Magnification and Your Telescope: Why Focal Length Matters

Many people in the market for a telescope for the first time want to find a scope with the highest magnification. This is usually expressed as a number followed by an “x” such as 50x or 100x. What this generally means is that an object will appear 50 times larger (50x) or 100 times larger (100x).

Our tendency is to assume that more and bigger is better. But as we’ll discuss, magnification has its limits. These limits are defined by measures that few amateur astronomers understand at the outset. It involves focal length, focal ratio and practical magnification.

Obscured by Clouds Alexey Kljatov

The focal length of a telescope is determined by a simple measurement. It’s the distance measured in millimeters from the primary lens or mirror to the point where the light comes into focus at the eyepiece. Think of it as a long and tall pyramid in the telescopes tube. The light is captured by the primary lens or mirror and focused at the eye piece. The actual length of this elongated pyramid is the focal length. The focal length is often printed on the scope or the box or in the instruction manual. Knowing your scopes focal length is important when it comes to buying additional eye pieces or a Barlow lens.

Why is Focal Length Important?

Understanding the focal length of your telescope will help you assess the practical limits of magnification for a particular scope. What many of us learn very soon as we drop eyepieces into our scope with higher levels of magnification is that two things occur. The object becomes dimmer as the magnification increases, and we begin to lose some detail in terms of focus sharpness. This is when you are reaching the limits of magnification with your scope.

How Does Focal Length Affect what I See?

There’s a phrase called “angular field of view.” This defines how much of the sky you see in your scope. Longer focal lengths tend to have a narrower field of view. Shorter focal lengths have a wider field of view. You might be looking at an object with two scopes with long and short focal lengths and see an object of the same size, but the amount of sky and other stars in a wide field will be greater with a short focal length than a long focal length.

It’s why catadioptric scopes or Cassegrains with their very short focal length present the widest field of view. Dobsonian scopes also offer a wider field of view due larger to the size of their mirrors even though they tend that have long tubes resulting in long focal lengths. This may seem confusing but the equations we’re working through will help you make sense of variations.


How do I determine the magnification of my scope and certain eyepieces?

Here’s an equation and it begins with knowing your focal length and the focal length of your eyepieces. The focal length of your eyepiece is often printed on the eye piece itself. If your telescope has a focal length of 800mm and you are using a 20mm eyepiece you divide the focal length of the scope by the focal length of the eyepiece: 800mm/20mm = 40. As a result you will get 40X as a your magnification. However, putting that same 20mm eyepiece in a scope with a focal length of 400 will give you a different magnification level of 20x.

400mm/20mm=20. This is another reason to know your focal length as you consider eyepieces for your scope.

What is Focal Ratio and Why is it Important?

The limitation related to the brightness and sharpness of an object is defined as the focal ratio. It’s the “speed” of your telescope’s optics. It’s actually measured in f.stops. If you’re familiar with photography you know that an f.stop is the size of the aperture on a camera that determines how much light enters the lens. It’s also one of the those curious situations where the bigger the number the smaller the aperture.

If your scope has a focal ratio of f.4, you have a larger aperture and can gather more light. This is good for deep space observing and photography but it will also mean a smaller magnification. It will also give you a wide field of view and capture light better.

A scope with a focal ration of f.11 is better suited for observing the moon and planets and high power photography. However, deep space objects will be both faint and dimmer.

A focal ratio of f.6 to f.10 is a good middle ground if you prefer to observe both near Earth and deep space. This is another determining factor when you’re choosing a scope. If your sole objective is to study the moon you’d probably want a scope with a focal ratio of f.11 or above up to f.15 or more. The converse is true for deep space. Most new astronomers are better served with a scope in the mid-range focal ratio from f.6 to f.10.


I Don’t Know my Scopes Focal Ratio. How Do I Figure it Out?

There are a couple of ways to do this. One way is to get on the Internet and see if the manufacturer or some other resource can tell you. There’s also an equation that is fairly simple if you know some other measurements related to your scope. You’ll need to know the focal length measured in millimeters and the size of the aperture also measured in millimeters. This is sometimes referred to as the “clear aperture.” This is a number you need to find either on the box, on the scope, in the instruction book or on the Internet. Many sites that sell scopes will give you the clear aperture in the “specifications” area for a specific scope. In order to determine the focal ratio you need to divide the focal length by the aperture.

For example. If your focal length is 100mm and your aperture is 25 millimeters 100/25 = f.4. That means you have a scope that is good for deep space viewing and a wide field of view although your magnification will be limited. Many people learn the hard way after purchasing a scope and struggling to observe a variety of objects that the focal ratio affects their ability to observe a wide variety of objects.

We’ve also all learned that the higher the magnification, the more difficult it is to track the object given the Earth’s rotation and the movement of some near Earth objects in the sky such as the moon and planets. A motor drive can compensate, but if you don’t have a motor drive you’ll probably find that your inability to consistently track an object is another limiting factor if your magnification is too high.

Nearly full moon Alexey Kljatov

How Can I Find the Magnification Limit on my Scope?

There’s a simple equation that will help you determine the actual magnification when you combine an eyepiece of a certain size with your scope. It involves knowing the diameter or your primary lens or mirror in millimeters and the focal length of the eyepiece in millimeters. If your scope was made in North America it may be measured in inches so you’ll need to convert the inches to millimeters. It’s not hard to do if you remember that 1 inch equals 25.4 millimeters.

To determine your scope magnification limit multiply the diameter of your primary lens or mirror by 2. For example. If your scope has a primary lens that is 100mm then 100×2=200x. That means that a telescope with a 100mm primary lens or mirror has a practical magnification limit of 200x. As a result, you might not want to purchase additional eyepieces or Barlow lens (doubles magnification) that would exceed 200x or 200 power. This is where some manufacturers misdirect amateur astronomers. The box proudly touts 875x as the magnification but its practical limit may be 150x. Sure, combining eyepieces and a Barlow lens might take you to 875x but the most you’ll see is a faint, blurry object.

There’s also another limiting factor at high magnification regardless of the focal length or focal ratio of a telescope. The night sky is not always clear and any haze, smog or pollution will also be magnified further blurring the object and diluting its brightness. You will also find that a cheap mount that does not hold your scope rock-steady will cause any instability to be magnified making it difficult if not impossible to simply find and observe an object in space.

Hopefully, this hasn’t been too complex. It’s worth taking the time to think about these factors and do some of the simple math. Whether you’re in the market for a new scope or are trying to understand how to use your scope better, understanding the impact of focal length, focal ratio and practical magnification can make a difference not only in terms of how you use your scope now, but how and which accessories to buy.

Images credits: 1, 2, 3, 4

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