The Binocular Blog - Part 4: Eye relief

Eye Relief (Pic courtesy: Olympus Asia)

One of the often ignored number when buying a binocular is the eye relief. This is the distance a binocular must be held from the eye to visualize the full field that is possible by that instrument. 

What you will see if your eye relief was appropriate

This is particularly important in spectacle wearers. If the distance from your eye to your spectacles is greater than the binocular’s eye relief (short eye relief) you will see only the central portion of the image being viewed.

What you will see if the eye relief is short.

If the eye relief is long, you will see a crescentic shadow moving around the periphery of the viewed area. This happens when you keep your eyes too close to the eyepiece.

 

For someone who does not need spectacles, this problem has a built-in solution as all modern binoculars have eye cups that can be twisted up (or out), so the eyes are positioned slightly behind the eyepiece lenses. The older models and some new ones have a foldable rubber cup. To avoid this shadow appearing, users who do not wear glasses should twist the eye cups up completely to get appropriate eye relief.

Eye cup twisted down (left). Eye cup twisted up (right)

If the user wears spectacles the eye cups have to be twisted down, assuming the eye relief of that particular instrument is in the adequate range.

Eye cups twisted down for spectacle user

 

Eye cup twisted up without spectacles

Otherwise, with spectacle, if your eye is too far from the eyepiece lens, the there will be a ring shadow on the entire circumference of the viewed area leaving you with a constricted field of view. This happens when spectacle wearers forget to twist the eye cups back in if it was twisted out or if you hold the instrument too far from the eye. On an average the eye relief, for most people, should be around 15 – 20 mm.  

This is one of the, often overlooked, piece of detail about the binoculars. I too had no inkling about this till I decided to go for my fourth pair. This is not usually found on the instrument like the numbers for magnification, objective lens size or field of view. You have to look for it on the box the binocular is packed in.

The Bushnell Trophy Bone Collector edition (10x42)

The Zeiss Terra ED (8x42)

The Vortex Viper HD (12x50)

To be very honest, the Bushnell 10x42 wasn't very comfortable for me and it is currently being used as a monocular telescope since the diopter adjustment ring is not working. The most comfortable of all my bins is the Zeiss. I got the Vortex primarily, for the magnification and that is also very comfortable, though not like the Zeiss. 

So, when you finally decide to buy a binocular, don't forget to read the small print on the box too.

For those who are impatient to go through the entire blog series, I'm giving below direct links to all the posts. You can go directly to the page you are interested in, though I suggest you go through each post as all of them contain some information that will be useful when you consider purchasing a binocular for your self.

1.       Introduction
2.       Part – 1: Anatomy of a Binocular
3.       Part – 2: Purpose and Portability
4.       Part – 3: Magnification, Objective diameter & FOV
5.       Part – 4: Eye Relief
6.       Part – 5: Exit pupil & Interpupillary distance
7.       Part – 6: Image brightness
8.       Part – 7: Focusing fundamentals
9.       Part – 8: Weather sealing
10.   Part – 9: Purchase decision
11.   Part – 10: Care & Storage
12.  Afterthoughts and accessories