There are lots of terms to discover when researching binoculars for bird watching. For example, what is exit pupil distance and why is it important?
We will explain how the size of an exit pupil measurement affects the quality of your viewing. This should help you to make an informed choice about which binoculars will suit your eyes, plus the type of bird watching that you like.
What is exit pupil in binoculars?
The exit pupil is the cylindrical beam of light that comes out of the eyepiece.
To see what it looks like, hold your binoculars orientated as if you were going to look through them. Instead of bringing them close to your face, hold them at arms’ length.
When you look at the eyepiece, you’ll see a bright circle of light. This small circle is the beam of light coming out through the eyepiece – in effect, the magnified image as it exits the binoculars and enters your eye.
How is exit pupil measured?
The diameter of this circle of light is the exit pupil. It is usually measured in millimeters.
How do you calculate exit pupil distance?
To calculate exit pupil, divide the diameter of the objective by the power/magnification.
For example: for a 10×50 binocular, we divide 50 by 10.
So the exit pupil is 5mm.
An exit pupil of 5mm is towards the upper end of our range of examples.
What determines the exit pupil?
These examples are listed in order of size of exit pupil so you can see how the ratio affects the exit pupil:
For an 8×21 binocular, exit pupil is 2.6mm.
For an 8×25 binocular, exit pupil is 3.125mm.
For a 10×32 binocular, exit pupil is 3.2mm.
For an 8×32 binocular, exit pupil is 4mm.
For an 8.5×42 binocular, exit pupil is 4.9mm.
For a 7×35 binocular, exit pupil is 5mm.
For an 8×42 binocular, exit pupil is 5.25mm.
For an 7×50 binocular, exit pupil is 7.1mm.
The larger the objective lens, the larger the exit pupil.
The larger the magnification, the smaller the exit pupil.
How do you find exit pupil?
Many manufacturers will list exit pupil in the spec, though, to save you doing the calculation yourself.
How important is exit pupil in binoculars?
To be able to see the full width of the field of view, your eyes must be in exactly the right position. This allows the binoculars to most effectively transmit the maximum amount of light that they can gather through the lens at the other end.
For this to happen, the exit pupil must be equal to or greater than the diameter of your pupil at any given moment. In ordinary light, a human pupil normally measures about 2-3mm.
It is also important to make sure that the eyepieces are lined up correctly with your eyes, so that the light is able to enter your eyes and give the best possible view.
Is a wider exit pupil better?
A wider exit pupil delivers a brighter image.
Because a bigger exit pupil has a greater capacity to gather light and deliver it to your eye, the image will be brighter. This is true – but only up to the limit of the maximum diameter of the eye’s pupil, which is around 4mm to 7mm.
This diameter varies, depending on your age and the amount of light available.
How does age affect what exit pupil we need?
As we get older, our eyes are not able to adapt as effectively to dim light. The pupils are not able to dilate quite so well.
In later years, your pupils might open a couple of millimeters less than three or four decades prior.
Exit pupil and age: what binoculars are best for older birders’ eyes?
In practice, this means that binoculars with large exit pupils may be of limited help to older bird watchers.
Since our pupils dilate less in dark conditions as we age, compact binoculars with smaller exit pupil sizes can be ideal for older eyes whose pupils stay quite narrow even as the environment gets darker.
The best bird watching binoculars for older, ageing eyes
For a more senior person, compact binoculars, such as an 8×32, will provide an image that is equally as bright in a lightweight, more manageable binocular. You will probably find another advantage – that a compact binocular is often cheaper too!
What is the best exit pupil for binoculars?
During the day, the human pupil might contract to just 2mm. When the diameter of the pupil is this narrow, the pupil does not process the full width of the beam of light entering the binocular.
Instead of entering the pupil, some of the light falls on the iris instead. In this case, even if you have a binocular with larger objective lenses letting in more light, that extra light falling on the iris is in effect wasted.
So in bright daylight, even a pair of binoculars with smaller objective lenses, like an 8×32 will deliver as bright and clear an image as we would see through an 8×42.
How big an exit pupil do I need for bird watching?
For most bird watching, in average light conditions, a pair of binoculars with an exit pupil size in excess of 4mm should be sufficient.
For bird watching in areas where there is more shade, like dense woodland or forests, in dim light (owl watching), or on boats, look for an exit pupil larger than 5mm.
An exit pupil higher than 5mm would be more than enough to allow a full field of view with good resolution.
Only in very poor light conditions would binoculars require an exit pupil wider than 6mm. In these conditions, the pupils in the human eye dilate to approximately 7mm.
The best exit pupil for a dark environment
To get wider exit pupils for bird watching in a dark environment, you could look for specifications with a magnification of 8x, to minimize image shake, but increase the size of the light-gathering objective lenses to over 50mm.
This solution is also suitable for those looking for binoculars to watch birds from unsteady boats. The wide exit pupil can counteract the disorienting effect of the water movement.
What you need to know next: is exit pupil distance the same as eye relief?
No. Eye relief is also measured in millimeters, but it is different to exit pupil.
Eye relief is an equally important measurement to know about when you are choosing binoculars for bird watching, especially if you wear eyeglasses or sunglasses.
We suggest you move on to one of our other articles that clarifies what eye relief is. It’s an important measurement for people who wear glasses, or even sunglasses, when bird watching >>> All about eye relief: choose and use binoculars for glasses wearers
- 1 What is exit pupil in binoculars?
- 2 How important is exit pupil in binoculars?
- 3 Exit pupil and age: what binoculars are best for older birders’ eyes?
- 4 What is the best exit pupil for binoculars?
- 5 What you need to know next: is exit pupil distance the same as eye relief?