Binocular lens sizes. Why it matters and what's important

How (slightly) larger binoculars can help you see more 

We had a talk in some previous blogs about how binocular and spotting scope magnifications are described and why this is important, especially for bird watching. 

As a quick example, they take the form of:

8 x 42

Where the first number, in this case 8, is the magnification. The second number is the diameter of the front lens in mm. You'll also see spotting scopes and zoom binocular described something like:

20-60x80

This means the magnification "zooms" or ranges between 20 to 60 times, and the front lens is 80mm in diameter. 

While we covered magnification for birders before, one question that gets asked by our birding customers here at BINTEL is "how important is the size of the lenses?"

What main lenses in telescopes and binoculars actually do

If you can get 8x25 and 8x42 sized binoculars and they both have 8x magnification, why do I need the bigger and more expensive pair?

Many folks are under the impression that the front lenses in binoculars increase the size or magnify the view. That's true to an extent, but their real job is to collect all the light that falls on them, concentrate it and present it to your eyeballs. Apart from increasing the size of the image which we perceive as being "closer" to what we're observing, they also increase brightness and detail.

I like to use the analogy of it being like a bucket being left out in the rain. The bigger the bucket, the more water you'll collect. Bigger optics collect more light and you'll see more.

Exit pupil

Ultimately the brightness of your view will depend on what's called the exit pupil. This is the size of the image that lands on your eyeball. It's given as the size of the binocular's main lens divided by the magnification. The exit pupil size for a pair of 8x42 binoculars would be 5.25mm, while for 8x50 it's 6.25mm.

A 8x25 set of binoculars deliver an exit pupil of some 3.12mm.

Your exit pupil is approx. 3-4mm during the day. Is will expand and open up as conditions get darker to around 5 to 7 mm.  (Another "benefit" of ageing is that the pupils in your eyes don't open up a much as they used....)

During the day, an 8x25 pair of binoculars won't produce as bright and detailed image as an 8x42 (3.12mm vs 5.25mm) You'd notice the difference between them immediately. 

There also won't be much difference in between the 8x42 and 8x50 binoculars (5.25mm vs 6.25mm exit pupil) during the day as your pupils will be only 3 to 4mm and simply won't be able to see the extra light.

Ok, so what are the condition where larger binoculars are going to be give me better views?

However, if you're planning on observing when the light isn't as bright such as early in the morning or dusk or even when weather conditions are poor, your wider pupils will pick up the additional light collected by larger, 50mm diameter binoculars. It's also handy when scanning around for what you'd like to observe. 

It's worth mentioning that we're comparing binoculars of similar quality. The total amount of light let through and the level of detail that premium optics can deliver will be quite noticeable when compared to less expensive models. 

On the down side, 50mm binocular are slightly heavier and this can be a consideration if you are hiking or travelling with a lot of other equipment in the field. 

Bottom line: If you're mainly nature observing and birding during the day, an 8x42 or 10x42 pair of binoculars will work well. There will be a noticeable advantage in image quality, field of view and brightness compared to say 8x25 or 10x25 binoculars of similar optic quality.

If you are planning on observing early in the morning or as it's getting dark in the evening (and this is often when some of most interesting wildlife behaviours occur) then upgrading your choice of viewing to an 8x50 or 10x50 size might be worth considering. 

As always, there's no once size is best for XYZ activity.  That's why our expert team at BINTEL is always happy to chat at length about optics what would suit you and your lifestyle.

Cheers,

Earl White

BINTEL

13th March 2026