Foresight Optical - Binoculars, telescopes and magnifiers

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Binoculars, Astronomical Telescopes, Spotting Scopes,
Night Vision, Microscopes, Magnifiers, Tripods & Accessories

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Choosing A Binocular

BINOCULARS are perhaps the best of all of the observing instruments. They are simple to use, can be carried easily and are not too expensive. They must also be sturdy enough to survive years of heavy use.

The more powerful binoculars will magnify any unsteadiness when you are holding the instrument (shake), so choosing a smaller magnification may give you an apparently sharper image than a larger magnification.

They must be easy to hold steady. They must resolve delicate details and reveal subtle colors with accuracy. They must focus quickly and up close and work well in dim light. They must be sealed from dust and moisture. And they must show the whole picture even users wearing spectacles.

The whole range of Foresight Optical binoculars have
been chosen to offer the best of these qualities for
whatever image you are looking at and whenever.
We have something to suit you needs.

Types of Binoculars

There are two major types of binoculars. They look slightly different and are classed by the prism type that they use. They are the Porro Prism Binocular and the Roof Prism Binocular.

The Porro Prism Binocular is the most common type of binocular. They consist of two lenses in the front with prisms that bend the light into the eyepieces in the back. This type of binocular is said to be a better performer for astronomy.

The Roof Prism Binocular are generally more expensive and much more rare than the other type. They look quite simply like two telescopes mounted next to each other. In essence that is what they are with a prism system to correct the image.

Power and light

Binoculars have a formula, such as 7 x 35 or 10 x 42.

The first number (7 or 10) is the power, the number of times the image is enlarged. With hand held binoculars, there is a practical limit to power beyond which it is not useful. Depending on the individual, as the power increases, hand tremor begins to degrade the image. Binoculars over 10 power usually require tripod mounting.

The second number in the formula (35 or 42) is the diameter of the objective lens in millimeters. The bigger it is then the more light can enter, and the greater the resolution of the image.

Exit pupil

Dividing the diameter of the objective lens by the power of the binoculars gives a measure of how bright the image will be when looked at through the binoculars.

For example, a 7x35 binocular has an exit pupil of 35 divided by 7 equalling 5mm.

It's in low light of night, evening or morning that the larger exit pupil is an advantage. For astronomy, an exit pupil of 7mm is standard. For bird-watching a 6mm exit pupil is usually large enough for even the most demanding low-light condition. For daylight viewing, even smaller exit pupils may be more than enough.

At noon, the pupils of your eyes contract to 2 to 4mm, and at night they may open to 7mm. If the beam of light exiting the binoculars is wider than the pupil of the eye, the excess doesn't get in: the eye can't see it.

As we age, the eye loses its ability to adapt to the dark. While a young person's pupils might open to 7mm, at 50 years the pupils may open only to 5mm. Therefore, binoculars with large exit pupils may not help the older birder

Eye relief and spectacles

Eye relief is how far back from the eyepiece your eye can be and yet still see the whole field of view: important for spectacle wearers, because glasses hold the eyes back from the eyepieces.

If the distance to your eyes is greater than the binoculars' eye relief, you will see only the centre part of the image. Like sitting in the front seat of a cinema but looking at the screen through a small tube. Normal eye relief for binoculars ranges from 9 to 13mm. Most binoculars have eyecups which lets spectacle wearers get closer. If you wear glasses, you need binoculars whose eyepieces are specially designed with long eye relief, 14mm or more.

Binocular Eyecups

When using your binoculars, be sure you understand the correct use of the eyecups. All binoculars will allow you to adjust the eyecups. There may be a rubber eyecup that can be folded down or a newer type that either twists or slides up and down on the eyepiece of the binocular.

Spectacle wearers always have the binocular eyecup rolled back (placed in the down position). This allows the widest possible field of view through your binoculars.

Conversely, if you don't wear spectacles, you'll enjoy your binoculars more if you leave these eyecups fully extended (placed in the up position). This will allow the binocular eyecups to just comfortably touch your face and help block out lateral light.

Lens Coatings

Each time light enters or leaves a piece of glass, about 5 per cent is reflected back. Binoculars may have 16 air-to-glass surfaces, with light lost at every surface. In early binoculars, less than half the light got through to the eye. The rest bounced around inside the binoculars, making the image hazy and hard to see, like watching a film in a cinema with the lights on. It was found that coating the lens with magnesium fluoride would let more light through.

So today’s optical coatings stop reflection and allow more light to reach your eyes and improve the image:

The type of coatings are.

Coated (C):

This is the lowest level of coating available. This consists of usually one layer of coatings on the front lens element and the eyepiece. When viewed from an angle the lens will have a very faint bluish tint.

Fully Coated (FC):

Every glass to air surface has one layer of coatings. These are usually the standard in lower priced binoculars.

Multi Coated (MC):

This is the most common coating system for affordable binoculars. Every glass to air surface is at least coated, and some of them are coated with multiple layers (usually the front lens and eyepiece lens). When viewed at an angle these lenses show a significant blue or green tint.

Fully Multi Coated (FMC, SMC ):

The highest level of coating generally available. Basically every glass to air surface has multiple layers of coatings. When viewed at an angle these lenses show many different colored reflections.

It’s possible to give even better viewing, for example, by the use of ruby coloured anti-UV coatings, which help to cut through haze, as well as protecting your eyes against the harmful effects of ultra violet light.

Prism Types

The two major glass types used to make prisms are BAK-4 and BK-7. Forget about what they stand for all you have to know is that the BAK-4 prisms tend to be brighter and better for astronomical viewing.

Field of View

The widest dimension of circular viewing area that you can see is described as the field of view.

So a binocular with a description:

Field of View at 1000 yards: 52 metres (171feet)

So a building 52 metres wide at 1000yards (over half a mile) will fit in your view. Increase the magnification and you will get closer.

Field of view is measured either in terms of feet (metres) at 1000 yards or in terms of angular degrees. Each degree of field corresponds to 52.5 feet at 1000 yards and binocular fields of view will generally range from 5º (263 feet) to 11º (578 feet).

As a general rule, the field of view will decrease as the magnification increases so a 10 power binocular will usually have a smaller field of view than a 7 power.

Field of view, however, is mainly determined by optical design of the eyepiece and rarely a function of the size of the objective lens.

For observing at close quarters in deep woods, or for picking up fast moving objects, a wide field of view is desirable.

Twilight factor

Used to describe the performance of binoculars and spotting scopes in low light conditions. It is a mathematical formula: the square root of the sum of magnification times the aperture. It should be noted that the size of the exit pupil and the overall light transmission (sometimes defined in terms of its brightness index) of the instrument are also important criteria for using binoculars in low light use.

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IMPORTANT Please Read - Disclaimer: Even though every effort has been taken to ensure accuracy of all data, descriptions and specifications, we cannot guarantee that they are accurately correct. If you are unsure of any aspect of our products, please do not hesitate to CONTACT US and we will gladly assist you.

All of our merchandise carries a manufacturers’ guarantee against defective materials and workmanship. Foresight Optical will refund the cost of goods purchased where the client is dissatisfied, provided that the goods and packaging are returned in their original condition, undamaged within seven days. Carriage charges are not refundable.

Foresight Optical, 7 Sawmill Yard, Blair Atholl, Perthshire UK. PH18 5TL
Tel: +44 (0)1796 482119 Fax: +44 (0)1796 482111 Email:

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