In Part 1 of this three-part series, we discussed the advantages of using binoculars for astronomical viewing of the night sky and we talked about the different types of targets you can enjoy on clear, dark nights. At the conclusion of each section, I mentioned “binocular magnification considerations.” Here, we will get into that subject, as well as other characteristics of binoculars that make them better, or worse, for studying the stars. Then, in Part 3, we will look at different recommended sizes of binoculars for stargazing fun.
Binoculars are assigned a pair of numbers that indicate the magnification of the optic and the diameter of the objective lens in millimeters. If you see a binocular that is listed as an “8x42”, the “8” is the magnification and the “42” is the diameter of the optic’s objective lens.
The magnification and objective lens size factor into how the binocular performs under different environments and lighting conditions.
Optics Magnification and Camera Lens Focal Length
Because many B&H Explora readers are photographers, let’s put magnification into camera lens focal length terms.
In photography, with a full-frame 35mm camera (135 format), a 50mm focal length lens represents, roughly, a 1x magnification. A 100mm lens would be 2x. Using that basic math, a 7x binocular has approximately the same magnification as a 350mm lens.
Magnification and Astronomy Binoculars
Magnification is one of the most, if not the most, important buying considerations when getting binoculars for any type of viewing and astronomy certainly falls under the “any type of viewing” category.
The higher the magnification number, the larger the magnification. The higher the magnification, the “closer” you will get to an object—usually a better thing in the world of binoculars. However, the downside of high magnification in optics is: the higher the magnification, the more your hands, arm, body will impart shaking into the image—this is one reason why high-magnification optics, like telescopes and spotting scopes, are mounted on stable supports.
There are no hard-and-fast rules, but many people can hold an 8x or 10x binocular reasonably steady. I feel I have fairly stable hands and find that I favor 8x over 10x for birdwatching and general binocular use.
There are no set rules, but “travel” binoculars are usually between 6-10x, birding binoculars live in the 7-10x range, and marine binoculars are usually around 7x. Confusingly, a search on the web for recommended “astronomical binoculars” will give results from 7-25x—a very wide range!
Cross-referencing the list of night sky “targets” in Part 1, there are uses for both general (7-10x) magnification and high/very-high magnification (>10x) binoculars when viewing celestial objects.
When considering binoculars for astronomy, remember that the physics of the steady image holds true when looking at the night sky—the higher the magnification, the more shake your hands/arms will impart to the view. The consensus is that anything above 10-12x will require a tripod or alternative support to achieve a steady view.
Higher magnification = More stability needed for a shake-free image
Higher magnification = More detail for celestial objects
Higher magnification = Narrower field of view
Objective Lens Size and Astronomy Binoculars
In general, the larger the objective lens, the more light is gathered by the binocular. During daytime binocular viewing, this is not a huge consideration because the sun gives us a lot of light with which to work. Compact 8x25 or 10x25 travel binoculars have sufficient light-gathering abilities for viewing when the sun is up.
Night poses a huge light-gathering challenge, and you’ll see that binoculars made for astronomical viewing have objective lenses that are on the larger size—with some being well over three times the objective lens diameter of a standard 8x42 birding binocular!
When viewing distant and dim celestial objects, the larger the objectives, the better the image you will get. The downside of a larger objective is weight. Optical glass is heavy and large objectives mean heavy binoculars. Even if you have handheld viewing-friendly magnification, a pair of large objective lenses might make your arms fatigue quickly and have you thinking about a support for your glasses.
Larger objectives = More light gathering power to see fainter objects
Larger objectives = Heavier binoculars. More fatiguing to hold and carry
It is important to note here that the light-gathering ability of an optic is not solely based on the size of the objective lens—the quality of the optical glass and lens coatings also have a profound effect on light-transmission capabilities of the optic. Diving into that subject would take us way off course here but, generally speaking, more expensive optics transmit light better than less expensive ones.
Of course, there is a lot more to consider than magnification and objective lens size when purchasing binoculars for astronomical use. I will touch on those considerations here but, again, if you want to take a deeper dive to any of these topics, please click over to the B&H Binocular Guide.
