How do binoculars work?

Humans have always harbored the desire to see further than is possible when using the naked eye. This was first made possible in the 1600's when a Dutch optician invented the telescope. Since then, improvements in lens and body design, and the parallel tube arrangement have contributed to make binoculars what they are today. The modern compact binoculars are generally small, yet with stereoscopic vision and depth of field far greater than before.

This guide aims to introduce you to the complex field of binoculars and help you make an informed decision when purchasing for your requirements.

Types of binoculars

Before describing the various configurations used in binocular design, a few terms should be introduced:

• Objective lens - this is the large lens at the far end of the binocular
• Ocular lens - the lens in the eyepiece
• Prism - a block of glass with triangular section having accurately polished edges, used to reflect light

Prisms are introduced into prismatic binoculars to enable the body to be physically reduced in length compared to that of a telescope. They also ensure that the image is correctly orientated.

Porro Prism

This consists of two right-angled prisms joined so as to reflect the light path 3 times, resulting in a corrected image. The conventional shaped binoculars use porro prisms


Picture 1: An example of porro binoculars

Dach (or Roof) Prism

This is technically more complicated than the Porro prism design, with prisms requiring very precise angles and polishing. The light path is reflected 4 times. This design is more compact, but as the light path is longer and the prisms more complicated, the glass and manufacturing need to be of higher quality than the Porro prism design for equal performance. Roof prism binoculars tend to have straight barrels as can be seen by the picture


Picture 2: An example of roof binoculars

What is magnification?

Picture 3: Differences in magnification

Magnification is the degree to which the object is enlarged. With 7x35 binoculars, the object will appear to be seven times closer than without the aid of binoculars. The number immediately following the "x" is the diameter in millimeters of the objective (front) lens. The larger the front lenses the more light that is allowed to enter binoculars. 10x50 binoculars have 10 times magnification with a 50mm objective lens.

What is the Objective Lens?

Picture 4: Measuring the objective lens

With a 7x42 The number immediately following the "x" is the diameter in millimeters of the objective (front) lens. The larger the front lenses the more light that is allowed to enter binoculars. 10x50 binoculars have 10 times magnification with a 50mm objective lens

How do you focus you binoculars?

Individual eye strengths vary and the desired focus is therefore different per person. Additionally some of the binoculars offered at binoculars-expert.co.uk have instant focus, meaning that these binoculars are automatically focused to infinity. However if you wish to focus your binoculars you have to carry out the following steps:

1. Adjust the interpupillary distance.
2. Set the diopter setting (normally on the right lens) to zero and view a distant object.
3. Keep both eyes open at all times.
4. Using a lens cover or your hand, cover the objective (front) lens of the side with the diopter setting ring.
5. Using the focus adjustment, focus on the distant object being viewed.
6. Cover the other objective lens and view the same object as above.
7. Using the diopter setting adjustment ring, focus on the same distant object.
8. Your binoculars should now be adjusted for your eyes.

What is the field of view?

Picture 5: Field of view at a 1000 meters

The field of view is the width, measured in degrees or meters, of the viewing area you would see at 1000 meters. Field of view is generally affected by the power of the binoculars compared to the objective lens size. A larger power (if the objective lens size stays constant) will lead to a reduced field of vision. A larger objective lens (if the power stays constant) will lead to a larger field of view.

What is the close focus?

The Close focus is the closest distance from the object that the viewer must be before they can focus the binoculars on the image.

What is exit pupil?

Picture 6: Measuring the exit pupil

The exit pupil refers to the size of the shaft of light transmitted to the eye. The larger the exit pupil, the brighter the image will appear. The exit pupil is an important indicator of the binoculars low light performance.

You can actually see the exit pupil by holding the eyepiece of the binoculars approximately 12 inches from your eye. It is the bright circle of light in the center of the eyepiece. Exit pupil is expressed in millimeters and is normally derived by dividing the power into the objective lens diameter.7x35 binoculars have an exit pupil of 5mm (35/7). 15-powered binoculars with a 60mm objective lens diameter have an exit pupil of 4mm.

What is relative brightness?

The relative brightness index is used to compare how well binoculars with different size exit pupils will perform under dark conditions. This index reminds us that as the size of the exit pupil increases, its area and ability to transmit light grow geometrically.

To find the relative brightness, square the exit pupil. Binoculars with an exit pupil of 5mm have a relative brightness of 25 (5x5=25). Because relative brightness does not consider factors such as optical quality or coatings, it should be used only as a rough guide.

What is eye relief?

Picture 7: Distance between the eye and the eyepiece

The distance between your eye and the eyepiece is called "eye relief." Extended eye relief is one of the three most critical performance factors, along with magnification and field of view, especially for those who wear eyeglasses in the field.

Which binoculars do I need?

The type of binoculars you need depends on the purpose of your observations. If you are going to observe planets with your binoculars you need different binoculars than if you are going to observe birds. We have made categories that will allow you to select the right kind of binoculars for your purpose.

Furthermore it also depends on the amount of money you are willing to spend. You will get better binoculars (higher quality) if you spend more money. With better binoculars we mean better lenses (better coatings and better glass) but also features such as full water proof (the binoculars are inserted with a gas), the design of the eyepieces and the size and type of the prisms will make binoculars more expensive.

You can contact our Customer Service to help you decide what the best binoculars are for your situation and purpose.