Long Eye Relief Binoculars (Why & How To Choose Them)
There are many terms used in the world of binoculars that sound confusing, until you find out what they mean. Then they make perfect sense like “close focus” which actually means how close you can be to an object and still focus clearly on it. Long eye relief is another such term. Let’s start with eye relief and take it from there.
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What Is Eye Relief?
There is a perfect distance your eyes should be from the binocular lens and see all of the image through the lens, that distance is called eye relief. If you position your eyes too far from the eyepiece, you won’t be able to see the whole image, you’ll lose the outer edges. Too close and you’ll get eye pains or the muscles in your face will hurt because of the constant pressure from the binoculars being pushed into your face.
The eye relief will be recorded in the specification for the binoculars (if they are of good quality at least) usually in millimeters. An average eye relief of 13 to 16mm is OK for most people. If you need to wear glasses continuously however, you’ll need more eye relief than 16mm otherwise you won’t be capable of seeing all of the image through the lens. Any eye relief above 16mm is called long eye relief.
What Is Long Eye Relief?
As we said above, long eye relief is an eye relief greater than 16mm, this means that allowing for your glasses, you will still have that perfect distance between your eyes and the ocular lens (the lens closest to your eye) and still see the full image (FOV).
While on the subject of eye relief, steer clear of those ridiculously cheap (£15.00) binoculars, as these usually have extremely short eye relief which will definitely not be suitable for glasses wearers.
What’s The Relationship Between Magnification And Eye Relief?
Binoculars that have low magnification usually have a longer eye relief and a larger exit pupil. This allows glasses wearers to see the full Field of view (FoV). So lower magnification, say, 8x with an objective lens diameter of 40 (8×40) will have a larger exit pupil than a 12×50.
What’s The FoV?
Here we go with the binocular terminology again, the FoV or field of view is the width you can see through the lens of the binoculars from left to right without moving your head and still see the image clearly. The FoV is quoted in the specs as either angular Fov or linear FoV; they both tell you the same thing, but in different ways.
This is always quoted in degrees of a circle and anything between 6° and 8°is considered to be a good FoV. 1° is equal to 52.5 feet or 17.5 metres so to work out the linear FoV just multiply the angular FoV by either 52.5 feet or 17.5 metres depending on which system you prefer.
The linear FoV is expressed either in feet per 1,000 yards or metres per 1,000 metres. 105 metres per 1,000 metres to 140 metres per 1,000 metres or 315 feet per 1,000 yards to 420 feet per 1,000 yards is the same as 6° to 8°.
The above numbers give you the average FoV for binoculars, you will find some with a lower and some with a higher FoV but anywhere between those parameters are good enough for most binoculars users.
Adjustable Eye Cups
The eye relief can be adjusted on binoculars using the eye cups. The eye cups are the rubber cups right at the end of the ocular lenses which fit against your face. They will either twist up/down or fold up/down and give you some adjustment on the eye relief. But don’t assume they will allow enough extra space for glasses, for that you will definitely need long eye relief binoculars.
Is It OK To Share Binoculars With Other Users?
If you’re sharing binoculars with someone who doesn’t wear glasses, you’ll need to readjust the eye cups every time you use the binoculars. In a similar way to adjusting the driving seat of a shared car. So if possible get a pair of binoculars for each person to save having to readjust the eye cups after each use.
What Is A Diopter?
The diopter is an individual focussing twist knob you’ll find on one of the lens tubes, usually the right but not always. To set the diopter correctly just follow the following steps assuming the diopter is on the right eye tube;
- Block the right lens with the lens cap, focus the binoculars using the central focussing knob, until the image is clear.
- Unblock the right lens and block the left lens with the lens cap, then adjust the diopter on the right lens tube until the image is clear.
- Unblock both lenses and look through the binoculars and fine tune with the central focussing knob. Job done.
Some Glasses Wearers Might Not Need Long Eye Relief Binoculars
Just because you wear glasses doesn’t mean you’ll necessarily need them for looking through binoculars. Being long sighted or short sighted is a nuisance but binocular lenses will correct this without the need for wearing your glasses. It’s only if you use glasses for something else like astigmatism that you’ll need long eye relief.
If you can use the binoculars without your glasses, be sure to put your glasses somewhere safe. Possibly on a cord around your neck, but this might become a problem if you start using binoculars with any frequency. Or if, for instance, you’re on a wildlife safari, where speed is of the essence. Those animals won’t necessarily wait for you to take off your glasses and lift the binoculars to your eyes.
You might be better off getting binoculars with long eye relief to save time and make the most of your wildlife viewing experience.
If you wear glasses buy binoculars with long eye relief for convenience
What Is The Exit Pupil?
If you hold the binoculars at arms’ length and look through the objective lens (the largest lens, the one closest to the object), the circle of light that’s visible through the lens is called the exit pupil. Generally, the larger the exit pupil, the brighter the image will appear when viewed through the binoculars. If you plan on using your binoculars at dawn or dusk or even on dull days, a larger exit pupil will be advantageous.
To calculate the exit pupil just divide the objective lens size by the magnification of the binoculars. For instance, 8×40 binoculars will have an exit pupil of 5mm because
In our earlier comparison we said that a lower magnification has a lower exit pupil and we used 12×50 as our second example,
This gives us an exit pupil of 4.2mm with a pair of 12×50 binoculars as opposed to an exit pupil of 5mm with a pair of 8×40 binoculars.
The pupil of the human eye varies from 2mm to 9mm depending on how much light is available. Pupils tend to get smaller as the years go by, with the average 30 year old having a pupil size of 7mm. Which decreases by 1 mm every decade so by the time the average pupil reaches 40 it will be 6mm and so on.
Frequently Asked Questions
Long eye relief binoculars are considered to have an eye relief of between 16 to 24mm.
If you need glasses for either short or long sightedness then you could remove your glasses and the binocular lenses will account for your condition. If however, you have an eye condition that means you cannot see without your glasses, then you will need a pair of binoculars with long eye relief. This will allow you to see the full field of view through the binocular lenses.