All you need to know about bird binoculars
Exit pupil
The exit pupil, measured in millimeters, refers to the diameter of the shaft of light that exits each eyepiece and reaches the pupils of your eyes. Calculate this number by dividing the diameter of the objective lenses by the magnification. A 7 x 35 model, for example, has an exit pupil of 5 mm (35 divided by 7). In other words, a bigger exit pupil means more light reaches your eyes, producing a brighter image. A large exit pupil also makes it easier to maintain a full image if your hands move or shake. Keep in mind that your eyes' pupil size varies from approximately 2.5 mm in bright light to 7 mm in low light. For viewing at dusk or dawn, look for binoculars with an exit pupil of 4 mm or more.
Focusing system
Another important factor is the speed at which you can focus the binocular. The reason is that birds are fast moving and they quickly change the distance from us. Binoculars with good focusing mechanism can go from close focus to infinity in one full turn of the focus ring, but for focusing at close distance, this is a problem. So some manufacturers offer as option focusing with variable speed of rotation. When the ring rotates rapidly, the focus also changes rapidly. A fine tuning can be done at lower speed of rotation. However, that needs some practice. What also matters is ring position, it should be easy to reach. Convenient option is binocular with auto focus.
Waterproof
Regarding to waterproof, binocular is good to be at least water resistant, that would keep it from light rain. But it is obligatory to be fogproof. This prevents from internal fogging, caused by high humidity, fog and temperature extremes.
Weight
Weight does matter and it is not something that you can easily ignore. We have the binoculars with us all day long, perhaps we need to climb or walk on uneven terrain. So, it is better to think of well balanced binoculars, comfortable to carry around and of course a strap, which can be very useful both for your neck and for your binoculars safety.
The exit pupil, measured in millimeters, refers to the diameter of the shaft of light that exits each eyepiece and reaches the pupils of your eyes. Calculate this number by dividing the diameter of the objective lenses by the magnification. A 7 x 35 model, for example, has an exit pupil of 5 mm (35 divided by 7). In other words, a bigger exit pupil means more light reaches your eyes, producing a brighter image. A large exit pupil also makes it easier to maintain a full image if your hands move or shake. Keep in mind that your eyes' pupil size varies from approximately 2.5 mm in bright light to 7 mm in low light. For viewing at dusk or dawn, look for binoculars with an exit pupil of 4 mm or more.
Focusing system
Another important factor is the speed at which you can focus the binocular. The reason is that birds are fast moving and they quickly change the distance from us. Binoculars with good focusing mechanism can go from close focus to infinity in one full turn of the focus ring, but for focusing at close distance, this is a problem. So some manufacturers offer as option focusing with variable speed of rotation. When the ring rotates rapidly, the focus also changes rapidly. A fine tuning can be done at lower speed of rotation. However, that needs some practice. What also matters is ring position, it should be easy to reach. Convenient option is binocular with auto focus.
Waterproof
Regarding to waterproof, binocular is good to be at least water resistant, that would keep it from light rain. But it is obligatory to be fogproof. This prevents from internal fogging, caused by high humidity, fog and temperature extremes.
Weight
Weight does matter and it is not something that you can easily ignore. We have the binoculars with us all day long, perhaps we need to climb or walk on uneven terrain. So, it is better to think of well balanced binoculars, comfortable to carry around and of course a strap, which can be very useful both for your neck and for your binoculars safety.
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