Therefore I have put this guide together to give you a clear understanding of what the differences are, what the markings and classifications mean, and hopefully a better idea of what type of binocular you might need.
Type of Prism?
In order to create an upright image, all binoculars have glass prisms within each of the barrels and the prisms flip the image in the same way a Digital Single Lens Reflex (DSLR) viewfinder does. The quality of the prism and its coating greatly affects the clarity, sharpness and overall quality of the image.
Higher quality prisms are made from a high density glass called BAK4, with cheaper but lower quality prisms are made from a glass called BK7. If the manufacture does not state the prisms are made from BAK4 then it is very likely to be made from BK7, but as a rule of thumb for best quality always look for BAK4 prisms or for low cost look for BK7.
There are two types of prisms types used in binoculars; Roof and Porro prisms. You’ll find that Porro prism binoculars are less streamlined in their construction as opposed to Roof prisms. The advantages of Porro prisms is that they offer a greater three dimensional viewing that is provided by the offset objective lenses, as well as generally being less expensive. It is worth noting that due to being heavy in weight, the prisms are more prone to being knocked out of alignment if dropped.
Roof prism binoculars are more compact in size, are easier to seal against dust and water but due to tight manufacturing tolerances are subject to higher production costs.
It is also worth stating that in regards to image quality Roof and Porro prisms are very much on par with one another and the real difference is in size, weight and cost.
Definition of the numbers, what do all the markings mean?
All binoculars are classified with a few different numbers and markings, such as 7x35, 8x42 etc. These numbers provide the measurement of the magnification and objective lens of the binoculars and are key to establishing which type of binocular you need.
The magnification number is the first number, if we take a 10x30 binocular for instance this magnification number would be '10' this means that this binocular has a magnification of 10x that of the naked eye.
At first glance when choosing binoculars, you might think that the higher the magnification, the better the binoculars. However this is not always the best option for every user as to increase the magnification you also increase the weight of the binoculars and the effort required to keep the binoculars steady for a clear image.
|Image Stabilization off|
Some binoculars (such as the range from Canon) now have image stabilisation built in that reduces the movement and handshake to provide a more smooth and steady viewing, this is achieved by employing a more sophisticated prisms that can automatically adjust to counter the effect of slight movements. Other larger magnification binoculars come with tripod threads on the base to
|Image Stabilization on|
There are a few binoculars that utilise a zoom magnification opposed to the standard fixed magnification; these binoculars allow you to change the magnification by using a level near the eyepiece. Zoom binoculars are generally more expensive than fixed binoculars and usually offer a lower image quality with a reduced field of view but the bonus is that they are more flexible.
The number after the 'x' is the diameter of the objective lens which is the lens on the opposite side of the eyepiece; this is a measurement given in millimetres. The size of the objective is a key characteristic of how the binoculars will perform in regards to their brightness. The larger the diameter is then the brighter the image will appear, therefore a 8x40 binocular is brighter than a 8x25 binocular but it is worth noting that the 8x40 will be a much heavier and larger pair of binoculars.
It is also worth noting that the objective and magnification are in proportion to one another, therefore a 7x35 and 10x50 pair of binoculars have the same brightness, this is worked out on the exit pupil diameter which is the Objective/Magnification=Exit Pupil. Therefore for both of the binoculars mentioned before this would be 5. This is sometimes referred to as the RBI which is the relative brightness index and is simply the square root of the exit pupil diameter, therefore a binoculars with an exit pupil of 5 will have RBI of 25.
Other Binocular Features
What other elements do I need to consider when buying binoculars?
Binocular eyepieces are an important consideration when choosing your binoculars. The eyepieces are constructed from three to six lens elements dependant on design and magnify the image after it has passed through the prisms. Wide-angle binoculars have wide field eyepieces that shorten eye relief - this is the distance from the eyepiece to the eye. The smaller the distance, the more difficult it becomes for eye glass wears to see the entire field of vision. Most binoculars are also supplied with rubber eye cups. The eye cups block out unwanted peripheral light and can normally be folded flat along the barrel of the binoculars.
Almost all binoculars are water resistant. However, water can enter the optical system if you are planning to use them in heavy rain or if dropped into water. Some binoculars are built with waterproof O-ring seals to keep water out. The truly waterproof binoculars are filled with nitrogen to keep out air and moisture and this also prevents internal fogging. It is highly recommend choosing a pair of binoculars that are fully waterproofed for any activity that is near water or for use in all weather situations such as boating or hunting.
The majority of binoculars have two adjustments for the focus, one type is to use a centre focusing knob which moves the eyepiece, this eyepiece movement can be either internally or externally, this focuses the binoculars over the range of the object distance.
The second focus adjustment is for the correction of your own eyes. This is done by the dioptre adjustment where there is usually an option to adjust each eye for variations therefore create a balance to allow a corrected vision for both eyes.
You can see our range of binoculars in either our stores in West Sussex or Central London, or online at www.parkcameras.com.