What phenomenon is described as always present with a constant size?

The phenomenon that is always present with a constant size is halos. Halos are visual effects that appear around light sources and are particularly noticeable when using night vision equipment in low-light conditions. They occur due to diffraction of light, where light rays bend around edges or through small openings. This effect remains fairly consistent in size regardless of the brightness of the light source, making it a distinctive feature when using night vision goggles.

In contrast, patterns, glare, and ghosting can vary in their appearance and perception based on external conditions, such as the intensity of light or the specific lens characteristics of the devices being used. While these phenomena can affect visibility and clarity in night vision, they do not maintain a uniform size like halos do.