Beginners Guide to Binoculars.

To aid your choice of instrument we offer a basic guide to binoculars.

Numbers (8×42,10×42,10×50, etc.)

The first figure 8x, 10x, etc. is the magnification. Thus the subject is larger or closer by the magnification used. The second number is the diameter of the objective lens (the lens furthest from the eye). The size of this lens gives a good indication of the size, weight and light transmission of the instrument. Dividing magnification into the objective lens diameter gives a guide to light transmission by what is known as the exit pupil, the diameter of the beam of light projected into the eye.

8×20 = small compact     exit pupil 2.5mm

8×32 = smaller full size   exit pupil 4mm

8×42 = medium full size exit pupil 5.25mm

8×50 = large full size       exit pupil 6.25mm

This is only mathematics and gives no indication of the quality of image.

Porro prism binoculars are a traditional style of instrument which has largely been superceded by the roof prism style. Although a little more complicated to make, the roof prism has several advantages over the porro. It is easier to waterproof, has closer focusing capabilities, is lighter in weight and more rugged. These are all generalisations.

Spectacles are accomodated by eyecups which lift up for non-spectacle use and twist, push or fold back for use with spectacles.

In general terms lower magnification and objective lens sizes, around 30/32mm, offer the widest field of view. A wider field can make it easier to find a subject within the field, but all conventional sizes will give an acceptable field of view.

Lens quality and coating influence the image quality most of all. Coatings aim to allow maximum light at any given size and lens quality for sharpness. Engineering also costs money but will give better long term service.

So the higher the cost, the better the instrument; but above all, try a range. There should be no black shadows, milkiness or obstruction within the view, which should be a complete circle. Discuss your requirements with a knowledgeable person (not always your birding buddy) and test in suitable conditions.