Just Say No to Zoom Binoculars
by Kevin Busarow
What are zoom binoculars?
Binoculars that have continuously variable magnification are called zoom binoculars. Zoom binoculars are designated by their magnification range, such as 10-30×60 or 12-36×70. The hyphen indicates that the magnification is variable from the first number to the second. This is done by moving a zoom lever to increase (“zoom in”) or decrease (“zoom out”) magnification. Conversely, a binocular with a single magnification is called a “fixed-power” binocular, and is designated with just one magnification number, such as 10×50 or 20×80. Note that some binoculars have multiple magnifications, but are not zoom binoculars. These are designated with a slash, such as 25/40×100. The various magnifications (in this case, 25x and 40x) are obtained by switching in different pairs of eyepieces.
Many of us have high-quality zoom lenses for our cameras, and they work very well. Why not take advantage of the same technology with our binoculars? The primary problem with using zoom lenses in binoculars is due to the fact that a binocular is two independent telescopes connected by a hinge. Zoom binoculars therefore must somehow maintain synchronization of the zoom mechanism (which consists of moving lens elements with each eyepiece), between both telescopes. This is accomplished by the use of a flexible metal band that passes through the ocular arms, connecting the zoom mechanism on the right side, to the zoom mechanism on the left side. Each moving part of the system has a certain amount of backlash, or “slop”, that is unavoidable. Because there are a number of parts of the zoom mechanism, each adding a small amount of “slop” to the system, it is impossible to maintain perfect synchronization. Each of the two telescopes will always be leading or lagging the other, depending on which direction the zoom lever is moving. The result is two telescopes that are never exactly at the same magnification.
Because zoom binoculars use moving lens elements in each eyepiece, each image will shift slightly in some direction as the lens elements move. This would not be noticeable with just one eyepiece, but because a binocular uses both eyes, collimation (the alignment between the two telescopes to each other) changes slightly (sometimes more than slightly) as the zoom lever moves from one end of the magnification range to the other. The image shift typically moves the opposite way as the direction of the zoom lever changes. The result is a binocular that is never in “perfect” alignment. As magnification increases, good alignment becomes increasingly important. Compounding this problem is the magnification synchronization problem mentioned previously.
Zoom binoculars FOV (field of view)
Due to the inherent design limitations of using moving elements within a zoom eyepiece, the field of view at the low end of the magnification range is going to be severely limited compared to a fixed-power binocular. One reason people choose low-magnification binoculars is because they offer a wide FOV- but viewing through zoom binoculars at low power is like having tunnel-vision. Compare the following FOV’s between zoom and fixed-power binoculars-
10-30×60 zoom, FOV at 10x- 3.4°
10x60 fixed-power, FOV- 5.7°
12-36×70 zoom, FOV at 12x- 3.2°
12x60 fixed-power- FOV- 5.7°
25-125×80 zoom, FOV at 25x- 1.25°
25x100 fixed-power, FOV- 2.5°
So you can see that zoom binoculars at the low end of the magnification range sacrifice as much as half of the field of view of fixed-power binoculars at the same magnification. In general, the higher the upper level of zoom magnification, the smaller the FOV at the low end of the range.
Manufactures push the magnification levels of their zoom binoculars to unrealistic levels in an effort to make their product appear to be the most “powerful”, therefore “the best”. Some zoom binoculars zoom to impossibly-high magnifications- some even over 100x! Not only will the image quality be very much degraded at these levels, good alignment is hopeless. In general, a zoom binocular that has upper magnification greater than half of the objective diameter in millimeters (eg. the “60” in 10-30×60), is more of a novelty (toy) than a usable optical instrument. Some manufactures label these “military binoculars”, which is deceptive marketing hype. Here’s an example of a 20-140×70 “Military Zoom Binocular”. No military would ever use zoom binoculars, let alone models that zoom to such ridiculous magnifications. The hype doesn’t stop there- this importer also claims that “only one factory possesses the technology to manufacture binoculars that can zoom up to 140 times closer!” This is probably the same factory that possesses the technology to double your gas mileage and horsepower by clipping a magic device to your fuel line. Not only that, “70 times more light comes into its gigantic 70mm lens”! These hucksters are clueless…
World’s Most Powerful Binocular
There’s a reason a binocular capable of 75x magnification weighs 28 pounds. The picture (at right) of a disassembled Oberwerk BT-100-45 shows what it takes to build a binocular that is truly capable of extreme magnification, with the combination of image clarity and precise alignment required to provide high-quality viewing. The BT-100-45 contains 7 lbs. of optical glass, and for good reason. Good image quality at extreme magnification is simply not possible with an inexpensive light-weight zoom binocular.
In general, zoom binoculars have a host of problems that can’t easily be overcome, and will always be out-performed by fixed-power binoculars (of similar size and price). At low power, the field of view is roughly half the size of a comparable fixed-power binocular. At high power, image quality is highly-degraded and most likely poorly-merged. Consider also that as magnification increases, hand-holdability decreases. The highest magnification that can be used without a mount is perhaps 15x, possibly as high as 20x for some. Beyond that, a mount of some kind will be required to obtain a usable view. So if hand-holdability AND high-magnification is desired, choose a good fixed-power binocular, such as a 15×70, or 20x80LW. Fixed-power binoculars such as these will offer as much magnification as can be held, and will have better image quality, and with none of the problems unique to zoom binoculars. If variable magnification is required, choose a binocular that has the ability to use different eyepieces to obtain different levels of magnification, such as the 25/40×100 or BT-100-45. These offer the best of both worlds- the versatility of multiple magnifications, but with none of the performance limitations of zoom binoculars.