Binoculars, Monoculars, Day or Night, Zoom, Eye Relief....Your questions answered!

by Devon Huning August 7, 2021

When you're looking for binoculars, there are a few things to consider before you buy. Will they be for general use or something specific (like birdwatching, for instance)? Things like weight, magnification, fog or water proof, one or two lenses (binocular or monocular) are important considerations. We’ll do our best to filter through the jargon and still make you your own bino expert!

First of all, binoculars vs monoculars. This may seem obvious, but the distinction is whether you have one or two lenses (tubes). Some describe them as binoculars cut in half. That’s it. Application, though, is another thing. The biggest consideration is YOU and your eyesight. If, like some, you have vision in only one eye, then a monocular would be the better choice for you. That doesn’t mean, though, that because you have dual vision you’ll automatically prefer binoculars.

Here are the basic pros and cons of binos compared to monoculars.

Binoculars	Monoculars
Use roof or Porro prism systems	Most often uses Porro prism system
Many magnification options	Many magnification options
Two viewing tubes-better 3-D replication	One viewing tube
Less eye strain for longer viewing periods	More eye strain when using for long periods
During night-time use, both pupils will be affected, maing for longer readjustment times to natural light.	During night viewing with 2 eyes, only one pupil is affected, leaving the other exposed to natural light. Readjustment will be quicker for the eye using the monocular
Heavier	Easier to adjust quickly
Wider field of view	Narrower field of view
More expensive (2 tubes = 2 sets of optics)

Other than recommended use for those with vision in only one eye, some suggestions for the use of each include:

Binocular Activities	Monocular Activities
Hunting, scouting and tracking	Birdwatching (unless for long periods)
Sports	Night vision
Boating	Hiking

Now let’s talk optics. After all, that’s what it’s all about!

Binoculars make things bigger to the naked eye. But they also make the image brighter and more detailed. The larger the objective lens, the more light that will enter the tube. Light also affects resolution or clarity of the image. Magnification and resolution have an inverse relationship – i.e. higher magnifications = lower resolution. (One reason why binos with lower magnification are recommended for bird watching. You don’t want to miss the detail in those feathers!)

Porro Prism vs Roof Prism

Essentailly, without getting too technical, it’s about prism placement. Both systems use two prisms to reflect light but arranged differently. Why do you need prisms at all? Because without them, the image you see would be upside down and backwards (sort of like what a mirror does – you’re not upside down, but the image is flipped). Prisms will correct that orientation so that your image is right side up and facing you....hence, the name “erecting” prisms and named after Italian inventor Ignazio Porro.

When you look at a pair of porro prism binoculars from the top, you’ll notice that the eyepiece is offset from the objective lens at the other end of the tube. Just like in this picture –

Light enters through the wide lens (at the bottom of the picture in this example), travels through the first prism, bends to travel through the second prism, bends again to travel through the eyepiece.

Porro prisms can be “standard” – the eyepieces are closer together than the objective lenses. The picture shows an example of standard Porro prisms. “Reverse” Porro prisms have eyepieces that are farther apart than the objective lenses. Any binoculars with an objective lens of 60mm or more will be of the Porro prism design. Because the prisms have to be perfectly aligned to transfer the reflected light and image correctly, dropping them can cause costly realignment.

In the next picture, you’ll see a side x side comparison of Porro prism design vs roof prism design, and you’ll notice the prism arrangement is now straight on top of each other. While you might think because they are in a straight line, they are easier to align, but this would be wrong. The manufacturing process is actually more complex. As a result, roof prism binoculars are typically more expensive than their Porro prism counterparts.

As you can see in this picture, light travels in a straight line from objective lens to eyepiece. Because of this, roof prism binos are also typically slimmer and lighter than their Porro prism brothers. They are also more durable, as dropping them doesn’t carry the same danger of offsetting the prisms (they are often glued together).

Roof prism compared to Porro prism – pros and cons

ROOF PRISM		PORRO PRISM
Pros	Cons	Pros	Cons
Optics are superior to Porro prism-especially w/ coatings	More expertise to manufacture	More of a 3-D like projection-more realistic	Bulkier and heavier than roof prism
More streamlined (less bulky) which some find more attractive	Uses more internal reflections	Easier to manufacture = less expensive	Harder to weatherproof
Lighter and more compact	Highest quality = most expensive	Basic optical quality is generally superior to roof prism design
Easier to weatherproof		Dollar for dollar, you’ll get a higher quality Porro prism system for the same amount you’d spend to get a mediocre roof prism system.
Recommended for hunting, bird watching, observing wildlife and sporting events, hiking, star gazing. In other words, you can’t go wrong with either for anything outside. Consider all the factors.

Another description you’ll often see with binoculars and monoculars refers to coatings. We’ve talked earlier about things that affect the amount of light entering through the objective lens. However much that is, you’ll want all of it to reach your eye for the best image. No matter what type of lenses you have, there is going to be some light loss due to reflection. Coatings help prevent this and will definitely affect the end result of the image you see. Coatings are typically made of magnesium fluorite. There are typically 4 designations.

Coated (C) = one thin layer of anti-reflective coating on one or more lenses.

Fully Coated (FC) = one thin layer (at least) of anti-reflective coating on each side of the objective lenses, both sides of the ocular lenses and along the long side of the prism.

Multi-Coated (MC) = One of more of the optical surfaces have multiple coatings.

Fully Multi-coated (FMC) = All lens surfaces (objective lens, ocular lens and prisms) have multiple coatings.

More coating means less light loss means a better, truer image, which of course comes with a price.

The last important feature to consider (there are more but this list is what you’ll look for most often) is eye relief.

This refers to the distance the viewing lens is from your eye. This becomes important for those wearing glasses. Look for eye relief of at least 14mm. For those who don’t wear glasses, this should be at least 10mm. Any closer will cause eye fatigue over long viewing times, as well as possibly tickling your eyelashes, causing you to blink frequently.

So, to sum things up for you, here are your major considerations:

Monocular vs Binocular
What’s your intended use
What magnification do you want
What’s your price range. (The best advice is spend as much as you can – your purchase will last you for years!)