I think it’s well understood that what we experience in life often boils down to the choices we have and the consequent decisions we make. There is a fork in the trail at the county park where I frequently bird that will take me down to a small wetland and adjacent lake or up through an oak woodland and into a nice stretch of restored prairie. On most mornings, I can either bird the wetlands or the prairie, but seldom do I have time to explore both. Then there are the mornings where the decision at hand is whether to go out bird watching at all or to get some extra sleep and see who’s coming to the feeders while enjoying a cup of coffee at my kitchen table.
Decision-making always comes with an opportunity cost, and regret can creep in when we don’t know what we are missing. If I decide to sleep in, I won’t know what birds I missed that morning in either prairie or wetland. If I do trek over to the park, will I miss an encounter with a new yard bird? Fortunately, not all choices we make as birders are fraught with such uncertainty. Better knowledge of the trade-offs at hand will generally lead to better decision-making and, ultimately, a better birding experience. Now let’s move away from the less-certain scenario of what bird species I might miss to one where the consequences of our decisions are more predictable.
Choosing a binocular can feel like another one of those fork-in-the-road moments. Let’s say you’ve been able to mentally settle on a brand and budget. You’ve still got a few forks in the purchasing road ahead: Do you go with the 8x or the 10x model? Is it worth carrying the extra weight of a 42mm binocular or does something smaller and lighter better suit your style? What about a Porro-prism model versus a roof-prism model? We can look at these three variables for added clarity (and maybe brightness) as we navigate the path toward your ideal birding binocular. It all begins with understanding and knowing the trade-offs.
Porro vs. Roof Prism Design
Let’s first consider two different styles of binocular, Porro prism and roof prism. A binocular’s prism is the piece of glass inside that corrects an image’s orientation when viewed through the binocular’s lenses. Without prisms, the birds we look at through binoculars would appear to be upside down and backwards. You can identify a Porro- versus a roof-prism model by the shape of the bin-ocular. A Porro-prism binocular is the “traditional” type of binocular, with a jog in the barrel. When looking at the silhouette of a Porro-prism binocular, it would have the shape of the letter “M.” A roof-prism binocular can be identified by its two straight, parallel barrels, and its silhouette is the shape of the letter “H.”
The traditional Porro-prism binocular is a simpler design and so generally easier and cheaper to produce than a roof-prism binocular. Birders who are looking for an entry-level binocular may find that a Porro-prism model presents a slightly higher quality image than a roof-prism model at entry-level prices. Less money invested in design and construction means more of the cost goes to glass and lens coatings. The other advantages of a Porro-prism binocular relate to the jog in its barrel, increasing the distance between the two objective lenses. As a result, Porro-prism models generally have a wider field-of-view (in which you can see a greater area at once) than roof-prism designs. Additionally, the ocular lenses (the small end of the barrels that you put up to your eyes) can be brought closer together in a Porro. If you find that the barrels of most roof-prism binoculars won’t squeeze close enough to accommodate the distance between your eyes, check out a Porro-prism binocular. I’m willing to bet that solves the problem.
With their barrels spaced farther apart, Porro-prism models will have a much more limited close focus than roof-prism models. If you want to focus on a bird, bug, or plant within 10 feet of you, you’ll need to be using a roof-prism binocular. The final trade-off in the Porro versus roof decision has to do with durability and waterproofing. Because of their design and construction, roof-prism binoculars are less prone to becoming out of alignment if jostled or jarred. Unlike Porros, which have an external focus bridge, the focus mechanism on roof prism binoculars is housed internally, within the binocular’s chassis. This feature allows for a better sealing of the binocular against water. Better impact resistance and waterproofing make roof prism models the binocular of choice among birders who travel with optics and are likely to find themselves birding in a variety of environments.
