What is a common issue experienced with NVG optics?

Fogging of the lenses is indeed a common issue experienced with night vision goggles (NVGs). This phenomenon occurs primarily due to temperature changes and humidity. When the NVGs are exposed to a cooler environment after being in a warmer area, condensation can form on the internal or external surfaces of the lenses. This fogging can obstruct the user's view and impact the effectiveness of the device, leading to potential safety hazards in low-light situations.

Other options like lens cracking, loss of color perception, and slight blurring are issues that may occur under certain conditions but are not as prevalent as fogging. For instance, lens cracking usually results from physical damage or impact rather than a common operational setback. Loss of color perception is inherent to NVGs because they typically provide a green image due to the amplification of light in a specific wavelength range while blurring can be caused by misalignment or external factors such as improper focusing. However, fogging is a direct result of environmental conditions and is frequently encountered by users.