The size of the objective lens helps to determine the resolution of the image being viewed. More light is gathered with a larger lens so a brighter image is obtained. The type of glass and quality of the optics also determines how sharp the image will be. So generally speaking, the larger the objective lens, the more light is let in and the sharper the image.
Field of View and Eye Relief
The field of view gives the width of the image being viewed at a stated distance. This mainly depends on the design of the eyepieces. A higher magnification normally gives a smaller field of view. A wide field of view has a shorter eye relief, which can be more difficult for spectacle wearers. The eye has to be nearer the eyepiece lens in order to see everything in the field of view.
Linear or angular measurement is used to work out the field of view, depending on where the binoculars are manufactured. Linear field of view gives the width in feet of the area being viewed at 1000 yards. Angular field of view gives the width in degrees of arc, of the area being viewed at 1000 yards. As 1 degree is 52.5 feet at 1000 yards then 5 degrees is 52.5 x 5 which equals 262.5 feet at 1000 yards.
The optical quality of the glass will affect the clarity of the image, and this is especially important around the edge of the field of view.
The optical quality of a pair of binoculars depends on a number of factors:
- The accuracy of alignment of the optics.
- The type of glass used for prisms. BK7 is good but BAK4 is better due to being a higher density glass.
- The type of glass used for eyepiece lenses. This should be of extra low dispersion glass.
- Coated glass. Multi-coated optics is best as this reduces glare and light dispersion.
Under normal circumstances, light passes through a glass surface from the air, but over 5% of it is lost. There are many air to glass surfaces in binoculars, so the loss of light is considerable, and this seriously reduces clarity and brightness. A larger objective lens can compensate for this loss but adds to the weight of the binoculars.
Since the 1940’s, single layer anti reflection coatings of magnesium fluoride has helped to reduce this light loss. However we now have multi-layered coatings, especially on more expensive instruments, that reduces light loss down to 0.25% per glass surface.
There are two advantages with anti reflection coatings:
- The image is brighter due to the increase in light transmission.
- The definition and contrast is improved because of a reduction of stray light reflected from glass surfaces.
With top quality roof prism binoculars, anti-phase shifting coatings are now used on some of the surfaces. These correct the path of light rays through roof prisms, and considerably help to give higher contrast images.
There are two main types of prism design used in binoculars. The porro and roof prism. The porro prism has been used since binoculars were first manufactured, but it was only in the 1960’s that the roof prism was designed. There is little difference in performance, but the roof prism is more compact and copes better with rough usage. They are more complex with higher optical precision than the porro prism, but are more slim line and comfortable to hold. Because of these reasons, roof prism binoculars are more expensive to buy. However, lower and middle priced porro prism binoculars often give a better performance, while roof prisms are best at the top end of the market. This is particularly so with bird watching.
The porro prism design has two pairs of prisms in order to turn the image the right way up for viewing. The roof prism design is quite different to the porro, but does the same thing.
Centre focusing is the most common means of adjusting the image until it is sharp. By the turn of a wheel, it allows the user to quickly ‘home in’ on the object being viewed, and this is vital when watching a rapidly moving subject such as a bird. Some older binoculars and specialist models are focused by adjusting each side separately. These are more commonly used for watching slow moving objects, and are particularly good for astronomy. More accuracy can be obtained by individual focusing and where speed is not too important.
A non-focusing system or fixed focused binoculars, do not allow for variations in eye strength. However, for certain usage they are useful and preferred, although viewed detail is often poor. Horse racing enthusiasts do not have time to make focusing adjustments, and may only need to see the horse that they have placed a bet on. Near focusing with this type of binocular is often a minimum of ten metres, while a centre focusing model can go down to a distance of four metres or less.
Near focus means how close the binoculars will focus and still maintain a sharp image. Three metres is very good while four to five metres is average with modern instruments. There are however, close focus binoculars that focus down to just 50 cms. That’s 20 inches, so these are excellent for observing insects such as butterflies and bees.
The dioptre is an integral part of the focusing system, and almost all centre focusing binoculars have it. This is the knurled adjusting ring located on the right hand side, usually behind or around the eyepiece. On some newer roof prism models, the dioptre is situated directly behind the centre focusing wheel for easier adjustment with the fingers. On some more specialist models the dioptre is situated on the left hand side, and this is usually the case with zoom binoculars, where the zoom adjustment lever and mechanism is on the right side.
The purpose of the dioptre is to fine tune the focusing of one side to the other. So with the standard centre focusing system, the focus wheel is rotated to obtain a sharp image through the left side, and the dioptre is then turned to focus the right side to the left. The end result should be a sharp image through both sides.
Collimation (alignment) of binoculars is achieved when the light path through the optics in both barrels is parallel.
There are various ways of detecting errors in collimation, but the simplest way is to look carefully at an object through the binoculars, first with your left eye only and then with the right eye only. Repeat this a few times, and if the collimation is out, a double image will be seen through the eyepieces. The cause of this is usually a prism that has moved out of alignment to the other prisms. The binoculars will almost certainly have been knocked or dropped for this to happen. Eye strain or even headaches can be caused by binoculars that are only slightly out of collimation. A more accurate test can be made by looking at a distant object, and this is especially important with more powerful instruments.
When the image in the right hand eyepiece is slightly to the left of the image in the left hand eyepiece, the image with both eyes usually comes together and is acceptable and eye strain free.
When the image in the right hand eyepiece is slightly to the right of the image in the left hand eyepiece, this horizontal divergence is almost always not acceptable.
If the two images are on a different horizontal plane i.e. one higher and one lower, this will be totally unacceptable and adjustments will need to be made.
To correct these errors, re-collimation of both sides should be done by adjustment of the objective lenses if possible. But many binocular models do not have this facility, so instead, the prisms or eyepieces will be used to correct collimation errors.
The best modern binoculars are labelled waterproof. They are filled with nitrogen and sealed. This prevents moisture entering the instrument and causing misting and fogging. Some models are described as being water resistant or shower proof, but these are not totally sealed against water. The rubber armour helps to keep water off the binoculars but does not help to keep moisture out.