Changing eyepieces on a scope: how to increase magnification

Lots of people wonder if they can switch the eyepiece on a spotting scope to increase or decrease magnification. The good news is that with many models of spotting scope it is possible to change the eyepiece.

We’ll look at your different options that will allow you to adapt your spotting scope to a variety of birding situations. You’ll understand how to increase the versatility of your scope and make it useful in more circumstances.

The two types of spotting scopes

When looking at spotting scopes, there are two options:

1. A spotting scope with a single, in-built eyepiece. When the eyepiece is built in it, it usually has zoom magnification. This is also known as multi-magnification.
2. A spotting scope with an interchangeable or detachable eyepiece. You can then choose alternative eyepieces. These come with either fixed magnification or zoom.

A spotting scope with a built-in eyepiece will indicate this in its name. For example, a 15-45×60 model or a 20-60×80 model. Both of these are popular sizes for birding.

A spotting scope with an interchangeable or detachable eyepiece will only list the size of its objective lens. This is likely to be something like 60mm or 80mm. Then you add whichever ocular lens you prefer.

Can I increase the magnification of a spotting scope?

If you already have a spotting scope with variable magnification, then using the zoom facility, you can increase or decrease the power as you wish.

Alternatively, if your spotting scope is the sort where the eyepiece is detachable, then it is possible to increase or change its range of magnification. You could do this simply by adding a different eyepiece.

How do you increase the magnification of a spotting scope?

In order to increase the magnification of a spotting scope there are a few options. You could add:

• an eyepiece with a higher fixed magnification
• an eyepiece with a higher variable magnification
• an extender

A number of spotting scopes on the market today have an eyepiece that detaches from the main body of the scope.
So to increase the power of your spotting scope, in theory all you have to do is get a new eyepiece of a higher magnification.

However, there are quite a few things that are worth considering before you take the plunge in purchasing a new eyepiece.

Do you want a fixed or a variable magnification?

This is a big question. It warrants deep consideration as there are various implications for image quality and practicalities. We have a whole article to weigh up the choice, so please consider taking a look at this:

Should I get a fixed or zoom eyepiece for my spotting scope?

Changing a scope eyepiece to upgrade magnification

As we have discussed, the eyepiece on a spotting scope is the part that determines the magnification. In other words, the eyepiece and its power of magnification dictate how many times closer the objective that you are viewing appears.

Some spotting scopes have an eyepiece that cannot be changed. It is built into the body of the scope. This means that whichever range of magnification you choose will be the one that you use permanently with that scope.

These kinds of spotting scopes normally come with a variable or zoom magnification, for instance of 20-40x. In this example, the zoom or multi-magnification eyepiece covers a range of magnification between 20x and 40x.

Other scopes have an eyepiece that can be switched over to an alternative magnification. These changeable eyepieces give you a wider range of magnifications to choose from. So you might have one eyepiece of 11-33x, that you can take off and switch over to another eyepiece with the magnification range of 22-48x.

How to increase power with a magnification extender

Another option to increase power on your spotting scope is a magnification extender (ME). An ME will typically increase the magnification from between one and a half to two times.

Only the premium manufacturers make these products. So it is worth finding out whether a magnification extender is something that might be a future purchase for you, before committing to buy a particular brand. In this way, it is possible to keep your options open when it comes to increasing magnification.

This is a really interesting option for easy extra power. It is worth knowing in advance if it’s something that might appeal to you. Then you can research if the brand you are considering offers this type of extender.

We cover the topic in more detail in this article >>>

Is a magnification extender worth it?

Will the size of spotting scope accommodate the increase in magnification?

When an eyepiece is described as having a particular magnification, it doesn’t mean that that eyepiece will always give that same magnification. The true measurement is dependent on the body of the scope that the eyepiece is attached to.

This is because the eyepiece contains one lens – the ocular lens – which has a focal length. And the scope contains another lens – the objective lens, which also has its own focal length.

Why is focal length important?

The focal length is the distance from the lens to the point where the image is able to be viewed sharply. The focal length is measured in millimeters. The magnification is the result of the combination of the two lenses.

Formula to find true magnification

To work out the true magnification, divide the focal length of the objective lens by the focal length of the eyepiece.

This formula uses both the size of the image (created by the objective lens) and how many times it is made bigger (by the ocular lens in the eyepiece).

This measurement can also be expressed as a ratio. Focal ratio is the objective diameter in relation to the objective’s focal length.

Technically, it is more accurate to say eyepieces have a focal length rather than a magnification. In reality, though, most people are interested in how many times larger an object appears in the image that they see, not in the focal length. This is why eyepieces are commonly described in terms of magnification.

In practical terms, this means that the same eyepiece used on a spotting scope with a smaller objective lens will have less magnification power than if it were joined to a spotting scope with a larger objective lens.

You could boost the magnification power of a smaller scope by attaching an eyepiece of a higher power. But because a bigger objective lens lets in more light and has a longer focal length, it creates a better initial image. So the larger scope (in other words, a larger objective lens) would result in a better quality final image reaching the eye.

What to think about when adding magnification

A popular size for a birding spotting scope is around 20x to 30x. Starting with this basic size in the range of 20x to 30x in your kit, it is always possible to add a stronger power, such as 40x.

The most advisable way to approach an increase in power is to consider the large magnification eyepiece as an alternative piece of kit rather than a replacement.

These high magnification eyepieces are well suited to long-distance birding, especially on stationary birds. You’re likely to see the best results in strong daylight.

Different distances: How far can I see with a spotting scope?

Are spotting scope eyepieces interchangeable or universal?

Spotting scope eyepieces are interchangeable in the sense that many detach from the main body of the scope. This allows the user to swap one eyepiece for another one in order to increase or decrease magnification, or to switch from a fixed to a variable magnification.

In many cases, companies produce eyepieces that only fit their own spotting scopes. So in this sense eyepieces are not universal nor might they be used across different makes of scopes.

However, one eyepiece would usually fit several sizes of scope all from that same company. So for example, there might be a 20-60x eyepiece that is suitable for use with either a 60mm or a 80mm scope within the same range offered by the same manufacturer.

On occasion, there are some brands that produce eyepieces suitable for use with scopes made by other manufacturers. Some claim to have a standard bayonet or screw-in attachment that fits with the scopes of multiple brands.
This is rare, though. Normally you need to choose from the eyepieces produced by the same brand of your spotting scope.

Final thoughts on increasing magnification by changing eyepieces

As is the case in so many situations where people are choosing optics according to their own particular bird watching requirements, there is no one simple answer. But the major advantage of the spotting scopes with interchangeable lenses is that there can always be multiple answers.

Your usage might vary according to the environment in which you go birding – landscape and light conditions – and the kind of birds you prefer to spot.

Your options for different eyepieces would also depend on the type of spotting scope that you already own. Therefore you might need to choose from whichever eyepieces that manufacturer makes. Or it might depend on how you are able to balance budget constraints between the scope itself and the variety of eyepieces.

The beauty of these kinds of optics is that you can choose whichever lens suits best now. Over time you can see how that works for you. Then it is always possible to choose another kind of magnification later. You could either trade in your old lens or add another one to your collection. Now that is what we call keeping your options open.

How to choose a spotting scope for birding