Back To Knowledge Bank Index

The primary role of a binocular is to permit the user to view a scene, or an object in greater detail and supply us with more information than we can see with the naked eye alone. It's as simple as that.

That intuitively means that the object we want to see, ideally needs to be represented in the image in a way that is detailed, stands out in contrast with its surroundings, and bright.

What aspects of a binocular permit this goal?


Image magnification can be used in two ways, which appear at first glance to be contradictory in nature. Higher magnification and lower magnification. We intuitively feel that we need as much magnification as we can get in order to view the detail in distant objects. Greater magnification seems intuitively to be a goal in itself, as we have all seen those images on TV of portrayed binocular views with hugely magnified images. It seems very simple to achieve highly detailed highly magnified images if we believe what we see in the movies. However, remember our initial goal of image detail. In the real world, magnification should be thought of as a single tool to help provide us with this goal, and that means that magnification should used in context with other factors . Hollywood magnification is relevant only in Hollywood.

As much magnification as we can get is not the answer. There must always be a balance between magnification and image brightness. As much magnification as we can get providing we have the necessary image brightness would seem a better answer.  Lets imagine we have a pair of 40mm binoculars (binoculars with 40mm objective lenses). During the day, when it is bright outside, we can use magnifications of 10X or 12X. In the evening or at dawn when the light is low, we need as much image brightness as the binocular can deliver, so do we use a lower magnification binocular such as 7X or 8X even though the image scale is smaller, or is there something else that dictates what is best?

We could also use higher magnification to see our object in greater detail, simply because higher magnification can reveal more resolution detail that could not be detected at low magnification. Of course we need a bright enough image in order to do this, and we have to take the stability of the image into consideration. Any movement of the binocular due to hand shake will blur image detail. This is less pronounced with lower magnifications. However, it is still there. Is there really more detail visible at 7X with the same amount of image shake as there is at 10X? The image shake is less but the detail is smaller. Have we actually got anywhere? At the level of magnifications we usually opt for with hand-held binoculars (7X to 10X), unless we are using them from a boat on choppy water, or from a moving vehicle, we may as well choose the higher magnification. We can always employ a tripod for higher magnifications if we require a still image, but carrying a tripod round for a small or standard size binocular is rarely convenient. 
So is less magnification ever useful? Yes. When we require a wider field of view, or if we want to follow a moving object easily. We want image magnification but want to see over a wider area or not lose our bird or aircraft out of our field of view. It must be understood though that the use of low magnification here is not working in the interest of our initial goal of only seeing more detail, but for a joint goal.     
A balance between objective lens size and magnification is more important, particularly for the use of a binocular in different environments. For astronomy and for low light use during the day, a larger aperture is required to collect more light. This is largely because we want to collect more light so that we can use more magnification.  With larger objectives, we can use higher magnification, whether we wish to detect low contrast objects in poorly lit surroundings (e.g. wildlife in its natural environment during dusk or dawn), or whether we have enough illumination (e.g. the moon) but require more magnification to detect greater detail.

During bright daylight, we can use the higher magnification/larger objective combination binoculars such as 10X50 or 12X50, but also we can get away with using the same magnifications but with smaller objectives (40mm or even 30mm) providing the light level is high enough. 
The determining factor is not the binocular at all, but the eye, or rather the brain controlling the pupil of the eye. The brain controls the diameter of the pupil by expanding or constricting the iris, because it requires the eye to provide image information from which it can maximise detail and contrast, by adjusting brightness levels. At night, with no moon, our pupils can reach 7mm or slightly more. In bright sunlight, our pupils can reduce to 2.5mm or slightly less. It makes sense then, that it may be beneficial for our binocular to deliver image information that closely matches what our brain asks our eyes to deliver.

If you divide the objective lens diameter of a binocular (in mm) by the magnification, (e.g. in the case of an 8X30 binocular, we are dividing 30 by 8), the figure reached will be the diameter (also in mm) of the pencil of light exiting the binocular eyepiece. This is the Exit Pupil, the small bright disc of light in the eyepieces we see when we look atthe eyepieces rather than through them. The exit pupil diameter (and thus its area) determines how much light is presented to the eye. Increasing magnification decreases the exit pupil diameter. If the exit pupil is larger than the pupil of the eye, then some of the light from the binocular does not enter the eye. If magnification is increased (but keeping the objective diameter the same), the diameter of the exit pupil can be reduced until all of the exit pupil light enters the eye. The observed brightness of the image will not appear to decrease whilst we are increasing the magnification, until the exit pupil becomes smaller than the pupil of the eye.

