The human eye adjusts to changing light conditions by the iris dilating and constricting. Without it, think of what it would be like – it would be either too bright or too dark depending upon how your iris was fixed.

Similarly, the camera needs to adapt to changing light conditions. One of these adaptations is performed by a function known as “auto-iris”. It works similar to the human eye allowing more light in when it gets dark, and reducing the intake of light when it's bright out. A manual iris serves no purpose in changing light conditions, as you are not going to climb up on a ladder and make changes to the lens throughout the day. A manual iris becomes applicable where you have fixed lighting conditions, such as in an interior office or hallway, which is not subject to ambient light.

Low Light

So, are all auto-iris the same? No. Some have better abilities to adapt to lower light conditions than others. Maybe you have noticed when you bought your digital movie camera; they talked about “lux”. A lower lux number camera is capable of functioning in lower light; some better than others.

So, how do I know if a camera is a low lux camera? In surveillance cameras, a low lux camera is one with a designation of 0.05 or less in color and 0.003 in black and white. When you see a referral to a camera being “ExView”, which is a Sony trademark, it identifies this as a low lux camera. What an ExView camera does when it gets dark is automatically switch from color to black and white to give the images better contrast and depth.

Infra-Red

Low light should not be confused with zero light. Outdoors, there is always some light. Indoors, you may have situations of zero or near zero light. A room with no windows and the lights turned off as an example. In these cases, a low lux camera won't produce any results and you would require an “infra-red” (IR) camera if you want to see anything. An infra-red camera has illuminators. These illuminators send out a beam, of sorts, creating visibility for the camera. The more illuminators you have, the further it reaches and the larger the area it can cover. Think of it as a flash light of sorts.

Therefore, if you have a very large dark area, the IR camera may not reach the farthest end of the area.