Close Focus Distance in Binoculars: A Comprehensive Guide
February 20, 2005
If you’re an avid birdwatcher, a nature enthusiast, or just someone who enjoys clear, crisp views through binoculars, understanding close focus distance (sometimes called near focus distance) is essential. In this comprehensive guide, we’ll walk you through everything you need to know about close focus distance, how it’s measured, and why it matters for different applications—from birdwatching and nature observation to astronomy and spectating.
What Is Close Focus Distance?
Close focus distance refers to the shortest distance at which a binocular can focus on an object while still providing a sharp and resolved image. A short close focus distance is particularly important for entomologists, botanists, and birdwatchers, who all study small creatures or plants up close.
How Close Focus Distance Is Measured
Optical laboratories measure close focus distance using specialized testing apparatus. Testers place a viewing target closer and closer to the binocular until they find the nearest point where sharp focus is achieved on a sensor attached to the binocular’s eyepiece. For larger binoculars that cannot fit into standard lab equipment, their close focus distance is calculated instead.
The data obtained through these tests or calculations is published as the binocular’s official close focus distance.
Real-World Use vs. Laboratory Measurements
In real-life settings, your personal experience with close focus distance may vary slightly from the published specs. Factors like individual vision (especially if you don’t have 20/20 eyesight) and environmental conditions can result in a difference of about a foot (0.3 meters). Fortunately, this minor variance rarely impacts the overall usability of the binoculars.
Why Close Focus Distance Matters
Close focus distance is more or less important depending on how and where you use your binoculars. Below is a breakdown of various activities and the ideal close focus distances for each:
Nature Observation (Birdwatchers, Botanists, Entomologists)
- Minimum close focus distance: Less than 10 feet (3.0 m)
- Very short close focus distances enable nature observers to get close to their (often tiny) subjects for the most detailed views.
Larger Mammal Observation (Hunters, Mammal Biologists, Ranchers)
- Minimum close focus distance: About 15–20 feet (4.5–6.0 m)
- Since these viewing subjects are larger, an extremely short focus distance is less necessary.
Event Viewing (Concerts, Sporting Events)
- Minimum close focus distance: About 25 feet (7.5 m)
- Performers or sports action are rarely right in front of you, so extremely close focus isn’t typically needed.
Whale Watching and Seawatch Birding
- Minimum close focus distance: About 50 feet (15 m)
- Because you stand on headlands or beaches to watch subjects far offshore, you’ll want to prioritize focus control at the far end of the range, not the close end. The binoculars you use for these activities need not be well suited to close viewing.
Astronomy
- Minimum close focus distance: 100 feet (30 m) or more
- When you’re stargazing, targets are very distant, making close focus specs almost irrelevant. Choose an astronomy binocular based on its ability to focus far away.
Key Questions About Close Focus Distance
Q: Is there a downside to having a very close focus distance in a binocular?
A: This depends on the binocular’s intended purpose. Binoculars designed for short close focus distances (e.g., for nature observation) may sacrifice precision at the long-range end of the focus spectrum. While they can still focus at a maximum distance, far-focus control may be reduced, which could be a drawback for astronomy or other long-distance viewing activities.
Q: Can a binocular have a close focus distance that is too close?
A: Yes. When the subject is closer than 5 feet (1.5 meters), you may fail to get a stereoscopic image (the 3D-like depth effect). Binoculars rely on each eye receiving a slightly different image, which merges into a single, 3D view. At distances under 5 feet, this alignment can fail, leading to double vision. In these scenarios, choose a monocular instead.
Q: Can binoculars with a short close focus distance be used for other purposes?
A: You can usually use binoculars designed for nature observation for general activities like whale watching or even astronomy, as long as their magnification and resolution compensate for their lack of far-focus control. On the other hand, binoculars specifically designed for astronomy are unsuitable for close-range use.
Choosing the Right Close Focus Distance
If you need binoculars for diverse activities, choose one with the shortest close focus distance required for your primary purpose. This ensures versatility across different scenarios. Binoculars with longer close focus distances are generally specialized, designed for far-distance clarity, and won’t perform as well in close-range viewing.
By optimizing your binocular choice according to close focus distance, you’ll be well-equipped for whatever viewing adventures lie ahead.