Understanding Magnification

This article reviews some of the absurd claims and outright lies regarding magnification sometimes seen on the packaging or advertising of low-end telescopes. We’ll review what magnification is used for, how you calculate it, and what are its practical limits. You’ll see it’s not a significant factor in distinguishing one telescope from another, and you’ll understand how that misleading claim on the toy telescope is justified.

Lies Toy Telescopes Tell You

You’ve probably seen shiny telescope boxes in department stores, toy stores, camera stores, and on eBay, with colourful space pictures, and boldly proclaiming the “power” of the telescope. 450-power and 675-power are especially common claims.

They’re lying.

Ok, they’re not lying in a mathematical sense. As we’ll see in a moment, they have correctly multiplied two numbers together to get the answer 675.

The lie is the implication that you care; that it means something; that it is a mark of quality; that it is a reason to buy that telescope. It would be like a box containing a clock saying “Buy this clock! It has over 3500 seconds in each hour!”

Customers who do buy such a telescope (often as a gift for a child) will never be able to use the claimed high magnification. And even if they could, they never would, because using such high magnification requires other conditions and equipment not available to most mortals. Most such scopes end up not being used at all, or being used only at the lowest available power.

What’s Magnification For Anyway?

Magnification is important, though, right? Your telescope’s job is to make things bigger so you can see them?

Actually, no.

Or at least not always. Many of the things we like to look at (like nebulae and galaxies) are actually not small, they are dim. Your telescope’s job isn’t so much to make them bigger as to make them brighter. You need some magnification — like 20 to 100 power — but you don’t need a lot.

An exception is viewing planets, like Saturn and Jupiter. They are already bright, but they are small, and magnification is needed. But what you need and can use is 200 to 300 power, not 600. More on this in a moment.

How To Calculate Magnification

Calculating magnification is simple.

Your telescope’s main tube has a “focal length”, which is essentially the distance the light travels from the first lens or mirror to the eyepiece. It’s usually measured in millimetres, and a number like 1000mm is common. (Small refractors tend to be in the 400 – 1000mm range; entry-level reflectors in the 1000 – 2000mm range; and Schmidt-Cassegrains in the 1500 – 3000mm range.) It will be specified in your telescope’s manual and may be marked on the lens.
You also put an eyepiece in your telescope, and the eyepiece also has a focal length. It will be clearly marked on the end or the side of the eyepiece. Numbers between 3mm and 30mm are common for eyepieces.

Magnification is just

So, in the examples shown in these photos, a 816mm telescope fitted with a 25mm eyepiece gives a magnification of

816 / 25 = 33 power.

If we replaced the eyepiece with a 4mm eyepiece in the same telescope, we would get

816 / 4 = 204 power.

Change the eyepiece and you change the magnification — it is not a fixed property of the telescope. Note that smaller eyepiece numbers give larger magnifications.

You can also get add-on optical devices called Barlow Lenses that act as amplifiers. They multiply the magnification by an amount marked on the lens. For example, that 816mm telescope, 4mm eyepiece, and a “2x Barlow”, gives a magnification of

(816 / 4) x 2 = 408 power.

But there’s a catch. That telescope would not produce useful results at such high magnification. Its main lens is 102 mm4 inches in diameter, and its useful magnification is limited to about 200 to 250 times. Let’s discuss that.

Limits of Magnification

As you can see, any telescope can be made to produce any magnification just by mixing eyepieces and Barlow lenses. However, there is a limit to the amount of magnification a given telescope can usefully provide. Beyond this limit, the image will be blurry — like trying to zoom in a digital photo beyond what the resolution of the camera supports.

The limit of useful magnification is a function of the diameter (not the focal length) of the lens. This limit is complex, but a simple rule of thumb that works in many circumstances is that the maximum magnification a given telescope can usefully provide is about 2 power per millimetre50 power per inch of diameter. So a telescope with 100 mm4 inches diameter might be able to produce around 200 power magnification before going fuzzy.

There is a second limit, too. The fact that we have air around us, turbulent, churning, and dust- and moisture-filled, also limits magnification. Beyond about 400 to 500 power, no small telescope located on the ground produces clear views. That’s why observatories are on mountains, or in space.

Finally, there is a third limit for many people: the stability of your mount will limit what magnification you can use. Budget-priced telescopes are often sold on lightweight mounts that can’t hold the telescope completely steady. At low magnifications this doesn’t matter, and may not even be noticed; but at high magnifications, the small vibrations of an unstable mount can make it impossible to find your target, focus, or observe.

The Toy Telescope Revisited

The department-store telescope that introduced this article has about 76 mm3 inches of aperture, so its maximum useable magnification is about 76mm x 23″ x 50 = 150 power. You might get 200 power out of it under ideal conditions. 675? Not a chance. Only mountaintop observatories, or the dozen-foot-high, multi-thousand-dollar, monster scopes used by very serious amateurs, will do that.

Where did the number 675 come from?

