Sizing & Fit Guide – How to Choose the Right Solar Filter Size

Sizing & Fit Guide – How to Choose the Right Solar Filter Size

Choosing the correct size is essential for safe, secure use. The sizing method depends on the type of filter you are using. This guide covers the practical sizing steps for slip-on filters, binocular pairs, and threaded camera filters.

1) Slip-On Solar Filters (Telescopes, Spotting Scopes, Some Camera Lenses)

Slip-on solar filters fit over the outside of an instrument. Slip-on sizing is based on the outer diameter (OD) of the tube or barrel where the filter will slip on.

How to measure OD

  1. Identify the front (sky) end of your optic (objective end / dew shield / front barrel).
  2. Remove any lens hood, cap, or cover. Slip-on filters must attach to the rigid barrel or objective housing, not to a hood.
  3. Check the first 1" (25 mm) of the tube. The filter typically slips on about this depth. Look for any flare, ridge, rubber armor, or trim that becomes wider within the first inch.
  4. Measure the outer diameter (OD) across the widest point the filter must slide over (within that first inch).
  5. Measure in millimeters (mm) if possible. If you measure in inches, multiply by 25.4 to convert to mm (for example, 4" ≈ 102 mm).

Choosing a size

  • Select a slip-on filter size whose fit range is just larger than your OD measurement.
  • If your measurement falls between sizes, choose the next size up.
  • Never force a filter onto an instrument. The filter must install securely and should not be loose.

Important: Do not use the camera lens Ø thread size as your only reference for slip-on filters. Slip-on filters fit the outer barrel, which is usually larger than the thread marking.

2) Binocular Solar Filter Pairs

Binocular filters are sold as matched pairs, with one filter for each barrel.

Compatibility requirements

  • Both binocular barrels must have the same outer diameter (OD).
  • Filters must be installed on both barrels before aiming at the Sun.

If your binocular barrels are different sizes, matched-pair filters are not compatible.

3) Threaded Camera Solar Filters

Threaded solar filters screw directly onto the front threads of a camera lens. Threaded filters do not use OD measurements.

How to choose your size

  • Look for the Ø (diameter) marking on your lens or lens cap (for example, Ø77 mm).
  • Choose the filter that matches this number exactly.

Final safety check

  • Inspect the filter before each use and discontinue use if it is damaged or compromised.
  • Ensure the filter is securely installed before aiming at the Sun.
  • Never look at the Sun through unfiltered optics.

Correct sizing is a critical part of safe solar observation.