In this modern era people use various optical sighting devices like the spotting scopes and few others for better sighting of the object they wish to target. Two major considerations come in the way. These are light transmission by your device and relief that your eye requires.

Commonly accepted jargons like “light gathering abilities” of the scopes does not carry any meaning. Scopes do never gather light but only transmit them through the lenses attached to them and in the process always loose a part of the light transmitted. Even the best of the scopes like the rifle scopes can ensure only 98% transmission and ordinarily most of the scopes will have transmission percentage around 90% of the light received only.

Remember that enhancement in magnification always causes loss of light in the eyepiece. However, this shortfall can be compensated with the larger objective lens in use. The age factor will also come into play in such cases. While younger people will have dilating power of eyes in the range of seven millimeters, the older can only hope for four millimeters or a slight more.

Determination of extent of light coming to your eyes through the eyepiece of the scopes or rangefinder is made using the calculations about the exit pupil. The circle of the light seen in the ocular lens is the exit pupil and its diameter indicates the size of the exit pupil. Similarly, the distance from the ocular lens that your eye requires to obtain a full and sharp image of the object is called the eye relief. Both eye relief and extent of light are important for sharp and accurate image of the object.