Brightness, in regards to binoculars and monoculars, is basically a measure of their ability to gather and transmit light to form the image.
Most modern binoculars and monoculars will perform well in daylight and even twilight, so unless you're going to use them at dusk or almost darkness, it is unlikely that you should be too concerned about brightness when choosing binoculars.
Light Gathering Capability
There are many factors that determine the brightness of the image. The size of the objective lens determines the amount of light gathered by the binoculars and so plays a significant part.
The size of the objective lens (front lens) is the second number in the binocular designation. An 8x50 binocular will have a front lens with a 50mm diameter.
So, an 8x50 will be brighter than an 8x40, even though they both magnify eight times.
Magnification & Brightness
Magnification also plays a significant part in determining the brightness of the image.
An 8x40 will be brighter than a 10x40, even though they have the same size front lens and therefore the same light gathering ability.
A little extra magnification can mean the loss of a lot of brightness.
It is quite complicated to work out the difference magnification makes, but as a rough guide: -
- A 10x50 can be four times brighter than a 20x50
- A 7x50 can be eight times brighter than a 20x50
Other factors that may affect the brightness are: -
- Quality of the lens glass
- Quality of the prism glass
- Type of lens coating
For normal daylight use, and even in twilight, most binoculars will be fine.
In bright light your eyes automatically adjust to limit the amount of light, so it is unlikely you'll actually be able to see the difference.