Optics

Optics

Introduction

Much has been written on optics for birders. Much of what has been written is repetitive. The truth is, there is only just so much to say. I don't intend to say a whole lot - just offer some basic wisdom and information on how to choose binoculars and telescopes. For information on tripods and care of your optics, visit "All the Gear".

Binoculars

There are two basic types of binoculars, and within those two types there are several variations. Each has its strengths and weaknesses.

Porro-prism Binoculars

"Porro's" are the traditional off-set binoculars that we all recognize. The eyepieces are not in line with the objective (big) lenses. This is because of an arrangement of prisms inside the binoculars that bends incoming light. Why is that necessary? If you didn't bend the light - fold it up a bit, so to speak - the barrels of the binoculars would have to be very long, making them unwieldy and difficult to use. It would be like taping two old-fashioned spy-glasses together.

There are a great many excellent porro-prism binoculars on the market. Their strong points are clarity, sturdiness, and relatively low cost. Their weaknesses are that it is difficult to make them truly watertight, they are a bit bigger and "clunkier" than the sleek roof-prisms (see below), and there are few models that focus really close (within 2.5m).

Roof-prism Binoculars

There is a school of thought (to which I do not subscribe) that says that no serious birder would ever use anything but roof-prism binoculars. Truthfully, there are some magnificent roof-prisms available. Offhand, I would say that every serious manufacturer touts a roof-prism model as its "top of the line" binocular. What's so special about roof prisms?

They tend to be more compact than porro-prism binoculars. This compactness has been achieved by putting more prisms in the design, causing the light that enters the binoculars to bounce around a few more times before it reaches your eye.

Roof-prism binoculars can be truly watertight. Not all are watertight, so be careful on this point. Some models can focus on objects as close as 1.6m - very useful for up-close birding, butterfly watching, and dragonfly watching.

The down side of roof-prism binoculars tends to be the cost. They are generally more expensive than porro-prisms of comparable optical quality. Cheaper roof-prisms often suffer from color infidelity due to inadequate coatings. The extra prisms make alignment of the optical components even more critical.

It is always worth your while to test a few pairs of binoculars before you choose one to buy.

How Do I Evaluate Binoculars?

Whether you are looking at porro-prism or roof-prism binoculars, there are several basic things to consider. This "check-list" will help you to find your way through the bewildering variety of binoculars on the market today. For convenience, I have arranged this discussion by topic heading.

Power

Power is the first thing that most people consider with binoculars. While it might not be the most important, it certainly needs to be discussed.

Most binoculars are either 7x, 8x, or 10x. A very few quality pairs have eccentric powers, like 8.4x, but for the most part, your choices are limited to three magnifications.

If you look at any pair of binoculars, you will see a number that might read "7x30" or "10x42". Pocket binoculars might say "8x24". What do these numbers mean?

The first number (to the left of the "x") is the power. A 10x binocular magnifies the image 10 times. A 7x binocular magnifies the image 7 times. And so on.

So what is that other number, and why should you care?

The other number is the diameter, in millimeters, of the objective lens. The objective lens is the big one - not the one you look into. Why is its size important? The objective lens is the lens that admits light to the binoculars. The smaller the objective, the smaller the amount of admitted light. This might not matter in bright light, but in reduced light conditions, it is a major consideration.

Bu dividing the power number into the objective size number, you can calculate the diameter of the exit pupil. What is the "exit pupil"? I'll show you.

Hold your binoculars a foot or so away from your eyes, so that they are pointed at a bright background like the sky. See the bright dot in the middle of the eyepiece? That's the "exit pupil". If you have a pair of 7x35 binoculars, its diameter will be 35 / 7 = 5mm. Five millimeters. So when your pupil (the one in your own personal eyeball) is 5mm in diameter or smaller, you will be getting the advantage of all the light your brain needs to properly view the image through the binoculars. In lower light, as your pupil dilates (gets bigger), there will come a point where it gets bigger than the exit pupil (in this example, bigger than 5mm in diameter). At that point, your brain needs more light than the binoculars can supply.

Lots of light transmission is a big key to satisfactory binoculars. For that reason, you will want to look for ratios that give you at least a 4mm exit pupil - 10x40, for example. Binoculars of very high quality can sometimes compensate for low light transmission with exotic coatings and exquisitely engineered and assembled optics, but generally, the ratio test is reliable. The bigger the exit pupil, the brighter your binoculars will be. Pocket binoculars suffer from poor ratios, meaning that they are often not up to snuff in low-light conditions.

So, in summary, power is important, but you have to consider it in the context of light transmission.

Are there other issues related to a power? YES.

High-power (10x and higher) binoculars can be HARD to hold steady. They might also be heavier than their lower-powered confreres. Heavier binoculars are harder to hold and more tiring to carry. Think about that - what's the point of having a jerky 10x image, if you could have a crisp 7x image?

High-power binoculars are more expensive (generally).

Alignment

Alignment refers to the ability of the two barrels of a binocular to merge the two images into one in your brain. For this to happen, the lenses and prisms must be aligned properly. A new pair of binoculars can be checked for alignment simply by looking through them at a horizontal line. If the line seems to jump up or down in the middle, the binoculars are not aligned - simple as that. Ask to look at another pair of the same make and model. Binoculars get knocked out of line during shipping, or when careless customers drop them on the floor. This can be fixed, but it should not be an issue with new bins. (That's a cutesy little word that birders use).

Misaligned binoculars cause eye strain and headaches. In extreme cases they are simply unusable. If you are buying used binoculars, make sure you check for alignment.

Focus

Focus is simply the ability of the binoculars to provide a crisp image. Under the heading of "focus" there is a further consideration - close focus. This refers to the ability of the binoculars to focus on an object that is close at hand. You might wonder why this would matter, but if you are looking at fall warblers in an alder bush, you will be glad if your binoculars can focus "down" to six or seven feet.

How do you check focus? First of all, choose something in the mid-distance - a road sign is a good choice. Focus on the sign. How crisp is the image? Look for small print. Can you read it? Look at an object that is farther away - now can you read the lettering? Look at brick walls. How clearly can you distinguish the bricks and mortar at a distance?

Then, pull the binoculars into the closest focus setting available. What is the closest object that stays in focus? Measure the distance from where you are standing to the object - this is the effective close focus length for the binoculars. Most binoculars disclose this feature in the specifications - check to see that what they say is accurate.

What is that funny ring for on the eyepiece? Everyone has different eye strength. The adjustment ring allows us to compensate for the inconsistencies in our eyes. Here's how you use it. It's usually on the RIGHT eyepiece, but if for some reason it's on the LEFT, just reverse this process. If it's on the axle of the binoculars (often the case with roof-prism models), just treat it as if it's on the right eyepiece. Best of all, read the manual that comes with the binoculars.

Okay. Here we go. Look at an object in the middle distance through the binoculars. Now, with only your LEFT eye open, focus on that object. Now CLOSE your LEFT eye and open your RIGHT eye. Re-focus using that little adjustment ring - NOT the main focus wheel. NOW open BOTH eyes, and re-focus using the main focus wheel.

You can make a note of the setting on the little focusing thingy - called the "diopter adjustment". It shouldn't change unless your eyes change. After you set the diopter once, you shouldn't need to re-set it very often.

Other Stuff to Consider

I won't discuss price - obviously, that's a consideration, but it's up to you how much you want to pay for your binoculars. In the binocular world you tend to get what you pay for, though, so keep that in mind if you're planning on buying your "last ever" pair of binoculars.

Eye relief This is the distance that you can hold the binoculars away from your eyes and still use them effectively. In fact, there is a fairly narrow range of optimum effectiveness when it comes to how and where to hold your binoculars. This is learned through experience. The specification sheet should report the "eye relief". Generally, long eye relief is better, especially for glasses wearers.

Field of View It's one thing to have powerful binoculars, but if they have a narrow field of view they can be very frustrating. A designation like "W", "WA", or "WF" after the power to objective ratio (8x35 WF), or a number (like 10x42 6°) indicates that the binoculars are "wide angle", meaning that they capture a broader field of view than other binoculars of the same power. Sometimes the field of view will be shown as "x ft. at 1000yds." meaning that in one field of view you could see x feet of a horizon 1000 yards away. The bigger the first number, the wider the field of view.

Waterproof-ness This is an important feature for many birders, but a hard one to find. Many pairs of allegedly "waterproof" binoculars cannot survive a good rainstorm, let alone submersion in water. The best way to approach the waterproofing issue is to plan not to use your binoculars when it's pouring rain. After that, check carefully to see what the manufacturer's warranty says.

Weight Heavy binoculars are hard to hold steady and they can hurt your neck and back. Consider this when you are looking at some of the high-power mega-bins available out there. If you do go with a heavier set, consider getting a better neck-strap. Wide foam or woven cloth neck-straps are available. They can take a lot of the pain out of a pair of big bins.

Coatings Coatings are applied to binocular lenses to correct for color shifts that happen as light passes through the glass. Regardless of what the manufacturers might tell you, a color check is easy to do. Look through the binoculars at a moderately lit white surface. Does it look white, yellowish, bluish, or pinkish? It should look as white as possible. Check a few different models, and have a look through some expensive ones even if they are out of your budget. This might help you judge color shifting.