Similarly we can increase exit pupil size by decreasing magnification. This would be useful if the eye’s pupil is opened wide and we want to maximise the combination of the light gathering power of the objective lenses with our maximum pupil diameter. From this we can see that perhaps the most important aspect of successfully using the optics in a binocular is the eye and the brain...................which are not part of the binocular. 
All this may appear a touch complicated, after all we want to do is to pick up a pair of binoculars and use them. However, knowledge of the basic operation and workings of a binocular permits a much more informed choice when purchasing a pair. You can, if you like, tailor the binocular choice to your needs.

So, in terms of light grasp a 7X50 binocular has some wasted aperture in uses other than darkness, such as astronomy because of our eye's pupil size in daylight. A 10X25 compact binocular is fine for a bright sunny day, but not really suitable once the daylight is low because of our eye's pupil size in low light.

Most binocular users choose either 8X30, 10X30, 8X40, 10X40, 10X50 or 12X50 pairs (or models of similar specifications). As the first figure in each case is the magnification, and the second figure is the aperture (objective diameter), divide one into the other and you can see the average binocular chosen for daytime use (dawn to dusk) has an exit pupil diameter of between around 3mm to around 5mm. This is ideal for most binocular uses. Only for astronomical use, are binoculars with large exit pupils (e.g. 11X80 giving 7.3mm exit pupil) worth considering. Most of us want to carry around a lightweight manageable pair.


Brightness of the image.

Image brightness is an important ingredient in our quest for greater image detail. There are several subtle and more direct contributions of the optics in binoculars (and telescopes) that determine the brightness of an image. In our quest for image detail, we need, in this case, to consider only the two basic contributors; objective lens diameter and magnification. We have seen how magnification can reduce image brightness, particularly if the exit pupil is much smaller than the pupil of the eye.  Larger objective lenses provide more light into the image because of the area of the lens.  Objective lenses of 50mm diameter provide considerably more light to the image than 30mm objective lenses. A 50mm lens has a diameter some 60% greater than that of a 30mm lens, but an area of over 170% greater. 
One aspect of binocular design that can reduce image brightness is poor light transmission through the optical train, due to absent or poor optical coatings on lens and prism surfaces. Modern multi-coatings applied to optical surfaces mean that today, this is less of a factor than it was a few decades ago.



In simple terms, in the context of our typical binocular, contrast is a word we use to describe the clarity of various adjacent areas in an image with respect to each other. The discreteness of areas within the field and whether areas are detectable as separate from each other. Rather like looking out at a scene through a slightly dirty window, and then opening the window to look at the same scene. The dirt and dust on the window glass serves to reduce contrast and hence the clarity of the whole view, as well as the contrast of one particular area of the view with respect to another area.  In a binocular, there are a few aspects of the optics, that we can point to as affecting contrast in the final image. One is poor transmission of light through the binoculars (lens and prism coating quality affect this aspect by a surprising amount), another is light scatter, reflections from internal surfaces within the binocular causing something termed Veiling glare, and yet another is the manufactured quality of the glass surfaces of all the lenses and prisms.  Fortunately, we don’t need any figures this time to tell us what we want to know when it comes to contrast. We simply need to pick up a pair and look through them. A clear high contrast image is recognizable by anyone. Our eyes all work in the same way.

Image detail and sharpness.

