Understanding Binoculars, Exit Pupil, Field of View, Eye Relief, Near Focus

Understanding Binoculars - Lens Coatings, Focusing

Understanding Binoculars - Brightness, Prisms, Contrast, Resolution

Understanding Binoculars - Magnification, Objective Diameter

The Exit Pupil Diameter is always quoted, in millimetres, in the binocular specification chart. The measurement represents the beam of light, in diameter, which leaves the eyepiece. A large exit pupil will mean the images will appear brighter so for example binoculars with an exit pupil of 4.2mm should theoretically deliver brighter images than binoculars with an ext pupil of 2.8mm.

If the exit pupil isn’t quoted, it’s easy to calculate by taking the objective diameter (say 42 if the binoculars are 10x42’s) and dividing it by the magnification power of the binoculars (10 in this example). The exit pupil of 10x42 binoculars is therefore 4.2mm.

Having a large exit pupil is useful in poor light conditions so astronomy binoculars are likely to have a larger exit pupil that binoculars designed for good light conditions.

The exit pupil should equate to the dilation of the pupil in your eye after your eyes have adjusted to the light conditions (greater dilation in poor light).

The Field of View (FOV) is the area that can be seen when you are looking through the binoculars. Binocular specifications will generally refer to three measurements in regard to the Field of View:

• Angular Field of View (Real)
• Angular Field of View (Apparent)
• Field of View at 1000mtrs

The Angular field of view is measured in degrees. You can roughly calculate the linear field of view (field of view at 1000mtrs) by multiplying the angular field of view (real) by 17.5. Take for example Canon 12x36 IS II Binoculars. The real field of view is 5^o so if you multiply 5 x 17.5 you come up with the linear field of view which is 87.5mtrs.

The Angular field of view (apparent) is the real field of view multiplied by the binoculars magnification power so continuing to use the Canon 12x36 IS II Binoculars as our example, they have a real field of view is 5^o  and a magnification power of 12, 12 x 5 = 60 ^o .   

As you can see the field of view is related to the magnification powers of the binoculars, the higher the level of magnification the smaller the field of view (as a general concept). If you are viewing fast moving objects it’s desirable to have a wide field of view which makes is easy to find the object that you want to view such as planes or birds.

Eye Relief is the distance in millimetres, from the eye, that the binoculars can be held with the complete field of view still comfortably visible. When you are looking to buy a pair of binoculars you will find that some are suitable for spectacle wearers while others are not. Obviously the further away from your eye that you can hold the binoculars the more comfortable it is, particularly for people who wear glasses. Most manufacturers only recommend binoculars with an eye relief greater than 15mm for spectacle wearers.

The Close Focusing Distance of binoculars refers to the nearest object you can focus on while maintaining a clear, sharp image. The close focus distance is quoted in metres.  The importance of close focus is dependent on what you want to use your binoculars for. Astronomy binoculars for example could have a close focus distance of say 60mtrs while birding binoculars may be able to focus on birds as close as 2.5mtrs and compact binoculars could focus on objects as close as a 1mtr.

Bird Watching Binoculars / Astronomy Binoculars / Compact Binoculars

Understanding Binoculars cont.........