Cómo medir un filtro solar

Sizing & Fit Guide: How to Measure for Solar Filters

Correct sizing is essential for safe, secure solar observation. This page explains how to measure and choose the right fit.

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

What “Size” Means for Solar Filters

Solar filter sizing is based on the attachment point. Unlike general accessories, solar filters must fit securely on the instrument’s front aperture.

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

Measuring Outer Diameter

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

How to measure OD correctly

  1. Identify the front (sky-facing) end of your optic.
  2. Remove any lens hood, cap, or cover.
  3. Check the first 1 inch (25 mm) of the tube for flares, trim, or rubber armor.
  4. Measure the widest point the filter must pass over within that first inch.
  5. Measure in millimeters (mm) when possible (1 inch = 25.4 mm).

Choosing a slip-on size

  • Select a size whose fit range is just larger than your OD measurement.
  • 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 marking as a slip-on reference. Slip-on filters fit the outer barrel, which is usually larger.

Device-Specific Sizing Notes

Telescopes & Spotting Scopes

Most telescopes and spotting scopes use slip-on filters sized by OD. Always measure the rigid tube or objective housing where the filter will seat.

Binoculars

  • Binocular filters are sold as matched pairs (one per barrel).
  • Both barrels must have the same OD.
  • Filters must be installed on both barrels before aiming at the Sun.

If barrel diameters differ, matched-pair filters are not compatible.

Camera Lenses (Threaded Filters)

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

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

Solar Film Sheets

Solar film sheets are intentionally oversized and trimmed to fit a custom front-mounted cell. Sizing applies to the cell you build, not the film itself.

The finished cell must fully cover the aperture and fit securely on the instrument’s front end.

Using the Fit Finder

The Fit Finder can help narrow options, but it does not replace proper measurement. Always confirm sizing manually before purchasing.

If your measurement is uncertain or falls between sizes, contact support for help choosing the safest fit.

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.