Man, this is a hard question to answer! It has tons and tons of answers. Frankly, it is too open-ended to answer realistically for every set of binoculars and spotting scopes. There are a lot of factors to consider.
What are you trying to see? You can look at the moon and it’s 240,000 miles away. You can see the sun, and it’s nearly 100 million miles away.
How far can you see without the aid of optics? Your personal eye health is a consideration when answering this question. Some people can see much further than others naturally.
Another consideration is the horizon. Past 3 miles at sea level, you can’t see much. Are you and what you are looking at higher than sea level? Then you can see beyond the typical 3-mile horizon. This varies by atmospheric conditions, weather, brightness, and more.
Let’s talk magnification. A spotting scope or a binocular will allow you to magnify an object at the approximate magnification of your optics. You can find this information in the owners manual or on the manufacturer’s website.
How does magnification work? The best way to explain it is using an example. If your spotting scope is 60X and you are watching a bird, the bird will appear to be 60 times larger than it would to the naked eye.
Higher magnification won’t give you the ability to see farther. Think of it as the ability to make things appear larger. A higher magnification allows you to see smaller things easier. Obviously, magnification is only as good as the optic it’s part of. To see something small and very far away, you need a high-quality optic.
You need light to maximize the distance you can magnify an object. Light is also key to seeing the image clearly. In fact, the farther you are looking the more light you will need. The light your optic receives is directly affected by magnification and objective lens size in the form of the exit pupil.
You can calculate the exit pupil by dividing the objective lens by the magnification level. This will give you the exit pupil size in millimeters. For reference, the human pupil is 2-3 millimeters. The higher the exit pupil number is, the clearer the sight picture will be.
Optimistically, you want your optic’s magnification and objective lens to work well together. This will provide you with a clear, large exit pupil. When choosing an optic, it’s important to pay attention to magnification and objective lens size.
Sometimes an optic with lower magnification gives you a clearer picture than an optic with higher magnification. For example, a 10×40 pair of binoculars gives you a fantastic 4mm exit pupil. This gives you a clearer picture than a set of binos that are 20×40. The 20×40 only have an exit pupil of 2mm.
If you want to maximize the distance at which you can clearly see a target, you need to ensure you have a solid exit pupil!
When you are scouting a target, it’s wise to know what works where. If you go birding, you know that birds are relatively small creatures. This means you aren’t going to be able to see one at several hundred yards with a regular spotting scope or binoculars. Instead, you’ll likely need a high-power optic to track them.
If you want to spot deer, you can get by with a set of 4x binoculars. Deer are considerably larger targets, so 4x binos are powerful enough to see them at hundreds of yards.
At a thousand yards away, you might be able to see your targets with a spotting scope. However, you won’t be able to see the tiny bullet holes at that range.
At a few hundred yards, with a powerful and well-made spotting scope, you can judge group sizes.
One Final Word About Distance
When it comes to binoculars and spotting scopes, you have to balance your expectations with your objectives. Don’t think, “how far can I see with binoculars or a spotting scope.” Instead, think, “what do I need to see and how far away do I need to see it?”
Identifying what exactly you are trying to do is the key to finding the best binoculars and spotting scopes for you or determining which one you need.