I'll be the first to admit I may be chasing a unicorn. My night sailing experience is very limited. All I know is, the lights I have used thus far always blind me when I use them, due to fog/haze/whatever, reflecting stray light back at me. I have to send the Admiral to the bow with the flashlight in order for me to see anything at all, due to the light being reflected back. She in turn is blinded, and I have to call out where to point the light. Not an ideal situation, even under the most benign of conditions.
Is not a narrow beam better than outright blindness?
Perhaps in low humidity/inland lake/extremely clear conditions, this would not be the case, and I've just been unlucky thus far?
And,
@Larry -- DH ... how the
deuce are you seeing anything at all through binoculars in total darkness?? You must eat a lot of carrots, or spend more on your binocs than my boat is worth!
Much to be learned here. I'm listening. Thanks, everyone.
Gene -
I have done a bit of night sailing. I was very specific when I wrote 7x50 binoculars. The lower magnification of 7x gives a wider field of view, and the larger objective lens diameter of 50mm (roughly 2 inches) has tremendous light gathering power, about 10 times compared with the naked eye. I admit that 7x50 binoculars won't help much on a cloudy night with a new moon, but they do help. Especially if you spend more you get better glass and coatings, but even moderate priced name-brand binos (around $150+) can be very effective.
I searched for a write up on this...
http://www.ebay.com/gds/THINGS-YOU-SHOULD-KNOW-BEFORE-YOU-BUY-BINOCULARS-/10000000007973565/g.html
Some info from that page:
D. LENS DIAMETER
The second number in the formula (i.e. 7X50) indicates the front lens diameter in mm
The larger the objective lens the more light enters the binocular and the brighter the image.
On a 6 – 8 magnification 20-32 mm is normal
On a 7 – 9 magnification 35 – 50mm is normal
On a 10 – 36 magnification 50 – 70mm is normal
Doubling the size of objective diameters quadruples the light gathering ability
But in reality the size of the lens must be considered along with the exit pupil and intended usage.
E. LIGHT TRANSMISSION
This refers to the quality of the optics or its ability to deliver a maximum of light
The greater the objective diameter the more light enters, but the heavier and bigger the binocular are.
F. BRIGHTNESS
The brightness of an image depends upon several factors:
Magnification applied to the image at the eyepiece;
Intensity of the light coming from the object being viewed
Diameter size of the objective as a light gathering lens.
The type and quality of glass used for the objective lens.
Transmission and reflection loss of light in passing through the instrument.
RELATIVE BRIGHTNESS INDEX (RBI)
Relative brightness index or RBI is used as a comparison of image brightness but it does have severe limitations as discussed below. It is determined by square root of the exit pupil. An RBI of 25 or more is considered useful in low light conditions. In bright light conditions an RBI of 9 would have no brightness advantage over one of 50.
RBI = Diameter / Power = exit pupil x square root.
Example : 7x50 binocular 50/7 = 7.14 x 7.14 = 50.98 = relative brightness.
TWILIGHT FACTOR
Twilight factor is a measurement of viewing sharpness and image detail in low light conditions. The larger the twilight factor, the more efficient the low light performance. This indication has some limitations. To calculate the twilight factor, take the square root of the power multiplied by the objective lens diameter.
Example on a 7X42 binocular.
TWILIGHT FACTOR 7x42 = 294 (square root) = 17.14 = twilight factor
This formula takes magnifications more into account than the relative brightness index and is a better indication for the quality of brightness. When observing in greater detail a distant object you will seek to obtain a higher contrast and therefore will have to compromise between the magnification and the objective lens diameter.
G. FIELD OF VIEW
This is the side to side measurement of the circular viewing field.
It is defined by the width in feet or meters of the area visible at 1000 yards or 1000 meters. The higher the magnification, the narrower the field of view.
A wide field of view is recommended to follow action ( i.e. sport)
Binoculars with a wide field of view usually have a magnification between 2 and 6. Some binoculars offer wide angle eyepieces which increase the field of view compared to the normal eyepieces.
This explains it better than I could.