That telescope has a focal length of 900mm. It comes with 20mm and 4mm eyepieces, and a 3x Barlow. The smallest eyepiece produces the highest magnification. 900 / 4 = 225 power. Add the 3x Barlow and you get 225 x 3 = 675 power. This is a very common combination, specifically set up to allow these misleading magnification claims. 450-power is another common one. That’s a 900mm telescope, a 4mm eyepiece, and a 2x Barlow.

On the other hand, those telescopes are usually 76 mm3 inches or 100 mm4 inches aperture. The better one, 100 mm, would be good for a maximum of about 2 x 100 = 200 power. Maybe 250 on rare occasions. Claiming 675 or 450 power is true in the sense that the arithmetic is correct, but it’s misleading and, in my opinion, fraudulent.

Needless to say I wouldn’t recommend anyone buy such a telescope. More than that — I tend to have strong feelings about being lied to. Personally, I won’t buy from a manufacturer that makes telescopes with such claims (which rules out even some mid- and high- end manufacturers), and I prefer not to buy from a store that sells them, although that’s not always practical. (If they’re willing to lie to me about that product, why should I trust them on other topics?)

How to Control Magnification

As we’ve seen, there are 3 ways to control the magnification of your system:

  1. The focal length of the telescope. Longer gives higher magnification. This is a fixed attribute of the telescope — you can’t vary it. Focal lengths are usually between 300mm and 3000mm. For a beginner, something around 600 to 1500mm is a good choice.
  2. The focal length of the eyepiece. You will want several eyepieces, to give low, moderate, and high magnification.
  3. Amplifiers like Barlow lenses. A good quality 2x Barlow is like doubling the population of your eyepiece collection, provided it doesn’t push the magnification of your highest-power eyepieces beyond the limits of your telescope.

Depending on the focal length of the telescope, a typical beginner might like to start with something like two eyepieces and a Barlow.

For example, let’s imagine you had a 150 mm6-inch scope with a focal length of 1200mm. The maximum magnification of your 150 mm scope would be about 2 * 150= 300 power, which you would get with a 4mm eyepiece. However, you might find it better to start with something like a 25mm eyepiece, a 10mm eyepiece, and a 2X Barlow. You then get 4 useful magnifications with only 3 pieces:

25mm alone 1200 / 25 = 48 power
25mm and 2x Barlow 48 x 2 = 96 power
10mm alone 1200 / 10 = 120 power
10mm and 2x Barlow 120 x 2 = 240 power

The lower powers will be great for large nebulae and clusters, and the higher powers will be good for the Moon, Saturn, and Jupiter.

Magnification versus Brightness

Photographers with single-lens reflex cameras know that, when they double the focal length of their lens, the image gets dimmer so they have to double the exposure too.

The same is true with your telescope. When you raise the magnification, the image gets dimmer. This means that selecting the right magnification to view a given object will often involve making trade-offs.

For example, suppose you are observing M57, the Ring Nebula. It’s interesting, but it’s quite small. At moderate power, you’ll see the tiny wispy smoke ring. Should you raise the magnification to get a better look?

Doing so will make the nebula bigger, but it will also make it dimmer. Whether this is better or worse for observing will depend on you and on observing conditions.

Personally, I tend to keep such objects smaller and brighter for observing, while others find magnification adds detail and that the dimmer view is still acceptable.

Reducing Magnification

Strange as it may seem, you may even find yourself wishing you had less magnification at certain times. For example, certain objects (e.g. M31 — the Andromeda Galaxy; M45 — the Pleiades) may be too large to fit in your field of view, even with your lowest power eyepiece. This is especially a problem with mid-sized and large Schmidt-Cassegrain telescopes, which tend to have very long focal lengths.

In such circumstances, you may wish to reduce the magnification of your telescope. You can do this by adding an accessory called a Focal Reducer. This is an extra lens which acts like the opposite of a Barlow lens — it reduces the magnification of the telescope – eyepiece set by a fixed factor, usually something like 1.4 times.

What Magnification to Use

Which magnification you should use at a given moment depends on many factors including the size of the object you are observing, its brightness, the surface detail, the aperture of your scope, the stability of your mount, and the observing conditions. I can’t think of any way to give you specific guidelines, except that I most often find myself using around 100 power when observing star clusters and dim objects like nebulae, and around 200-250 power when observing bright planets. If I had larger telescopes with greater resolving power (greater aperture) I’d probably use more, but not much more.

However, I recommend you should always start with your lowest magnification, or at least something on the low end. This will assist in finding the object and observing it against the context of the background stars. Then you can gradually increase the magnification on objects where you think it will improve the view.



    I was given a Celestron Powerseeker 70EQ missing the Barlow 3x and the 4 mm eyepiece. I was about to replace those missing pieces, found your article, and instead got a 5 mm eyepiece and a 2x Barlow, to better make use of the magnification the telescope can actually do.

    Interesting that the 5 mm costs more than the 4, and the 2x Barlow also costs more than the 3x.

    So, it is really is true: less is more!

