Optics Explained

Binoculars are usually referred to by two numbers, for example 8x40, 10x50. The first of these numbers is the actual magnification of the binoculars: the size to which the subject is enlarged. The second number is the size, in millimetres, of the objective (front) lens. For the same given magnification the bigger the objective lens the brighter the object will appear. Therefore an 8x42 will have the same magnification as an 8x25, but the image will appear brighter.

The brightness of binoculars of differing magnifications can be ascertained by dividing the objective lens size by the magnification. This gives us what is called "the exit pupil" diameter in millimetres. This can also be seen if you hold the binoculars a few inches away from your eye; the circle of light viewed through the objective lens is the exit pupil. In an 8x40 binocular this circle is 5mm in diameter. The pupil of the human eye can, in low light, open to approximately 7mm. There is little point having a binocular with an exit pupil bigger than 7mm as the eye could not take in the extra light. In brighter viewing conditions the eye closes down smaller than 7mm so it is not as important to have a large exit pupil in these conditions. It is also worth noting that the ability to open the pupil reduces to around 5mm as you get older, so an 8x42 or 10x50 are very good for most practical purposes.

Lens coatings may also make a small difference to the image brightness.

In summary: a 10x50 ( exit pupil 5mm) will have the same brightness as an 8x40 (exit pupil 5mm) and brighter than a 10x25 (exit pupil 2.5mm). This figure should always be kept in mind when purchasing binoculars, especially if they are to be used in poor light or at night.

Close [-]

The field of view of a binocular can also be important. A narrow field of view makes viewing a fast moving subject that much harder. The FOV may be given either in simple degree terms or by the measured distance seen at a range of 1000 metres or yards eg 129m @ 1000m (1 degree is roughly equivalent to 17m @ 1000m).

FOV is affected by factors in the design of the binocular; the most important being the magnification- higher magnifications give smaller FOVs. This is another reason why we do not recommend large magnifications.

Close [-]

Prisms are usually either Bak7 or Bak4. Bak4 is generally a higher quality. The difference may be seen by holding the binocular at arms length. The exit pupil in a Bak4 should be perfectly circular; in a BAk7 there may be slight "shading" or cut off of the edges.

Close [-]

There is often confusion on about lens coatings. All Magnaloux Optics have coated lenses. You will see a blue or green tint on the front lenses of Magnaloux binoculars. Good lens coatings, whatever the colour, reduces reflections and therefore increases the brightness and clarity. Lens coatings can be of the following types:

• Coated optics- one or more lens has a single-layer coating
• Fully-coated optics- all air to glass lens surfaces have single-layer coatings
• Multi-coated optics- one or more lens has multi-layer coatings
• Fully multi-coated lenses- all air to glass lens surfaces has a multi-layer coating
• ED glass-Extra low dispersion glass for fantastic performance

Multi-layer coating gives a higher quality image than single -layer coating.

In addition, DCF (roof prism) binoculars sometimes have what is known as phase corrected coatings, this is to overcome the slight polarisation of light due to the optical makeup of the prisms.

Close [-]

What makes the difference between a models costing £100 to one costing say £200? Generally the more expensive binoculars employ higher quality materials and can therefore produce higher quality images. The images are usually sharper and brighter and the binocular will probably be more ruggedly constructed.

Close [-]

This may be important if you are likely to be working in the wet. A true waterproof binocular should be able to withstand submerging in water. Some models will also be nitrogen filled; this stops the internal lens elements fogging up when used in cold, damp conditions.

Close [-]

Most modern binoculars may be used with spectacles on or off. If you are using them with glasses off, there is an adjustment around the right eyepiece to set them for your eyes. If you use them with glasses on, with a model with soft eyecups (eg GL3 series) you simply roll them back or with a twist eyecups (eg LC2 series) you twist them back; either way you get the binoculars closer to your eye. Some binoculars are designed with "long eye relief" which make viewing with spectacles easier.

Close [-]

Larger Binoculars

Compact Binoculars

Close [-]

Spotting scopes are primarily designed for viewing stationary or slow moving objects at longer distances. They require to be mounted on a firm tripod (all Magnaloux scopes have a 1/4inch thread thus fitting on any standard camera type tripod). Usually spotting scopes have a zoom lens: a scope marked 20-60x80 will have a variable 20 to 60 x magnification with a 80mm objective lens.

These scopes can be used for digi-scoping where a compact digital camera or a DSLR can be attached to turn them into very high powered lenses. These camera adapters are available at small cost.

Close [-]