Zoom binoculars and what the zoom costs you

What a zoom binocular is and what it gives up

Celestron defines zoom as an optic with variable magnification that you adjust with a zoom lever. One pair covers a range, such as 10-22x50 or 8-24x25, instead of a single power.

That flexibility has a price. Audubon says zoom models are heavy, give up a lot of brightness and have a much-reduced field of view. The chart holds 554 models, and the zoom ones sit at the narrow end of its field figures. The full chart is in best binoculars compared.

Why the field of view narrows

Nikon says that as magnification rises, the field of view will be narrower. A zoom binocular repeats that effect every time you turn the lever toward its top setting.

At 204 ft of field, the Nikon Action Zoom 10-22x50 is the narrowest of the three. The Celestron 10-30x50 UpClose G2 lists 225 ft, and the 8-24x25 Sportstar Zoom lists 241 ft. The median across all 554 models is 340 ft, and the range runs from 141 to 563 ft.

For a fixed power of the same size, the 10x50 binoculars have a median field of 315 ft. The 8x25 binoculars have a median of 357 ft. Each zoom model sits below the median of its comparable fixed size. For the full explanation of the figure, see how field of view is measured.

Why the image gets dimmer

Nikon defines exit pupil as the effective objective diameter divided by magnification, and says a larger exit pupil gives a brighter view in dim light. Its example is 8x42, where 42 divided by 8 gives 5.3.

Raise the magnification and the exit pupil shrinks. The calculated 5 mm for the Action Zoom 10-22x50 from Nikon and the Celestron 10-30x50 UpClose G2 matches their lowest setting. Its Sportstar Zoom 8-24x25 sibling works out to 3.1 mm at the lowest setting. At the top of each range the exit pupil is smaller still, which is the brightness Audubon says you give up.

You can run any size through the exit pupil calculator to see the figure for a chosen power.

Weight and holding the view steady

Nikon says a higher magnification makes a steady view harder to hold. On a zoom model that means the top of the range is the setting where hand shake shows most.

Weight adds to it. Weighing 33.5 oz, the Action Zoom 10-22x50 from Nikon is the heaviest, and the Celestron 10-30x50 UpClose G2 weighs 28 oz, against a catalog median of 24 oz. At 10.8 oz, the Sportstar Zoom 8-24x25 is the light one.

Both 50 mm zoom models are listed as tripod adaptable. A tripod adapter lets you hold the top settings still.

The zoom models in the chart

Three zoom models carry full records. Nikon Action Zoom 10-22x50 and Nikon Sportstar Zoom 8-24x25 both sit in the $100 to $200 maker list price band. Celestron 10-30x50 UpClose G2 sits under $100 and is listed as waterproof.

All three use porro prisms. The most common prism across the whole chart is roof.

Eye relief and close focus on zoom models

Eye relief is the distance from the eyepiece at which you still see the full view. The Celestron 10-30x50 UpClose G2 lists 20 mm, the longest of the three. Nikon lists 16.3 mm on the Action Zoom 10-22x50 and 13 mm on the Sportstar Zoom 8-24x25. The catalog median is 16.4 mm, and 337 of 529 models with a printed figure list 16 mm or more.

Close focus is where zoom models fall behind. Two of them list 26.2 ft and 26 ft. The Sportstar Zoom lists 13.1 ft. The catalog median is 8.4 ft, so none of the three gets as near as a typical fixed model. If you wear glasses, binoculars for eyeglass wearers are listed by eye relief.

Fixed power models to compare with

A fixed pair gives up nothing to a zoom mechanism. The 8x42 binoculars have a median field of 393 ft and an exit pupil of 5.3 mm. The 10x42 binoculars have a median field of 336 ft and 4.2 mm.

For a pocket size, the 10x25 binoculars weigh a median 11.1 oz with a 297 ft field. That is close to the 10.8 oz of the Sportstar Zoom 8-24x25, with a single magnification.

The magnification chart shows what each power does to field and brightness, and 8x42 against 10x42 shows the step between two common powers.

Who the figures suit

Zoom suits a reader who wants one low-cost pair for a range of distances and accepts a field of 204 to 241 ft. The Celestron 10-30x50 UpClose G2 pairs that with its under $100 band and 20 mm of eye relief.

Dusk viewing, long carries and wide scanning favor fixed power. A median of 29.3 oz for 10x50 and 11.1 oz for 10x25 shows how weight falls with size, and compact binoculars collect the lighter pairs.