Glass Fluorite glass is a better conductor of light. It doesn't color the light and it doesn't tend to cause fuzzy edges. It is (surprise, surprise) more expensive, but if the increase in cost is only a small percentage of your overall investment, it's a worthwhile consideration.

Nitrogen Purging Waterproof binoculars are often "nitrogen purged", meaning that the air inside the barrel has been replaced with nitrogen gas. This feature improves light transmission.

Peripherals Binoculars might be supplied with a number of different accessories. These include a case, a rain cap, lens caps for all lenses, a neck strap, a cleaning cloth, and even filters for the objective lenses in rare cases. Some models have a tripod mounting hole. Check the eyecups, too. If you are a glasses wearer, you will want roll-down or retractable eyecups. Check the list of supplied accessories that is on the box or in the manual. Sometimes these are left out by mistake. Get what you're paying for!

Out of the Main-stream

There are a few eccentric types of binoculars that require mention at this point.

Astronomical Binoculars These high-power binoculars with huge objective lenses are designed for star gazing. You can set them up on a tripod or some other support and stare through them at an essentially motionless object. They are not appropriate for birding because of their weight and size.

Marine Binoculars Marine binoculars are often of the 7x50 variety - designed to collect lots and lots of light. It is common for them not to have central focusing. Instead, each barrel is focused separately. This assists in waterproofing. Marine binoculars often incorporate a direction-finding compass, which gets in the way for birdwatchers. Not recommended.

Zoom binoculars Nice idea, but not very effective, I'm afraid. One pair of Sears zoom binoculars that I saw was not bad - in the brilliant noon-day sun of Buenos Aires - but they are generally to be avoided.

Fixed-focus Binoculars These are designed for sporting events and other places where it isn't critical to be able to focus really close on occasion. Their minimum focal length is much too "long" for them to be suitable for bird watching.

Image stabilizing Binoculars There are several models on the market. Some use mechanical stabilizers; others use an electronic process. They are not cheap, but if you have the cash, they can be worth the investment. The better ones provide razor-sharp images in typically frustrating circumstances, notably in windy conditions or when you are a bit fatigued.

Opera glasses Come off it. Are you a birder, or are you going to see Swan Lake? Opera glasses have no prisms and generally don't magnify beyond 4x. Do not be seen carrying these things in serious birding company.

Monocular A monocular is as it sounds - a single barrel magnifier. Some very reputable companies make monoculars. They do not reproduce three dimensions well, but the optics can be extremely good. They have the advantage of being fairly simple mechanically. You will look a bit more eccentric than other birders (and that says a lot), but hey - to each his or her own.

Telescopes

Birding telescopes are often called "spotting scopes" to distinguish them from astronomical telescopes. Spotting scopes are compact, rugged, simple devices without all sorts of knobs and baubles sticking off them.

There are a great many spotting scopes on the market. What should a buyer be looking for? Here are a few suggestions.

Power As with binoculars, power is an issue, but with spotting scopes you have a choice. Most models can be supplied with a number of different powered eyepieces. In fact, it is common to buy the 'scope separate from the eyepiece! Remember that when you're shopping for prices - ALWAYS ask for the price with an eyepiece! Typical powers are 15x, 22x wide angle, 30x, 45x, and on bigger scopes, 60x. There are also zoom eyepieces available.

To Zoom or Not to Zoom? Zoom binoculars aren't a good idea for birders. So why would telescopes be any different? Well, they are. Many zoom lenses for telescopes are superb. They cost considerably more than a fixed-power eyepiece, but many birders find them worth the investment. Don't be turned off by zoom lenses for telescopes.

Objective Size The Objective lens (the big end) is usually one of two sizes in a telescope. Smaller 'scopes have objective lenses in the 55mm to 60mm range. The newer, bigger 'scopes have objective lenses in the 77mm to 80mm range. The bigger objectives are brighter and allow for more powerful eyepieces. they are also more expensive and a lot bigger and heavier (generally) than their small-fry cousins.

Nitrogen Purging Waterproof scopes are often "nitrogen purged", meaning that the air inside the barrel has been replaced with nitrogen gas. This feature improves light transmission.

Angled versus Straight eyepieces Many models are available with angled eyepieces. While these seem like a brilliant idea, you may find them a bit tricky to get used to, especially if you're used to the "straight" eyepieces found on most 'scopes. Angled eyepieces introduce another piece of glass into the light path, too.

Peripherals As with binoculars, 'scopes have a few peripherals worth watching for. Lens caps, of course, are standard. A body cap (to protect the body of the 'scope when the eyepiece is removed) is a good idea, too. Cases for 'scopes are available. Some manufacturers include the case in the price of the 'scope, while others charge as much as $90.00 for a case. Check the box and the inserts to make sure you're getting all the bells and whistles.