How to Measure Your Lens/Scope for a Custom Ring Kit

Ordering a custom ring kit is easy — all you need are a few simple measurements of your lens. This guide walks you through each one step by step with photos, tips, and examples so you can feel confident you’re measuring everything correctly. When you're ready, you can return to the custom ring kit page and enter your numbers.

Basic Steps

1. Tips for Using Calipers 

• Start by zeroing your calipers before taking any measurements.
• Report the exact number your calipers display — don’t round up, down, or add “a little extra.”
• Whenever possible, use the flat portions of the jaws rather than just the tips; this gives a more stable reading.
• Apply only light thumb pressure to close your caliper jaws. If you’re squeezing hard, you’re distorting the reading.
• If you don’t already own calipers, these are the ones we recommend. Though most metal digital calipers will work fine if you prefer a different option.

2. Figure out where you want the front and rear rings. 

Identify two stable locations on the lens where the rings can sit:

Key guidelines:

• Pick spots that do not move when focusing or zooming.
  • Zoom lenses are fine as long as you’re comfortable locking the zoom ring in place once the rings are installed.
• Use a section that is straight (not tapered). Raised text or small switches for AF/IS/etc. are OK if they’re under 2 mm tall.
  • If they’re taller than 2 mm, reach out — we may be able to work around them with thicker foam or custom cut-outs.

• If the area is a bit narrower than the 15 mm ring width, that’s usually fine. The ring just needs 5–10 mm of usable width for a secure grip.
• If you cannot find a usable location for the rear ring and you’re using a cylindrical astronomy camera, you can put the rear ring around the astronomy camera body instead.
• If the front section of the lens has no suitable flat area, a cylindrical lens hood can often be used — just make sure it doesn’t taper outward.
• If your lens has a removable tripod foot, we strongly suggest taking it off; the round surface underneath is often the ideal rear ring location.

3. Measure the diameter of the front and rear ring.

Measure the diameter at each location you selected. This is the most important measurement for ensuring your rings fit correctly. Again, provide the exact number — no rounding.

4. Thick or regular ring size.

If your lens is heavier, longer, or you simply want a more rigid setup, consider choosing the “Extra Thick Rings” (30 mm vs. 15 mm). Use them only if you have the space to accommodate the extra width.

Extra Step for Optional Accessory Plates

5. Measure the Distance Between the Rings

If you plan to use accessory plates (findershoe, EAF bracket, etc.), we need the spacing between the centers of your two rings:

Extra Step for EAF Adapter

6. Measure the diameter of the helical focuser.

If you’re ordering an EAF adapter, measure the diameter of the helical focuser exactly as shown (in pink). Use the same caliper method you used for the ring diameters. 

7. Measure the distance to the focus ring. 

Measure the distance from the center of your front ring to the center of the helical focuser gear (shown in purple). The focuser should fall somewhere between the two rings. If your lens geometry makes this impossible (e.g., if you can't place the rear ring before the helical focuser), contact us — we can often work around unusual designs.

Put in the Measurements

Once you have your measurements, enter them on the product page and submit your order. You’re helping us build the rings to fit perfectly — thank you for taking the time to measure carefully!