Check

Inspect the filter before each use.
Ensure it is securely installed.
Never look at the Sun through unfiltered optics.