1. Download one of the ruler files here:
IMPORTANT: Make sure to print out the ruler at 100% size, do not scale to fit.
2. Cut out the ruler and wrap around the focus ring of your lens. Make sure to wrap it around the focus ring, not other parts of the lens such as the filter
3. Record the measurement of your focus ring's circumference. The following chart will help you understand which ring to purchase:
| If your lens circumference is within | Your Redrock Lens Gear Size is |
| 180mm-220mm | Size A |
| 211mm-250mm | Size B |
| 241mm-280mm | Size C |
| 280mm-320mm | Size D |
If your focus ring circumference can be used with more than one Redrock lens gear, you can select either lens gear.
Canon EF Lenses
Here are some common Canon EF (EOS) lenses and their corresponding lens gear size (if more than one is listed, you can use either):
| Canon EF Lens | Diameter mm | Lens gear |
| 14mm f/2.8 L | 75 | B |
| 16-35mm f/2.8 L | 76 | B,C |
| 16-35mm f/2.8 L II | 76 | B |
| 17-35mm f/2.8 L | 76 | B |
| 20-35mm f/2.8 L | 77 | B,C |
| 20mm f/2.8 | 75 | B |
| 24mm f/1.4 L | 80 | C |
| 17mm f/4.0 Tilt Shift Lens | 88 | C |
| 24mm f/3.5 Tilt Shift Lens | 79 | B,C |
| 45mm f/2.8 Tilt Shift Lens | 78 | B,C |
| 90mm f/2.8 Tilt Shift Lens | 73 | B |
| 24mm f/2.8 | 67 | A |
| 28mm f/1.8 | 73 | B |
| 28mm f/2.8 | 66 | A |
| 24-70mm f/2.8 L | 84 | C |
| 28-70mm f/2.8 L | 83 | C |
| 24-105mm f/4.0 L IS | 82 | C |
| 35mm f/1.4 | 79 | C |
| 35mm f/2.0 | 67 | A |
| 50mm f/1.8 | 65 | A |
| 50mm f/1.4 | 74 | B |
| 50mm f/1.2 | 86 | C |
| 50mm Macro f/2.5 | 67 | A |
| 100mm f/2.0 | 75 | B |
| 100mm Macro f/2.8 USM | 79 | B,C |
| 100mm Macro f/2.8 | 75 | B |
| 135mm f/2.0 | 76 | B,C |
| 85mm f/1.2 | 92 | D |
| 85mm f/1.8 | 75 | B |
| 70-200mm f/2.8 L | 83 | C |
| 70-200mm f/2.8 L IS | 86 | C |
| 100-400mm f/4.5-5.6 L IS | 90 | D |
| 180mm Macro f/3.5 L | 78 | B,C |
| 200mm f/2.8 L | 75 | B |
| Zeiss Distagon f/3.5 18 mm Lens with T* Coating for Nikon ZF mount | A | |
| Zeiss Distagon f/2.8 25mm Lens with T* Coating for Nikon ZF mount | A | |
| Zeiss Distagon f/2 28mm Lens with T* Coating for Nikon ZF mount | A | |
| Zeiss Distagon f/2 35mm Lens with T* Coating for Nikon ZF mount | A | |
| Zeiss Planar f/1.4 50mm Lens with T* Coating for Nikon ZF mount | A | |
| Zeiss Planar T* 1,4/85 ZF.2 Lens | B | |
| Zeiss Makro-Planar T* 2/100 ZF.2 Lens | B |