Basically zoom binoculars have a continuously variable magnification so you can look through them at a magnification of 8x or 10x for example and zoom in to an object at a higher power magnification, to get a close up view and observe the object in more detail.
Zoom binoculars are designated by their magnification range separated by a hyphen, for example a pair of 10-30x25 zoom binoculars has a magnification range of 10x to 30x and an objective lens diameter of 25mm and a pair of 12-60x70 binoculars have a magnification power range from 12x to 60x and objective lenses with a 70mm diameter.
It is important to point out that some binoculars have multiple magnifications, but are not officially zoom binoculars as the different magnifications are obtained by switching to different sets of eyepieces. You can tell these apart from boom binoculars as they are designated with a slash and not a hyphen, for example 25/40x70.
The idea of being able to scan an area and then zoom in to get more detail on a particular object sounds ideal, but do Zoom binoculars actually work?
The answer is both yes and no! Let me explain, they do work and yes you can zoom into an object, but you cannot compare them to the quality of a high end binocular. The view through the ones that I have tested tend to be a little fuzzy in comparison to high end fixed magnification binoculars and they often have a much smaller field of view. Meaning that they are far from ideal for uses like birdwatching.
I think that it is important to mention that of all the manufacturers that make binoculars, not many of the real high end quality brands like Steiner, Kowa, Swarovski and Zeiss make Zoom binoculars. Could this because they don't want to risk their brand on something that is a little gimmicky or is it because they are not progressive enough? Of the companies that do make Zoom binoculars, the best are Nikon, Bushnell, Olympus and Celestron.
As with most things nothing is perfect and there are some people feel that the ability to be able to scan for something using the low power magnification and then zoom in to observe the close-up detail outweighs the downsides. So what are the best Zoom binoculars and where can you get them?
Zoom binoculars are very popular in binoculars that are designed to look at the stars as you can scan the sky using the low power magnification and then zoom into a star or constellation once you have identified the correct part of the sky you wish to observe.
The way that Zoom binoculars work, is partly the reason for the view through them being a little fuzzy when compared to quality fixed magnification optics: Some people may wonder why a zoom lens on a digital camera or video camera works so well why cannot they just use this on a pair of binoculars?
The main problem with designing zoom lenses for binoculars is due to the fact that a binocular is basically two separate telescopes connected together, each perfectly synchronised so that you get a crisp clear image. This problem gets far more complicated if you want a variable magnification as the zoom mechanism consists of moving lens elements in each barrel and each 'telescope' has to somehow maintain the synchronization even whilst zooming. To achieve this, manufacturers use a flexible linkage band that passes through the ocular arms that connect the zoom mechanism on the left to the zoom mechanism on the right.
Each moving part of the system has a certain amount of "slack" that is unavoidable and because there are many parts in the zoom mechanism, that each adding a small amount of "slack" to the system, it makes it impossible to maintain perfect synchronization. The result is that the two telescopes are never exactly at the same magnification and hence the image can become a little fuzzy.
Reduced Field of View
The width of the image you view through your binoculars (field of view) will be far less on a pair of Zoom binoculars at the low end of their magnification range than that of a comparable fixed magnification pair. This is because of the limitations of having to use moving lens parts inside the eyepieces. As a general rule, the bigger zoom, the smaller the field of view at the low end of the range. This can be as much as half of the field of view when compared to that of a fixed magnification binoculars at the same magnification.
Collimation refers the optical and mechanical alignment of the binocular and if this is not perfect it can feel like the binoculars are trying to suck our eyes out! Good quality binoculars are very carefully collimated, often with the use of laser instruments. This requires time and expense at the manufacturing level, and raises the price at the retail level.
So what does this have to do with zoom binoculars? Well, because zoom binoculars use moving lens elements in each eyepiece, each image moves very slightly as the lens elements move to zoom. This tiny movement is not be noticeable with just one eyepiece in a telescope for example, but because you use both eyes with a binocular, the collimation changes as you zoom from one end of the magnification range to the other. This results in the binoculars never being in perfect alignment and so are not perfectly collimated.
If these issues concern you, but you still require binoculars that have variable magnifications, you may also like to consider a pair that use different eyepieces to obtain different levels of magnification, where you get the the versatility of multiple magnifications, but without the performance limitations of zoom binoculars.
Below is a selection of my most recent reviews that I have written on zoom bins:
If you have any comments or anything else to add about variable magnification binos, then I would love to hear from you: