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You’ve decided you need a pair of binoculars. That’s great! You’re joining the millions of people who have made binoculars one the most purchased optics on the market, second only to eyeglasses. In this article, I’ll discuss zoom binoculars: what they are, a few important terms, the advantage and disadvantages, and how to choose the best set for you.
What are zoom binoculars?
In my earlier article, All You Wanted to Know About Binocular Magnification, binoculars were defined as “two telescopes side by side, one for each eye.”
Binoculars with the ability to have a range of magnification are known as “zoom” binoculars. These continuously variable binoculars can change in both directions, from high power to low power, or from low to high. To do that, they have the difficult task of making those two telescopes crisply match up their images. And because movable ocular lenses are used to adjust the power level, more parts can create problems.
Zoom binoculars are identified by their numbers. The first two numbers, separated by a dash, give you the range of magnification. As an example, a 10-30 x 42 means that the range of magnification starts at ten times, and runs up as high as thirty times. Caution! Don’t confuse zoom with binoculars that use a “/” in place of the “-“. The slash indicates that the eyepieces change, often by rotating a turret. These specialty glasses carry a very hefty price tag!
How do Zoom Binoculars Work?
The quick answer is “not as good as fixed” unless you’re willing to pay a whole lot more. Why is that, though?
Simply put, each eyepiece lens has several lens elements in the housing to zoom. And, with each additional part comes additional room for error. On top of that, Zoom binoculars are trying to synchronize the actions of the two separate telescopes by using a flexible linkage band that goes through the ocular arms to connect the two zoom mechanisms. There is a certain amount of unavoidable “slop” or “slack”, resulting in a distorted or fuzzy image because the two telescopes won’t ever have the same magnification.
Many people choose zoom binoculars because they prefer the convenience of being able to quickly change the magnification. For example, they may be using them under bright light, following an object that is far away and moving. The variable magnification allows you to scan under the low power, for a greater field of view, and then to zoom in for more detail. Others may choose zoom binoculars with the use of a tripod for astronomy. They can scan the skies at a low magnification until they find an object that requires greater detail.
The disadvantage of zoom binoculars is the high cost for image quality. And it gets even more complicated, so let’s take a look at a few concerns.
- Objective Size
The objective lenses are the apertures. The bigger they are, the greater the amount of light there is available for the image. As a rule, the objective should be 5x magnification. This means that you are either going to have more size, and additional weight, for the upper levels of magnification, or you’re going to sacrifice light.
Let’s look at binoculars with a power of 7X and an objective lens of 35mm. Those lenses have a diameter of 1.38 inches. To improve the light you need at 14X, 70mm lenses are recommended. Those lenses measure 2.75 inches across. That’s double the weight. That’s something to consider if your binoculars are hanging off of your neck all day!
- Reduced Field of View
How much of an area you see through your binoculars is known as your field of view. As magnification goes up, your field of view goes down. For example, at 5x, you may see both sides of a tennis court 200 yards away. But zoom to 10X and you may not see anything more than one player.
The table, below, compares FOV’s between zoom and fixed-power binoculars:
|10-30×60 zoom, FOV at 10x- 3.4°
|10x60 fixed-power, FOV- 5.7°
|25-125×80 zoom, FOV at 25x- 1.25°
|25x100 fixed-power, FOV- 2.5°
As you see, zoom lenses FOV is considerably smaller compared to that of fixed magnification binoculars. The difference is much more significant as the power increases. In the example above, the FOV at 25X for the zoom binoculars is only half of the FOV for the fixed magnification binoculars.
Because each eyepiece within zoom binoculars uses a moving lens, the image of each will move a little as the lens elements move. This image shift is usually in the opposite direction of changes made in the zoom lever. Getting the images of the two telescopes to line up is referred to as collimation. Poor collimation could result in double-vision and a loss of image area. Proper alignment is even more important as magnification increases.
Ideally, you want the exit pupil to be larger than the diameter of your pupils. Binoculars with an Exit Pupil that is smaller than your pupils will make it seem like you’re looking through a peephole.
To find the Exit Pupil size of your binoculars, divide the objective by the magnification. At 8X40, the Exit Pupil is 5mm. That’s pretty big. Now, if we change over to binoculars that are 10X25, our Exit Pupil drops to just 2.5mm. If you intend to use your binoculars during bright days only you have nothing to worry about. But, if you’re a hunter, boater, or star-gazing, then you need to take a close look at the Exit Pupil.
Summary of Advantages/Disadvantages
Zoom binoculars offer the ability to quickly and easily change the magnification. As magnification goes up though, the available light is reduced, your already reduced Field of View drops even lower, collimation issues increase, and the issue of reduced image quality due to Exit Pupil size becomes a greater concern.
Addressing these issues at the factory is expensive. In general, zoom binoculars will cost twice as much as fixed-power binoculars. Additionally, to get to the high magnification levels, objective lenses have to be bigger, adding to the weight, and greater weight means a lowered ability to hold your binoculars steady.
Selecting and using zoom binoculars
You have learned that zoom binoculars offer the convenience of quickly changing magnification, but that convenience comes at a price. For many people, that convenience is well worth either paying more or accepting some of the limitations. So, before you buy your next set of binoculars, answer the questions that follow.
- What activity(ies) will they be used for?
- What are the conditions? Consider –
- Light levels – what time of day will I use them the most? Will I be in an open area with sunlight?
- Climate/Moisture – will I need waterproof or rubber coating to absorb the shock of being dropped?
- Will I be moving or stationary?
- Weight – how long can I hold them without shaking? Will I wear them around my neck? Can I take the time to set them up on a tripod?
- Field of View – how important is it to be able to see a wider area?
- Level of Magnification – How far away will I be looking? How much detail do I need?
- Eye Relief – will I wear glasses of any kind?
- How much does cost matter to me? What amount am I willing to pay?
- How good is the warranty?
Try Before You Buy
Take a little time at a local sporting goods store to appreciate the differences you’ll find with all binoculars, not just between fixed and zoom binoculars. The variations of objective sizes, magnifications, FOV, and weight have to be experienced. This step is crucial whether you order online or in a store. Consider the following questions before you buy.
- How do they feel?
- Do they feel balanced when you hold them up to your eyes?
- Can you comfortably hold them up without shaking?
- Are the controls easy to move as you hold the binoculars to your eyes?
- How is the image quality?
- Brightness may be hard to judge. Keep in mind the size of the Objective lenses and when you’ll use them.
- Clarity – is the picture crisp, even on the edges? What happens to the clarity as you increase the magnification?
- Collimation – Are you seeing one sharp, distinct image at high and low power?
Focusing Your Binoculars
Now that you have decided that zoom binoculars are the best choice for you, let’s see how to set them up.
Many of the binoculars have a focus wheel between the two scopes. A right-eye diopter to account for the small difference in the strength of each eye adjustment is found near the eyepiece. Pick out a target that’s about 45 feet away. With your right eye closed, focus the image by moving the center wheel. Next, close your left eye and open your right eye. This time only use the diopter to make the image clear. Now you can open both eyes and use the center focus wheel to make adjustments. Leave the diopter setting alone.
Manufacturers may have different ways to focus their zoom binoculars so be sure to read through your manual carefully.
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