If you have binoculars, you can fix the focus for different yardages. Take your archery target out to 20 and focus the binoculars until the image is perfect. Mark the focus dial. Repeat the proces out to 30 and 40 using different markings or colors for different ranges. Then you can just focus with your binoculars and look at your focus control for range. Not exactly precise, but not expensive either. I hope I explained that where it is understandable.