multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors. Determined by the optical properties of the lenses used and intended primary use of the telescopic sight, different coatings are preferred, to optimize light transmission dictated by the human eye luminous efficiency function variance. Maximal light transmission around wavelengths of 555 nm (green) is important for obtaining optimal photopic vision using the eye cone cells for observation in well-lit conditions. Maximal light transmission around wavelengths of 498 nm (cyan) is important for obtaining optimal scotopic vision using the eye rod cells for observation in low light conditions. These allow high-quality 21st century telescopic sights to practically achieve measured over 90% light transmission values in low light conditions. Depending on the coating, the character of the image seen in the telescopic sight under normal daylight can either "warmer" or "colder" and appear either with higher or lower contrast. Subject to the application, the coating is also optimized for maximum color fidelity through the visible spectrum. A common application technique is physical vapor deposition of one or more superimposed very thin anti-reflective coating layer(s) which includes evaporative deposition, making it a complex production process. Tube size The main tube of telescopic sights vary in size, material, the applied production process and surface finish. The typical outside diameters vary between and , although , 30 mm and recently 34 mm are by far the most common sizes. The internal diameter of the main tube influences the amount of space within which the relay lens group and other optical elements can be mounted, the maximum size of the erector tube, and the maximum angular ranges for elevation and windage adjustments. Telescopic sights intended for long-range and/or low-light usage generally feature larger main tube diameters. Besides optical, spatial and attainable range of elevation and windage adjustments considerations, larger diameter main tubes offer the possibility to increase the tube walls thickness (hence a more robust sight) without sacrificing a lot of internal diameter. Adjustment controls A telescopic sight can have several manual adjustment controls in the form of control knobs or coaxial rings. Diopter adjustment (also called the ocular focus) on the eyepiece — meant to obtain a sharp picture of the target object and reticle. Elevation control – meant to adjust (or "track") the vertical deviation of the optical axis. Elevation tracking to intentionally compensate for bullet drop