If it weren't for the prisms in binoculars, you would see an upside-down, mirror-reflection of the world through your binoculars. Prisms sort out the image so it reaches the eye in proper orientation without such surprises. There are several types of prisms that accomplish this:
- Roof prism - straight profile, eyepieces in line with objective lenses, fairly new design.
- Porro prism - classic design, eyepieces set closer together than the objective lenses.
- Reverse porro prism - eyepieces set wider than the objective lenses, common in compact binoculars.
Some trade-offs to consider when selecting prism type are:
- Porro prisms lose little light or image quality.
- Porro prisms yield better performance and image quality when compared to roof prisms of the same price (it simply costs less to produce a Porro prism).
- Roof prisms enable a binocular design that is easier to hold and use. Don't underestimate the comfort factor.
- Roof prisms are more complex and alignment is more critical than in Porro prisms. This requires roof prism binoculars to be very well made to reduce the chances of misalignment of the prisms.
- Roof prisms loose more light than Porro prisms and require special coatings to compensate (this in turn inflates the price tag).
- Roof prisms split the image and when it rejoins it is slightly out of phase, leading to decreased image quality. To compensate, roof prisms must have phase correction technology that eliminates this problem (again, this inflates the price tag).
- Reverse Porro prisms are usually only used for compact binoculars.
Binoculars are described by two numbers, for example 7x35, 8x40, or 10x50. The first number is the magnification (or power) and the second is the diameter of the objective lens (in millimeters). Binoculars used most commonly for bird and wildlife watching are 7x, 8x, or 10x.
Some trade-offs to consider when selecting magnification are:
- As magnification increases, field of vision decreases and it is more difficult to locate a subject through the binoculars.
- As magnification increases, image detail increases so field markings are easier to see.
- As magnification increases, the effects of a shaky grip on the binoculars are exagerated.
- As magnification increases, the amount of light transmitted to the eye decreases so you may not be able to identify subjects as the sun drops below the horizon.
- As magnification increases, the minimum close-focus distance increases. This means if you intend to watch butterflies at close range, you may want to consider a lower magnification.
Field of Vision
Field of vision describes the width of the viewing area visible through your binoculars. Field of vision is influenced by numerous variables in binocular design but magnification and objective lens diameter are the most important factors.
Higher magnifications reduce the field of vision for equal objective lens diameters. Larger objective lenses yield larger fields of view for equal magnifications. The field of vision is normally listed with the binocular specifications as the number of degrees or feet you would see in the viewing area at 1000 yards.
Some trade-offs that affect field of vision are:
- A larger diameter objective lens will give you a larger field of vision, all other factors being equal.
- A larger diameter objective lens will result in a heavier set of binoculars.
- A larger diameter objective lens results in a brighter image.
Minimum Close-Focus Distance
The minimum close-focus distance is a measure of the minimum distance you must be from an object before you can bring that object into focus through the binoculars. If you plan to watch butterflies, it is often useful to be able to focus on objects that are quite close, for example.
Some trade-offs that affect minimum close-focus distance are:
- A closer minimum close-focus distance usually means forfieting magnification.
- A closer minimum close-focus distance often means an reduction in overall image clarity.
Eye relief refers to the optimal distance between your eye and the ocular lens of the binocular. Eye cups are designed to hold the binoculars at the correct distance from your eyes but this only works if you don't have to wear glasses. Without glasses, you don't have to worry about eye relief since the eye cups are designed to give you the proper fit. But if you wear glasses, eye relief becomes an important factor to check before buying binoculars.
If you wear glasses, then you will need to turn down the eye cups on your binoculars. The distance your eye glasses sit from your eye must closely approximate the eye relief distance of the binocular design to offer you a good image.
Some trade-offs to consider about eye relief distance are:
- If the eye relief distance your binoculars are designed to accomodate is less than what your glasses will allow, you are more likely to experience tunnel vision when looking through the binoculars.
- If the eye relief distance your binoculars are designed to accomodate is greater than what your glasses will allow, you may end up holding the binoculars away from your glasses which will be uncomfortable and potentially unsteady.
The exit pupil is the shaft of light transmitted through the binoculars to the eye. The exit pupil is an indicator of how well the binoculars will perform in low light conditions. Larger exit pupils transmit more light and thus a brighter image.
Exit pupils also reveal the quality of the image. If you hold your binoculars a few feet from your eye and view the exit pupil, it will have sharp edges if the binos have high image quality. Finally, exit pupils provide you with a method of checking the alignment of a pair of binoculars. If you positioning the binoculars on a flat surface and point them at a horizontal line (such as a window ledge or shelf), you can then check that the line is even between the exit pupils.