Sunday, October 13, 2019

What is the best magnification for bird watching binoculars?

What size binoculars are good for bird watching?


Most bird watchers use binoculars that range from 7x to 10x ("7 to 10 power magnification"). But the magnification is only one part of the equation. The other important variable needed to answer this question is the size of the objective lens ("the big end"). These two things determine the size, weight, light gathering ability, field of view, cost, and your ability to hold the binoculars steady. There are several tradeoffs to magnification and objective lens size.

My first pair of binoculars (actually my father's "sporting goods store brand") was a 7x35 ("seven by thirty-five"). It magnified 7 times and had objective lenses of 35 millimeters. When I was able to afford my first pair of binoculars, I did what many beginning birders do. I bought a pair of 10x binoculars. 10x50. Fifty millimeter objective lenses are big...and heavy.

The size of the objective lens determines how much light your binoculars will gather, thus how bright the image appears. Larger objective lenses allow you to bird in overcast conditions, dark woods, and twilight. As a rule of thumb, full-sized birding binoculars should have a ratio of 1:5 magnification to size of objective lens. Thus, 7x35, 8x40, and 10x50 all have that 1 to 5 ratio, and are common binocular sizes. This ratio (objective lens diameter divided by magnification) is called exit pupil. This published spec allows you to determine generally how bright the image will be when light levels are low. Binoculars of 8x42 are somewhat brighter than this standard ratio. Other popular birding binoculars of 10x42 and 8x32 are significantly dimmer than this standard ratio. Rather, they are fine in bright light but give a dimmer display when light levels are lower.

The larger objective lenses make the binoculars heavier. Heavy binoculars are harder to hold steady and become wearisome after hours in the field. This unsteadiness is exacerbated with more magnification. The shakiness of your hand will create a shaky image. And 10x binoculars will have image shake almost 1/3 more than a 7x binocular, if they weighed the same. But they don't. The 10x binocular is heavier and will cause additional shakiness as the day wears on.

Greater magnification reduces field of view. Field of view is how wide of an angle your binoculars capture. A wide field of view helps you "get on" a bird flying overhead more easily. Think of a narrow field of view as looking through a straw to find the bird. Field of view on binoculars is measured as linear feet of width at 1000 yards distance. Thus, binoculars that show an image 330 feet wide at 1000 yards are considered on the narrow end of field of view. Binoculars that show an image 440 feet wide at 1000 yards have a very wide field of view and are very desirable.

Two other specifications of binoculars are important for bird watchers. If you wear eyeglasses, you'll want binoculars with longer eye relief. Eye relief of 15.5 to 20 millimeters is good for eyeglass wearers. Within that range, longer is better.

Bird watchers may view birds in woods at close range. They often also enjoy viewing butterflies and dragonflies. If this describes you, then look for binoculars with close focus of less than 8 feet, perhaps down to 6 feet.

Photo of binoculars and bird books at home
My recommended bird watching kit

8x42 ("eight by forty-two"): the best all-round magnification for bird watching binoculars


With all that we have considered above, most birders choose 8x42 binoculars. 8-power magnification brings even distant birds into view. Yet they are small and light enough that they can be held steady without undue arm fatigue.

The exit pupil of 5.25 mm (objective lens diameter of 42 mm divided by magnification of 8) means that this binocular is quite bright in even dim light. This is perfect for walking in the woods at dawn or under overcast morning skies.

8x binoculars also tend to have an inherently wider field of view than 10x binoculars. So you are able to scan more area and find birds off of the center view.

What about 8x20 or 8x25 compact binoculars?


Compact binoculars are those with the normal magnification (7x to 10x) but with smaller objective lenses of less than 30 millimeters. They are much lighter and some fold down to fit easily into a pocket, purse or backpack.

Their small size may be suitable for some children or adults with smaller hands.

But because the exit pupil is less than 5mm (2.5mm for 8x20 and 3.125mm for 8x25) they will be noticeably dimmer in low light. They will be great under sunlit skies. They will give a darker display in the morning gloom or under the canopy of trees. So they are not the best choice for general-purpose everyday binoculars.

These are suitable for a "second pair" of binoculars you may always keep in the car. They may be fine for watching birds at your feeder out your window. They are a great backpacking binocular. The 8x25 binoculars are better in low light than the 8x20.

Something to research: 8x32 mid-sized binoculars for birding


The mid-sized 8x32 binoculars are a good compromise between the full-size 8x42 and the smaller compact 8x25 binoculars.

For one thing, the 8x32 binoculars often have the same eyepieces (ocular lenses) and prisms as the larger 8x42 binoculars. Thus, they often have much better specs than the compact binoculars, which require smaller prisms.

They are lighter in weight and have a smaller form factor than full-sized binoculars. So they have many of the benefits of the smaller compact binoculars.

The exit pupil is 4.0mm. They will be bright in most birding circumstances. But you will notice they are dimmer in low light in a side-by-side comparison with an 8x42 binocular of the same model.

The 8x32 mid-sized binocular could be a good choice for someone with smaller hands or who wanted a lighter-weight pair of binoculars but didn't want to give up too much optical quality of most compact binoculars.

Are 10x42 binoculars good for bird watching?


We've already discussed how 10x magnification binoculars have a narrower field of view, are slightly darker in dim light (exit pupil of 4.2mm), and magnify your arm shake. However, they also magnify the bird's size by 25% over the 8x binocular. That is not insignificant.

If you have strong, steady, arms you may consider a 10x42 binocular as your primary birding binocular.

10x binoculars are especially good for open country birding. If you primarily watch more distant birds in good light, then these might be a better choice for you. If your birding consists primarily of watching ducks, hawks, and shorebirds, then these are your bins. However, you may consider a spotting scope for this type of birding, allowing for 25-60x magnification.

Are 10x50 binoculars good for birding?


I had this size binocular for several years as a young man. They have a bright 5.0mm exit pupil for excellent brightness in dim light. They are significantly heavier, though. And mine were poro prism design (not waterproof). [Some poro prism binoculars are waterproof, while most roof prism binoculars are.] The waterproof roof prism design would be even heavier than my old 10x50 poros.

The close focus distance is generally not close in these binoculars. They may not have good eye relief, either.

Can they work for birding? Yes. Would I recommend 10x50 binoculars for bird watching? No.

Are 10x25 binoculars good for bird watching?


No. The 2.5mm exit pupil is unacceptably dim. The field of view will be terribly narrow, making the arm shake even more noticeable. I can't recommend 10x25 binoculars for birding. If I saw a bird and these were the only binoculars available then, sure, I'd lift them to my eyes. But I wouldn't have them as my primary birding binoculars.

12x binoculars for bird watching? 15x?


No. For all the reasons 10x are difficult, anything more becomes unusable. If you want more magnification than 10x then you need to look into purchasing a spotting scope with a sturdy tripod.

Conclusion


  • The 8x42 binocular is the best magnification for bird watching under the widest variety of conditions.
  • The 8x25 compact binocular is good for bird watching on a backpacking trip or as a second pair to carry in your car's glove compartment. 
  • The 10x42 can be a good bird watching binocular if you have steady arms and (though we didn't discuss it) if you don't wear eyeglasses. 
  • The mid-sized 8x32 or 10x32 can be a nice compromise between full-sized and compact binoculars.



Check out our in-depth binocular buying guide. Compare all the specs in the handy tables.


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