Porro Prism vs. Roof Prism
Porro prism binoculars are the traditional configuration with the familiar angled housings and are the most popular configuration for astronomical binoculars. Roof prism binoculars are the compact, straight configuration that are popular in both entry-level and high-end birding binoculars.
Which is better? Traditionally, binoculars that were made specifically for astronomy, or pairs known for good astronomical viewing, were Porro prism models. The Porro design lends itself to larger objectives (it is rare to find roof prism models with objectives larger than 56mm), increased sense of depth in the view, and optical performance for the price. An inexpensive Porro is going to perform better than an inexpensive roof prism binocular because of the roof’s manufacturing challenges.
Inexpensive roof prism binoculars are ones you might want to avoid. If you follow the market, you’ll know that some of the world’s most expensive premier binoculars are roof prism models—so it is obvious that not all roof prism binoculars are inferior to Porro models.
In my opinion, when you are looking at quality optics, the advantages/disadvantages of each type are minor, and the binocular that fits the best in your hand will get way more use than a pair that doesn’t feel good to hold.
There are two general methods of focusing binoculars—central focusing (CF) and individual focusing (IF)—and both come with advantages and disadvantages. CF binoculars have a focus wheel that is used to adjust focus for both eyes. IF binoculars allow you to focus each eye at a certain distance to infinity (where celestial objects are usually found).
Properly configured IF binoculars will give you a sharper viewing experience because they are configured to a single viewer’s eyesight. If you are going to be sharing binoculars with friends and family while stargazing, a CF binocular might be the way to go.
Field of View
In general, the greater the magnification, the narrower the binocular’s field of view. This makes sense, right? A powerful telescope only shows a tiny soda-straw area of the sky above.
Often you will see the field of view, as an angular degree, published on the binocular’s housing (usually near the magnification and objective numbers). On specification sheets, the FoV might be listed as a lateral dimension at a distance of 1,000 yards. If you want to convert between the two different types of measurement: 1° equals 52' at 1,000 yards.
For an angular reference, stretch out your arm and spread your fingers. The angular separation between your thumb and pinkie is approximately 20-25°. The big dipper measures about 25° and the four stars of the Big Dipper’s dipper measure approximately 10°.
You will find that binoculars of identical power might have variations in their fields of view. For astronomical viewing, a wider field of view, for a given magnification, will mean that it is easier to find your night sky targets and, in the case of viewing star fields, a more immersive viewing experience.
The exit pupil of a binocular is a measurement (in millimeters) of the opening of the image circle of the optic. For binoculars, you can easily figure it out by dividing the objective diameter by the magnification. Example: a 7x50 binocular has a 7.1mm exit pupil.
Your eyes have pupils, as well. If the size of the binocular’s exit pupil is smaller than your own pupil (approximately 8mm as an adult human in darkness), when you look through a binocular, you’ll see an image surrounded by a dark circle. If the exit pupil is larger than your eye’s pupil, you’ll see a complete image but will also be missing the outer edges of the projected image—a waste of the binocular’s light-gathering power. In general, you’ll want binoculars with an exit pupil that is at or smaller than your own pupil. Note that, as we age, our eyes don’t dilate as much (bummer), so exit pupil measurements increase in importance.
The distance from the eyepiece to your eye can be a factor for eyeglass wearers. Many binoculars have adjustable eyecups that are designed to keep your eyes at the proper distance from the binocular’s ocular lens (eyepiece). Fold down or retract the eyecups when you are using eyeglasses.
A short eye relief (less than 15mm) can cause problems when viewing with eyeglasses on because you might not be able to see the entire image.
Binoculars are primarily used outdoors—and always used outdoors when stargazing. While you probably won’t be looking at the moon or stars during a rainstorm, to protect from unexpected precipitation, moisture, condensation, dew—all things that can be part of night sky viewing—it is advantageous to have waterproof and fog-proof optics.
You may see the type of binocular prism listed in a spec sheet or list of selling points. For any kind of serious viewing, choose the superior BAK4 prism over the less-expensive BK7 unless, of course, you are on a budget. BK7 prisms aren’t the end of the world; they just aren’t quite as good as BAK4’s.
I hate to crush different manufacturer’s dreams of selling magnification-changing zoom binoculars, but it is rare that you will find a serious positive review of any pair of zooming binoculars because their optical quality is just not on par with single-magnification optics. While zoom lenses for cameras can be great, no binocular manufacturer has cracked the code on making zoom binoculars perform optically as well as a single-magnification pair.
As a counterpoint, being able to change the magnification of your binoculars might make it easier to locate targets using less magnification and then “zoom in” by increasing the magnification. Also, a pair of zoom binoculars can increase the versatility of the optic—allowing for different types of viewing and studying celestial objects.
Tripods and Other Supports
A great way to employ binoculars for stargazing is to throw yourself (and your binos) into a comfortable lawn chair, lie back, relax, and take in the views. This is a great way to spend an evening with a handheld pair of glasses and reduce fatigue from holding the binos (and standing on your feet!).
Regardless of the amount of magnification of your binoculars, the view through any optic can be assisted by a steady support. Unless you are running into the backyard on a cold winter night for 30-seconds of stargazing fun, even moderately powered 7x, 8x, or 10x binoculars will find a rock steady view when placed atop a tripod.
While many different types of binoculars will accept a specialty tripod mounting adapter, there are a few “universal” mounting options for those binos that don’t accept a threaded or hinge-mounted adapter—basically a tripod-mounted tray on top of which you strap the binoculars.
Depending on your tripod and head combination, many standard photography setups will be perfectly adequate for mounting binoculars. The versatile photography ball head is great for binocular use, but you might find that a video fluid head is a great companion for stargazing, as well.
For those heavy and powerful high-magnification astronomical binoculars, there are parallelogram-style rigs that feature a counterweight setup.
The image-stabilized binocular’s position in the bottom half of this article belies its impact on the world of astronomical binoculars because it is most certainly a game-changing technology. While the bulk of the article is tailored to traditional optics, the not-so-new technology of image stabilization allows stargazers to explore the night sky while handholding binoculars that feature magnifications that previously required the stability of a tripod or parallelogram setup.
Note that the physics of light does not change here. You will still want a larger objective lens to gather as much light as possible. However, if you want the long reach of extra magnification but you want to travel light and leave the tripod (or parallelogram) at home, this option is the way to go.
Oh, and remember these things run on batteries (with the exception of the mechanically stabilized Zeiss 20x60s), so head out into the field with extra power so you aren’t left with a non-stabilized high-power optic.
Taking Photos Through Your Binos—Digiscoping
When we see something beautiful in the night sky, we often want to share that vision with our friends and loved ones. Yes, you can connect your smartphone camera or another photographic device to your binoculars for the purpose of digiscoping, but know that, even with a good adapter, the process is a bit tricky.
I have found that digiscoping is much easier with a spotting scope or telescope because they feature a more stable platform and larger eyepieces but, if you want to digiscope through binoculars, a tripod or support is almost a requirement since you’ll find that you need two hands to hold the binocular steady, a third and fourth hand to connect the photo adapter to the optic and the camera or smartphone to the adapter (getting the picture here?), and then a free finger to release the shutter.
One of the benefits of looking through binoculars is a stereoscopic view, and not everyone has good vision in both eyes. In cases where one eye has much better visual acuity than the other, the monocular is a fantastic tool for stargazing. All the numbers and physics for binoculars apply to the monocular as far as power, objective lens size, FoV, etc., but one advantage of the monocular is that it is considerably lighter, smaller, and less expensive than its brother binocular since we are dealing with half of the optics of a binocular and no complex chassis.
Get it? Got it. Good! To read more about stargazing binoculars, click here. If you are ready, Part 3 discusses what size binocular is best to meet your stargazing dreams. If you have questions about this part of the series of articles, please engage us in the Comments section, below!