Parfocal eyepieces are 2 eyepieces that maintain focus when you change from one to the other. This is done by creating lenses that keep the focal point on the same plane. Parfocal eyepieces allow you to change magnification without refocusing your telescope.
Learning to get good focus is one of the first challenges faced by the astronomy newbie. This is especially true when switching to higher magnification with a manually driven telescope.
Focusing takes time, and objects quickly move out of view at high power, which makes it easy to lose your target after switching eyepieces.
One way to get around this hurdle is to invest in parfocal eyepieces, which can save both time and frustration in the field.
You will pay more for this feature, but depending on your gear, your observing style and your conditions it may be a good investment.
In this article we’ll learn what parfocal means, what makes eyepieces parfocal, and how this can benefit the visual astronomer.
What Does Parfocal Mean?
The term “parfocal” means that a lens, or set of lenses, remains in focus when magnification is changed. This can apply to a zoom eyepiece, a range of fixed focal length eyepieces, and even to eyepiece-barlow combinations.
The key point is that when the focal length of a zoom lens, or set of fixed lenses, is changed, the focal point remains on the same plane and you do not need to re-focus your telescope.
So, for your parfocal eyepieces to be helpful you will need a set.
What Makes an Eyepiece Parfocal?
Parfocal eyepieces are a set of eyepieces that maintain the focal point on the same plane, so when you change between them they stay in focus.
Eyepiece designs vary greatly both in the arrangement of their internal components (lens elements, field stop), and external components (barrel, nosepiece).
The focal point of the eyepiece is determined by the specific arrangement of these components.
Manufacturers can control design specifications across a range of eyepieces to ensure they come to focus on the same plane, though this becomes more difficult when there is a large difference in eyepiece focal length, or when you move from a 1.25” to 2” barrel size.
You might find that eyepieces from different product lines and even different manufacturers are parfocal by chance. Conversely, you might also find that eyepieces which claim to be parfocal in fact require some re-focusing after switching between them.
These adjustments should be minor.
Do I Need Parfocal Eyepieces?
While having parfocal eyepieces is not a necessity, for some telescope users they can provide certain advantages.
When you are using high magnification it is easy to lose a target and the time it takes to change eyepieces and refocus could make a huge difference.
This becomes even more important when you have exceptional conditions or if you are visiting a dark site.
Parfocal eyepieces give you more time looking up and less time adjusting equipment.
After you have been using your telescope for a while you will know what type of astronomer you are. Do you use and enjoy a large eyepiece collection or do you stick with one or two all night? If you switch a lot, having those parfocal eyepieces may be worth it.
Anyone with a large, manually driven telescope can probably relate to the following scenario:
You have located a compact object of interest, such as a planetary nebula, and wish to study it at higher magnification. After switching to your shortest focal length eyepiece, everything is blurry and you are trying to find focus, but there are no bright stars in your small field of view to focus on and by the time you have it figured out, the object has drifted from your field of view and there’s no way for you to find it at such high magnification. So, you switch back to your low-power eyepiece and start over.
If this sounds familiar, you just might benefit from a parfocal eyepiece set where little to no refocusing is required. This will help you to stay on target when changing magnifications.
Parfocal Lense Sets
The most well known parfocal eyepieces are Tele Vue eyepieces. These eyepieces are available in a wide range of magnifcation and barel sizes. They are all parfocal.
I would love to try this set of Orion Parfocal eyepieces. It is affordable and comes with 26mm, 32mm, and 38mm. They are threadded to easily attach to 2″ Orion filters.
If you are new to astronomy and just building your eyepiece collection this set may be a good way to get started.
Can I Make My Eyepieces Parfocal?
A set of mismatched eyepieces can be “parfocalized” using special rings which clamp to your eyepiece barrels. These parfocal rings (affiliate link) are available from Amazon and specialty astronomy stores, and they act as spacers between the eyepiece barrel and the focuser.
You can even use them to make an astronomy camera parfocal with your eyepieces. If you plan on using a planetary camera, for instance, you will spend a lot less time fiddling with focus; with a manually-driven telescope, this means less chance of losing your target.
There’s also a way to achieve this on a shoestring budget: if you can find rubber O-rings of an appropriate diameter, you can slip as many of those as you need onto the eyepiece barrel to achieve the same effect, albeit in a less stylish fashion.
You can find the rings from Orion and the rubber bands on Amazon (affiliate links).
Can Barlow Lenses Be Parfocal?
Barlow lenses are added to eyepieces to change the focal length and change the magnification. When adding a barlow to your optical train you will need to refocus when you change eyepieces. Tele Vue makes a parfocal barlow called the Powermate. This can allow you to change eyepieces without refocusing.
Tele Vue makes barlow lenses which are parfocal with both 1.25” and 2” eyepieces, and there may be others on the market, so check the specifications and read reviews if this is a desirable feature for you.
If you are deciding between a barlow and a Powermate, read our Barlow VS. Powermate guide that can help you understand all the differences.
The barlow lenses I’ve used are not parfocal by any means; when you insert an eyepiece, the image goes completely blurry and a lot of refocusing is required.
My Final Word
Parfocal eyepieces are hardly a necessity for amateur astronomers. I’m more of a mix-and-match type of observer when it comes to eyepieces.
But if you’re looking for a reason to stick with one brand and line of eyepieces, parfocality is something to consider.