    Thanks again. Looking forward to seeing the Galilean moons…

  2. Brett, I suggest that you join a local astronomy club. They usually do outreach programs for people interested in astronomy. At times, they have star parties where members can examine various setups. Once you can see the variety of systems being used you can make an informed decision on what to buy. Also, do join cloudynights.com and post questions in the Beginners area.

  3. Don’t some of these department store telescopes also have a stopped down aperture because of the poor quality lenses, ie. single element glass? I read a review in recent years from a 1980s edition of New Scientist where two British astronomers reviewing a toy telescope that had an aperture stop and although it claimed to be 40×40 the aperture was topped down so much it only have a magnification of 10.

    I also had an awful toy telescope as a child very similar to the one I mentioned above. A stopped down aperture meaning all I could see was the Moon. Venus and Jupiter were just like fuzzy stars, and awful toy that actually put me off astronomy for a few years.

  4. I have a profile projector I use x50 lens for magnification , the objective that i can use is 20 millimeter I want use 100 millimeteter objective butI can not because the lens contact with objective . what can i do for increasing the focal leneght with this lens&|60;?
    my email

  5. Have a Meade 102 mm. F600. F/5.9 What is the MAX I could push my scope? I have a Barlow X 2, a 6.4 mm, 9 mm, 26 mm, 32 mm super plossl. Which should I use to view various planets etc…

  6. My daughter is interested in astrology. She’s asking for a telescope. She is interested in looking at constellations, planets, galaxies etc. I’ve read articles, and I’m very confused. Could you recommend what I need to look for, please? A brand/model no. I have no idea, and I don’t want her to loose interest, which is exactly what will happen if she can’t get any use out of one purchased that is inadequate for her. We live in a rural area of SD(in case that matteres). Thank you.

  7. Hi! Maybe you can help me with a question I don’t find an answer to. How can you calculate magnification when adapting a digital camera to your telescope? The camera’s T-mount replaces the eyepiece, so you don’t have an eyepiece focal length number to calculate. The camera’s actual lens has also been removed and replaced with the T-mount (you’re left with the body only), so you don’t have a camera lens focal length number either. So I’m missing the “eyepiece focal length” to calcule! Hope u can help, thanks

    1. Imagers don’t like to talk about magnification, because things like resolution per pixel and how many pixels are covering your target are more important. For reference, though, use the diagonal length of your camera’s chip as an eyepiece focal length. Many DSLRs like the Canon Rebel have a chip size of 22.2 x 14.8, which is 27mm diagonal. So divide your scope’s focal length by 27 to get an idea of magnification. For a full-frame 35mm (e.g. Nikon D3) use 43mm.

  8. Thanks! that was very informative. I am planning to buy a telescope and your information helps me a lot. I am also planning to buy a camera adapter, so that I can take photos. Are such adapters worth buying?

  9. I, like you, am irritated at the number of false claims on cheap scopes. See hundreds of them on eBay, and I fell into the trap as a child when I bought a “cheap”&|60;?75 scope from Argos. That talked of 500 or 600 times mag, but it was rubbish…all I could actually see what the moon, it let so little light in. I’m now older and wiser, and have two refractors…a 4″ with a Focal Length of 600mm, (which I want to use for Deep Space Objects), and a 5” with a FL of 1200mm, which is great for Prime Focus shots of the moon, and magnified images of Saturn. These were&|60;?150 and&|60;?200 respectively, so you really don’t need to spend a lot. The mount though…that’s where the majority of your money should go, especially if you want to try astrophotography. Tim

  10. That was really informative, thanks&|60;:). Btw, i just bought a telescope 3 days ago and its pretty much like the “Toy scopes” you were talking about. Its a 60mm aper. and 800 focal. Its max. is 660X lol but i’ve been using like 64x(12.5) and 200x(4mm) for the moon, saturn and mars. Although, i do have a 32x(25mm). Im wondering if i can see star clusters and nebula’s like Orions or anything. If i can, its with the low mag. like you said, so like anything under 100x is good for that stuff? And which would be better, 64x, 32x,(btw i have 2x barlow lens, 3.3 barlow lens and 1.5 barlow lens). Please tell me what is the best and if I am even able to see stuff like that with this telescope.

    1. Taha, For a high quality scope and eyepiece the rule of thumb is max magnification is about 50X magnification per inch of aperture. For seeing distant objects with the eyepiece, you need decent objective lens size. Also, very important is seeing conditions. Nowadays, light pollution is terrible. Getting out far from big cities helps. Also there are filters that can be used to block out the effects of air-glow caused by those street lights. Watch some videos on Youtube about astronomical topics.

  11. Richard, Thank you very much for your concise and easy to understand explanation of this subject and how it applies in a practical sense. I just purchases a Nexstar 4se telescope for my family (we have 12 year old budding astronomer and 4 younger siblings who will undoubtedly catch the bug) and was really surpised at how little useful information there is on the internet for new buyers. In real terms, those of us who are just starting out need clarity on usage. I have read numerous posts and articles and none were as helpful as yours. I even sent an Email to one of our local universities to get their opinion which did not answer my questions as well as this article. If I had read your article first, I would have definitely went for the 6″ aperature. Great article. I have it bookmarked!

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