Binocular Eye Relief Explained
Are you new to the world of binoculars? Do you keep hearing terms you’ve never heard before? Do you want to know what eye relief is? If you have answered yes to those questions, then you’re in the right place. Read on to learn all there is to know about eye relief.
What Is Eye Relief?
Eye relief is determined by the design of the eyepiece. All ocular devices, binoculars, monoculars, telescopes or spotting scopes have eye relief. When using any of the above optical devices, there is a perfect distance that your eye should be from the eyepiece, that distance is called eye relief.
If your eyes are further away from the binoculars eyepiece than the ideal eye relief, you won’t be able to see the whole image through the lens. You lose parts of the image. The farther away from the eye relief your eye is, the less of the image you’ll be able to see.
What Is The Exit Pupil? And How Does It Help?
The exit pupil is the diameter of the circle of light you can see when holding the binoculars away from your eyes and aimed at the sky. The column of light that is coming through the binocular lens to your eye, the diameter of that light is your exit pupil.
How To Calculate The Exit Pupil On Binoculars
It’s easy to determine the exit pupil on a pair of binoculars without ever having looked through them. Just divide the magnification of the binoculars by the diameter of the objective lens. For example, a pair of binoculars with a magnification of 10x and an objective lens diameter of 32 (10×32) will have an exit pupil of 3.2 mm.
So divide the magnification by the objective lens size to obtain the exit pupil number. Just one more example to get your head around it fully. A pair of binoculars with a magnification of 8x and an objective lens diameter of 42 (8×42) has an exit pupil of 5.25 and the sum looks like this:
It works like this – The larger the objective lens, the larger the exit pupil, but, the greater the magnification, the lower the exit pupil. So the exit pupil is determined by the magnification and the objective lens size.
How Big An Exit Pupil Is Best?
The size of the exit pupil should be greater than the size of your own pupils once they’ve become accustomed to the dark. An average 30 year old will have a pupil size of around 7mm. Every 10 years this will decrease by 1 mm so the average 30 year old with a 7mm pupil will have a 6mm pupil by their 40th birthday and so on.
So with binoculars, the higher the exit pupil the better, but as long as it is 7mm or larger it will be OK for 30 year olds. To be certain of our understanding of exit pupils, here’s a better example.
Eye Pupil Size Averages
|Age||Average Eye Pupil Size|
|30 year old||7mm|
|40 year old||6mm|
|50 year old||5mm|
|60 year old||4mm|
|70 year old||3mm|
|80 year old||2mm|
The size of an average pupil decreases by 1 mm every decade.
How Does The Size Of The Exit Pupil Help Choose Binoculars?
Now we know the average size of the human pupil at any given decade of their/your life, we can now purchase our binoculars with that knowledge in mind. For example a present of a new pair of binoculars for your 60 year old grandad will need an exit pupil of at least 4. This gives us plenty of choices 10×42 (exit pupil value of 4.2) 8×32 (exit pupil value of 4) 8×42 (exit pupil value of 5.25), let’s clarify this some more.
|Binocular Magnification and objective lens size||Exit Pupil Value (rounded down to the nearest whole number)|
The size of the exit pupil makes for more comfortable and better viewing when it comes to binoculars but as with all of these equations, there are circumstances where the eye relief and exit pupil can mean the difference between injury and not. Take rifle shooting for example, the high powered scope that’s used to view the target needs to be at the perfect eye relief for the user.
If not the target isn’t clear or the scope could give you a nasty whack because your eye was too close to the lens. So this information crosses over into any optical instrument.
The take away from this is that in low light conditions, you’ll struggle to see anything through the binocular lens with a low exit pupil. A high exit pupil under the same low light conditions will show much more of the image as the lens will capture more light – illuminating the image you are looking at.
What About The Field Of View (FoV)?
These terms crop up whenever optical equipment is the subject of conversation, and for good reason. The idea of using any optical equipment to magnify your image is to improve your view. For instance a shooting scope needs to be accurate for your shots to be on target, binoculars should give you a great image of whatever object you’re looking at and so on.
The field of view is basically the measured distance of all you can see from edge to edge through the lens. It’s either measured in feet or metres or as an angle. For instance a field of view of 105 metres per 1,000 metres or 6°. These two actually represent the same answer – 6° is the same as 105 metres per 1,000 metres. For Binoculars a good field of view is anywhere between 6° and 7.5° (105 metres to 132 metres per 1,000 metres). If you want to calculate the field of view from degrees(°) to metres per 1,000 metres just multiply the number in degrees by 17.5 and this will give you the value in metres per 1,000 metres. 1°=17.5 metres (roughly) so it really is a simple one to work out.
The easiest way to describe the FoV is how wide your view is through the binocular lens. A wide field of view makes seeing fast moving objects (like birds) easier. Low FoV = less than 6° means a narrower width of image as seen through the binocular lens. Higher FoV= above 7° means a wider width of image seen through the binocular lens and so on.
