Archive for Feb 13th, 2012


Bin it or not? Part 2

Last Time in this series we talked about Magnification and Aperture; this time we’ll kick off with exit pupil.

Exit Pupil: This is the beam of light that leaves the back of your binoculars containing the image you watch. The diameter of that exit pupil in millimetres is determined by dividing the aperture by the magnification. So a pair of 10×50’s will have an exit pupil of 5mm, a pair of 8×42’s will have an exit pupil of 5.25mm and so on.

The relevance of this is that if the exit pupil is greater that the diameter of the pupil of your eye, then part of the image is wasted as it will be blocked by your iris and not reach your retina; resulting in a dimmed image. If the exit pupil is smaller than your own pupil again the image brightness is falling short of what your eye could manage.

Your pupil can have a diameter of between 2 and 8mm, towards the larger end of this range in younger people, and varies dependent upon the lighting conditions. In brighter light your pupil will automatically constrict to allow in less light. In dull conditions it will dilate to let in more light. With the use of a mirror, torch and a ruler it is possible to measure your own range of pupil size. If you are regularly using binoculars in low light it may be wise to choose a pair with a greater exit pupil for optimum performance.

Design: There are two designs of binocular in common use; Porro Prism, which has offset barrels and Roof Prisms which have straight barrels. There are also reverse Porro Prisms which are used in compact binoculars to reduce overall size; personally I don’t like this sort of binocular. Porro Prisms are bulky and usually heavy. Roof Prisms are the way to go.

Many makes of binocular are now gas filled to make them completely waterproof. DO NOT pack this sort of binocular in hold luggage when you fly abroad. The unpressurised hold will ‘pop’ the seals rendering your optics useless. Not the sort of thing you want to happen when you are going on the holiday of a lifetime.

Balancing: You can’t hope to get the best from binoculars unless they are balanced for your eyes. Binoculars have a dioptre fitted to one barrel. This dioptre independently adjusts the focusing on one barrel only. This is because our eyes are not the same and optics require independent focusing. Sometimes the adjustment is on one of the barrels or sometimes where the barrels join together. To correctly adjust the dioptre follow the procedure below:

With both eyes open cover the right hand objective lens* and focus on a static object using the focusing wheel. Cover the left objective lens and focus on the same subject using the dioptre eyepiece adjustment only. The binoculars will then be focused and balanced for both eyes. Further adjustment is made only with the centre focusing wheel. (*reverse if the eyepiece adjustment alters the left hand barrel of the binoculars)

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Feb 2012


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