Whether you're new to birdwatching or a seasoned spy, there are always things to be learned.
- Most popular – 7-8x42, with light grasp or rubber coating.
- Close focus specification important for close up study
- Roof prism optics most popular
- Good detail
- Quick focus
- Look for a 6mm exit pupil size for low-light conditions.
- Avoid fixed focus (or so called auto focus) binos - they won’t focus up close.
What the heck is an ‘exit pupil’? Well, it has nothing to do with your eye falling out of your head!
Exit pupil refers to the small bright circles that can be seen through the eyepiece when it is held up to a light. Technically, the larger the exit pupil, the more light that is transmitted to your eye. The more light, the better the overall image quality. But image quality is also directly affected by the lens system and how big the objective lens is.
The formula for determining the size of the exit pupil is easy – Objective lens divided by magnification. These are two numbers you will always see in the description of a particular binocular. But you won’t always see the exit pupil measurement. Here’s an example... an 8x42 binocular (8x magnification with a 42mm objective lens) has an exit pupil of 42/8 or 5.3mm. If you’re comparing a 7x35mm system with a 10x50mm system, the math will tell you that the exit pupil is 5mm for both, BUT, because of the larger objective lens, the 10x50mm system will give you a brighter image. So, the lesson here is DON’T rely on exit pupil to determine how bright your image will be.
When choosing the right size of an exit pupil, consider this. The average size of the human pupil in daylight = 2mm, in the dark = 7mm. When the exit pupil of a binocular is equal to or larger than your pupil, the image will appear brighter. When the exit pupil is smaller than your pupil, the image will appear darker. So it’s important to consider when you’ll be doing most of your birdwatching.... during periods of bright daylight or periods of dawn and dusk?
It’s not uncommon for seasoned birdwatchers to have more than one pair of binoculars with different exit pupil measurements. Any new or experienced bird watcher will tell you it’s important to have the best light to get the best definition of the intricate details that can set one bird apart from another.
Magnification is another important consideration and can affect nearly everything else important to the end image, including depth of field, field of view and weight.
Assuming everything else is constant, the higher the magnification, the lower the light transmission. In other words, zoom in and the image gets darker, which will affect the amount of detail you perceive. Magnification also affects field of view (FOV) – the more power, the narrower the FOV. So, more power isn’t necessarily better or necessary. For example, a 10x would be good for distance, while a 7x will give you a wider field of view and be better in low-light conditions.
When thinking about depth of field, think of the picture you see through your bino (or monocular) lens as the “field”. How many of the objects in that field are in focus is what is termed “depth” of field. As the magnification increases, the region of focus in your “field” diminishes or becomes more shallow. As objects get farther away, depth of field increases. The closer an object is, the more work your eyes have to do to keep things in focus, which can lead to more eye fatigue.
Your next consideration is field of view. In general terms, it refers to the area that is visible through the lens. This, too, is inversely affected by magnification. As you zoom in, your “field” becomes smaller and smaller – think of it like cropping a picture. Field of view (abbreviated FOV) in binoculars is reported as the width of your “field” at 1000 yards. For example, binoculars with a FOV of 235 yards means that when looking through your binoculars at an object 1000 yards away, from edge-to-edge of your viewing field is 235 yards. Keep in mind the narrower your FOV, the more difficult it might be to follow a bird in flight. FOV is also expressed as an angle (such as 8 degrees). To convert an angle to a linear measurement, multiply the angle by 52.5.
Another consideration affected by FOV is eye relief, which is the distance from the eye piece lens to your eye, and is usually expressed in mm. A wider FOV generally means a shorter eye relief distance which may be important for those wearing glasses. Eye relief is one of the more important considerations to avoid fatigue. The minimum recommended eye relief is 10mm for those who don’t wear glasses, and 14mm for those who do.
Close focus is another important consideration. You may think that something only a few feet away doesn’t require binoculars, but you’d be missing out on an amazing amount of detail. Being able to close-focus on an object only 6-8 feet away will allow you to see the incredible feather outlines and beak details that you might otherwise miss. Generally measured in feet, know that generally, as magnification increases, so does the close focus range. Close focus ranges of 10 feet and under are ideal, 10-16 feet is ok, but more than 16 feet...you’re going to lose a lot of what you came out to see!
As most committed birders will tell you, weather doesn’t stop you! So you want binoculars that are rated waterproof, and ideally, nitrogen gas filled. This will protect the binoculars from developing internal condensation that will cause fogging of the lenses, especially when you’re out in colder weather.
The last but by no means less important consideration is weight. Unless you carry a tripod, you’ll be holding your binos up for possibly a long time – a few minutes can quickly become several and if your binos are too heavy, your arms will quickly become tired. Not only that, unless you are wearing a pack or harness (always a good idea!), they will be hanging around your neck, adding that weight to your cervical spine.