The Facts About Optical Physics
The way optical physics works is, as you increase the power of magnification, the field of view and the eye relief decrease. High power magnification means less eye relief, and lower magnification power means more eye relief. Binoculars with zoom lenses present more of a problem, unless they are a top quality pair of binoculars, the image loses some of it’s clarity at higher magnification so as you increase the zoom, you lose some of the image due to a poor quality image.
This probably explains why none of the top binocular manufacturers make any really high powered zoom lenses. But not all binoculars have to have zoom to affect the eye relief.
Adjustable Eye Relief For Wearers Of Glasses
Many binoculars offer adjustable eye relief, some for instance might have an eye relief of 16mm which might suit some glasses wearers. It is possible to adjust the eye cups and if you twist them to their full adjustment level, you will lose some of that distance and you could end up with an eye relief of 9mm.
You’ll still be able to see the entire field of view but due to how hard you’ll have to press the rubber eyecups to your eyes, you will experience a lot of discomfort. If you mess with the eyecups you’ll be able to reach a position of comfort and full viewing potential.
Having multi-positional eyecups and long eye relief will allow you more viewing flexibility, better comfort and eye safety too. This will be applicable for both glasses wearers and non glasses wearers.
Short Eye Relief
In the binocular industry, an eye relief of less than 13mm is considered to be short eye relief. For non glasses wearers, 13mm won’t be a problem, even though it could cause you to press your eyes too far into the eyecups to achieve a full field of view. For those of us who wear glasses constantly, a field of view of 16mm or less could be considered to be short eye relief.
Long Eye Relief
Long eye relief is anything above 20mm and allows for more comfortable viewing whether you wear glasses or not. Having greater eye relief will allow you to not need to press your eyes too close into the eyecups to see the full field of view which will definitely be more comfortable in the long term. If you position your eyes in the wrong place, either too close or too far away from the recommended eye relief distance you will experience some or all of the following;
- A Reduced Field Of View
- Lower Light Transmission
- Blurred Image Edges
- Images Cut Off At Edges
- Eye Strain
All binoculars will state the eye relief value in the specifications (usually in additional information), and this will give you some idea of where you should position your eyes for the best view.
What Is Close Focus?
This is great for viewing butterflies and bees at close range. The close focus is the minimum distance you can be from an object and still see it in clear focus. Usually expressed in metres, the lower the close focus is, the better the binoculars are for looking at insects etc up close and personal.
A General purpose pair of binoculars has around 20 to 25 feet close focus, a bird watching pair can expect around 10 feet and the closest focus pair has around 3 foot close focus. If you are looking at birds in the distance and movement in a bush close by catches your attention, having binoculars with close focus will assist you in identifying what that movement is caused by. Regular binoculars won’t be much help under these conditions.
A Summary Of The Main Terms Covered In This Article
We know how it is, with so much new information to absorb, it can sometimes feel like your brain will explode. You have read so many facts and figures that they all start to blend into one. So here’s a short summary of what we’ve covered to remind you of the important bits.
Eye relief is the perfect distance between your eye and the eyepiece to see the whole image through the binocular lenses. Many binoculars offer adjustable eye relief which will definitely be necessary for glasses wearers.
Anything below 13mm is considered short eye relief and anything above 20mm is considered long eye relief.
The exit pupil is the diameter of the light that shows through the binocular lens. It’s important to get a decent exit pupil to see the full image through the lens especially in low light conditions. The human eye has a pupil that, in darkness, is around 7mm in diameter at the age of 30. As our age increases, our pupils don’t dilate so much so by the time we reach 60 our pupil size in the dark will be around 4mm.
In low light conditions a high exit pupil will allow more light to enter the lens thus making the image you are viewing brighter, and easier to see.
Field Of View (FoV)
The field of view is how wide the image you can see is through the lens of a pair of binoculars. A wide field of view makes it easier to track fast moving objects like birds. The FoV is shown either as an angle or as a number of metres per 1,000 metres. Anything 6° (105 metres per 1,000 metres) or above is acceptable for binoculars, but more is better.
The problem is as magnification increases, FoV and eye relief decrease. It’s a compromise whichever way you look at it, but sometimes we think we need greater magnification than we actually do.
Close focus is the minimum distance you can be from an object and still see it in clear focus through the lens of the binoculars. It is usually depicted in feet or metres and a general purpose binocular has around 20 to 25 feet close focus, while a decent bird watching pair has 10 feet or less and the lowest or closest is around the 3 foot range.
Frequently Asked Questions
Eye relief is very important because if you have incorrect eye relief you will not see the full image. It is even more important for glasses wearers as they need to take account of the size of their glasses when trying to look through the binocular lenses.
Eye relief means the recommended distance you should have the binocular lenses from your eyes. Too close or too far away and you will not see the full image clearly.
With glasses, you need an eye relief of at least 16 to 17mm But more than 20mm is better.
Anything above 20mm is considered to be long eye relief when it comes to binoculars.
Eye relief does change with magnification, with binoculars typically, the higher the magnification, the shorter the eye relief.