A guide on Choosing Binoculars
Tuesday, February 19th, 2008
How to Choose the Right Pair of Binoculars for You
There are several factors to consider in making a good choice of binoculars.
How important each binocular factor is to your selection will depend on how you plan to use them.Important binocular properties;
Binoculars are almost always identified as number with X then a second number, such as ‘7X50?. Magnification in this example is 7X. Natural expectation for people is to want higher magnification but higher magnification comes at a price more than money. The higher the magnification the harder it is to hold what you are looking at steady. Since most uses of binoculars are hand held, having excessive magnification is not desirable.
The second number after magnification is the example above 7X50 is the diameter of the ‘objective’ lens in millimeters. The objective lens are the larger ones on the far side of the binoculars (see diagram courtesy of Nikon below). The size of the objective lens is important to brightness as the larger they are, the more light they are gathering for your eyes at the other end. If you think of photons as a quantity of ‘information’, the more photons the lens gathers and bring to your eyes, the more ‘information’ your brain will have to enjoy. However the trade off for larger objective lens are the binoculars will be more heavy and bulky.
The light that does get to your eyes has an effective area called the ‘exit pupil’. This can simply be calculated using the 7X50 specification if you divide the 50 (objective lens diameter in mm) by the magnification (7X), which in this case is slightly more than 7mm. The pupils of your eyes are less than 4mm wide in bright daylight so in this case there is ample efficiency. However in low light situations the pupil expands to 7mm when you are younger (not as much as you get older). So if you are older than 40 and using 7X50 binoculars in a low light situation you will likely see a darker view than with a pair of 7X35 binoculars with a 5mm exit pupil.
- Weight & Bulk of Binoculars
Weight is not an optical property but can be a crucial factor in how useful the binoculars you choose are. As mentioned above having high magnification won’t mean much if they are too heavy to lug and hold steadily up to your eyes. More expensive lighter lens materials can offset this if you can afford it. Also more expensive “roof prism” binoculars are more compact are an option again if you can afford it.
Field of view for binoculars is most often expressed as ‘X’ number of feet at 1000 yards. The higher the magnification the smaller the field of view will be. You can improve field of view for a given magnification with more expensive “wide angle” binoculars. For birdwatching or Astronomy 300 to 400 feet at 1000 yards will be acceptable. It is a challenge for 10X magnification binoculars to be above 300 feet at 1000 yards.
- Optics (lens, prisms, and coatings) Quality
- Ideally you want to avoid poor quality optics , start with good quality, and then decide later if you want to move to superb quality optics. There is no point to buying bargain basement binoculars or poorly maintained used binoculars as your experience will be disappointing. Preferably all optics are coated and better still multi-coated. Coated optics improve light transmission. Look for the phrase “multi-layer coated optics” and be wary of the phrase “fully coated optics” as this might mean that only external air to glass surfaces are fully coated but other internal lens and prisms may not be. Light transmission loss occurs at all surfaces without anti-reflective glass coatings. I will explain how to visually check for coatings in a later post.
- Resolution - High quality lens provide good color fidelity, are free of optical aberrations, and provide excellent detail.
- Stabilization
Expensive electronic ‘gee-whiz’ adjusting system that noticeably improves the view, especially in hand held use targeting moving objects. An interesting experiment is to take a ‘value priced’ pair of binoculars and look at the moon holding them as steady as you can. Then mount the same binoculars on a steady camera tripod and look at the moon again. You will be shocked at the extra detail that you can see due to stability.
Having rubber overmold on the binocular body does not always mean they are rugged. Search for drop specifications if rugged is important for how the binoculars will be used. Waterproof is important for hunting and marine use as well as fogproof. Waterproof is specified usually in a JIS or IEC 529 rating (click on the link to understand more about waterproof ratings). An IEC rating of IP64 will suffice or look for IP67 if you want to take your binoculars swimming. Fogproof binoculars have the air inside replaced with a gas such as Argon or Nitrogen.
Binoculars you are most likely to be familiar with are porro prism with a prism system moving the two light paths from the objective lens closer together on the way to the two eyepieces you look into. “Roof” prism binoculars are “straight through” looking. This lens system is more expensive but has the advantage of allowing for smaller compact configuration and storage. They also look better as a fashion statement if that is important.