Let’s move away from binocular design for the moment and look at the give and take involved in choosing what size binocular to carry around your neck. In this instance, size is referring to the diameter of the objective lens (measured in millimeters), not the binoculars magnification (more on that in a bit). Objective lenses vary from as small as 20mm on compact binoculars up to and even beyond 50mm on full-sized binoculars. Given that the job of the objective lens is to gather light and collect information going into the binocular, it’s easy to see how a bigger lens means more light and detail getting transmitted to your eyes. With that in mind, the reality of binocular use is that we spend more time carrying and holding our optics than we do looking through them, and with those bigger lenses comes a bigger and heavier instrument. The comfort and ease of use of your binoculars is critical to your enjoyment, so I would urge you to resist the temptation of putting all your eggs in the maximum-light-transmission basket with the biggest lenses you can find. If hauling heavy optics around with you out in the field annoys you, it doesn’t matter how much light they collect; you’ll avoid using them. On the flip side, I would suggest steering clear of compact binoculars. In low light, such as at dawn and dusk—when birds tend to be most active—they just won’t provide the detail that bigger binoculars offer. It’s been my experience that the sweet spot for binoculars will be in the 30 to 42mm range. The bottom line is when you are outfitting yourself with a new binocular, you need to be mindful of not just the view through it, but also holding, handling, and wearing it.
The other element of binocular size that comes with its own set of trade-offs is magnification. In this regard, size isn’t about how big or heavy your optics will be, but rather how much they will magnify the bird you are observing. With magnification, more isn’t always better. In fact, I would generalize that lower-power binoculars will be more user-friendly than higher-power models. One example is with field of view. We know that this can be affected by Porro- versus roof-prism models, but a binocular’s magnification also plays a role. As you increase a binocular’s magnification, you’ll get a closer look at your subject, but you will be decreasing your field of view. This can create difficulties when trying to locate birds that are in dense habitat or when trying to keep your sights on a bird in flight or otherwise moving around. Another aspect that can affect the usability of higher-power binoculars is your ability to hold them steady. A high-power binocular not only magnifies the object you’re looking at, but also magnifies any movement or shakiness in your arms or upper body. After a few cups of coffee, or if you’re breathing a bit harder because you just hiked uphill, or if you’re birding in the wind, your view will be shakier through a 10x binocular than through an 8x model. Holding your binocular steady allows you to see more detail even with lower magnification.
Finally, as we increase magnification in binoculars, we limit the amount of light that gets to our eyes. We touched on brightness as being affected by the objective lens size, with bigger being better, but power plays a role, too: The lower the power, the brighter the image. This is illustrated by a binocular’s exit pupil. Understanding exit pupil is to understand the relationship between lens size and magnification, and it is a useful concept for understanding optics in general. Grasp this, and you’re well on your way to being an optics expert.
I generally try to avoid math when discussing binoculars, but exit pupil is where I make an exception. You can observe a binocular’s exit pupil by holding it at arm’s length and pointing it toward a light source, such as a window or the sky (but never point them directly at the sun). Look at the tiny dot of light in each ocular lens. The exit pupil is the shaft of light that hits your eyes as you look through the bin-oculars, like a projector putting an image on a screen in the theater. It is determined by both the magnification and objective lens size in this formula: Exit pupil (mm) = Objective lens size ÷ magnification.
So, an 8×42 binocular (the most commonly used configuration among birders) has an exit pupil of about 5mm. That’s going to be a good amount of light coming into your eyes, even at dawn or dusk. Now take that same 42mm objective lens, but increase the power to 10x and you’ll see that even though we still have the same size front lens, the exit pupil drops to 4mm, resulting in less light getting to your eyes. An ideal birding binocular will have an exit pupil of 4 to 5mm for good performance in low light. This is exactly why 8x32s, 8x42s, 10x42s, and, to a lesser extent, 10x50s make up the majority of binoculars used by birders.
It’s never easy making choices, knowing that obtaining one thing requires giving up another. The sting of not being able to have it all can be softened a bit when we make our decisions knowing what we are giving up in exchange for what we are acquiring. With binoculars, this is certainly the case. If we could only be this fortunate with all the choices presented to us in life, it might be easier to decide if it’s a good morning to lace up the boots for a stroll in the prairie or sleep in a little longer.