We talk about image sharpness and detail (or resolution) when we refer to how much fine detail we can see in an object through our binocular when the image is in perfect focus. Theoretically, resolution is increased when the aperture of the optical instrument is increased, and we realise that resolution with higher magnification. In reality though, with small hand held binoculars, we don’t see the resolution a binocular can deliver unless we use a higher magnification, and binoculars generally don’t have changeable eyepieces that will allow this. They are always fixed magnification eyepieces. There are always zoom binoculars, but zoom hand-held binoculars have optical issues that prevent good resolution. The same errors or conditions that can affect image contrast, also affect image sharpness and the visibility of fine detail. Another is ensuring that focusing is accurate. Another is factory assembly errors, such as poorly centred or mis-aligned lenses or prisms in either half or barrel of the binocular. One common cause of reduced resolution is when there is poor alignment of one barrel or telescope (remember that a binocular is two telescopes joined at the hip), with the other. Binoculars should be aligned so that when focused to infinity (such as focusing on a star), both barrels point exactly at the same object. If they don’t, then we lose a little resolution because the brain is trying to make sense of two images in slightly different positions laterally, or radially, in the field of view. The brain attempts to merge the images together, just like it does with our stereo information supplied by our eyes, but with binocular images we can feel the strain on our eye muscles because the brain has to ask our eyes to look at different directions, like going cross-eyed. A binocular can have correctly collimated (aligned) optics within each half or barrel of the binocular, but still have poor alignment between the two barrels. The former will cause loss of resolution due to optical aberrations, (image degrading optical errors), the latter due to problems merging the images. The instrument may not have lost resolution, but our brain has trouble seeing it.   
Three dimensional images. A binocular is simply two telescopes joined at the hip, a telescope for each eye. Our two eyes provide our brain with images that give us information about distance and space between objects. A binocular does the same, in some cases even more so, as many binoculars have objective (main) lenses that are further apart than our eyes. This spatial awareness within the image adds to us being able to discriminate between objects in the image, as separation of objects in the image field, whether by brightness or colour differences, two dimensional spatial separation or three dimensional spatial separation is discrimination between the objects in the image. There is also the added benefit of greater awareness of object discrimination and detail in a binocular image over that produced by a monocular or telescope, as the brain has more information with which to compose an image. Two telescopes are better than one just as two eyes are better than one.


All of the above is interesting and useful to know, because it can help with initial binocular choice. The best way to choose however, is to visit us here in Northallerton and simply try a few pairs.

Your eyes and brains have the remarkable ability to differentiate miniscule alterations in image quality between different binoculars, and choosing based on comparative testing is fun. We can advise on models best suited to your needs and budget, which binoculars are ideal for spectacle wearers and which are best suited to a particular use (general use, bird watching, astronomy etc.). Once you have established the models best suited for your needs, then simply compare a few pairs for their optical performance, choose the one that you consider to have the best image clarity, and you have your new binocular. Its as easy as that.

There are two basic types of binoculars: porro prisms and roof prisms. You can identify the prism construction of your binoculars at a glance. Porro prism binoculars have a right-angled bend between the objective lens and the eyepiece, which makes them form an M shape when standing on their objective lenses. Roof prism binoculars are typically straight with the objective lens in line with the eyepiece, so they take on an H shape when stood upright.

Porro Prism Binoculars


Porro prism binoculars (also known as “porros”) first appeared in the mid-1800s, the design of an Italian optician named Ignazio Porro, and feature two right-angled prisms in each binocular barrel. Today, porro prism binoculars are considered “traditional” binoculars, since they were the most common design until roof prism designs gained popularity in recent years.

Porros rely on an external focus mechanism, which causes the eyepieces to slide forward or backward along an external tube. This type of focusing allows for sharp images of close subjects, as well as precise focusing on close-proximity objects as near as 6 to 10 feet.

For low- to mid-range priced binoculars, porros offer the best value.

The advantages of porro prism binoculars are:

  • Brighter images due to greater transmission of light
  • Fast focusing
  • Close focusing
  • Wider field of view

The disadvantages of porro prism binoculars are:

  • Weight (due to the large prisms)
  • Bulky design because of the angled prisms, making them harder to hold if you have smaller hands
  • Less durability, as the external focusing mechanism is more easily jarred out of alignment

Roof Prism Binoculars


Roof prism binoculars were also developed in the mid-1800s, but by a German manufacturer who oriented the light-directing prisms inside straight barrels. Because of this design, roof prism binoculars produce more reflections than porros, so special coatings are applied to enhance the final image’s brightness. These coatings also increase the binocular’s cost.

Most of the focusing hardware is located internally, with an external focusing knob or wheel. Recent design advances have enabled roof prism binoculars to focus as close as, if not closer than, their porro counterparts.

Roof prism binoculars have grown in popularity in the past few decades, primarily because many leading optics manufacturers are producing high-quality roof prism optics. At the mid- to high-price range, roof prism binoculars dominate the optics market.

The advantages of roof prism binoculars are:

  • Ease of handling
  • Close focusing in advanced models
  • Increased durability due to fewer external moving parts
  • Better power-to-weight ratio (a pair of 10x roof prism binoculars weighs less than a pair of 10x porros)

The disadvantage of roof prism binoculars is:

More expensive due to the need for special prism coatings to increase image brightness