I greatly prefer 10 power binoculars, but many other butterfliers find it easier to use 8 or even 7 power binoculars. Brightness Binoculars vary widely as to how much light is transmitted to your eye, the brightness of the binoculars. The brighter the image, the easier it is to distinguish color and detail. While the brightness of binoculars is a meaningful factor to consider for butterflying, it is not nearly as critical as it is for birding. This is because it is very likely that you will be butterflying under conditions of bright light.
Several elements affect the brightness of a binoculars.
The most important is the diameter of the objective lens which determines how much light is let into the binoculars. Thus the objective lenses of 7x42 binoculars are 42 mm in diameter. The larger the diameter of the lens, the more light will be admitted. Also affecting brightness is how well he light is transmitted once it reaches the binoculars. Optically more efficient binoculars have their lenses coated to allow greater transmittance and less reflectance.