Even the best binocular will cause eyestrain if it's not focused properly. Here are some easy steps to ensure that the focus on your field binocular is tack sharp.
1. Cover the diopter-adjustable lens and focus the non-diopter lens first, using the center focus wheel.
Focus on something distinct, like a fence line.
2. Now cover the non-diopter lens with your hand and
focus the diopter-adjusted lens using the diopter adjustment on the
3. Using both hands (so that nothing obstructs either lens), check your
focus once more using the binocular's center adjustment wheel.
4. Make a final check of your focus by viewing various objects at 50, 75 and 100 yards. Now the binocular should be perfectly adjusted for your eyes.
Check out my binocular test with Steiner Binoculars: