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How does lens F-stop rating (aperture) relate to low-light and night performance?

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The F-stop rating or aperture of a lens describes how bright the image generated by the lens is as compared to the brightness of the object itself. For example, if you are using a lens rated F/2.0, then the image will be 1 / (2.0^2) = 0.25 or 1/4th as bright as the object. Thus, a lower F-stop number means that the lens is more light sensitive.

For night and low light imaging, the F-stop rating of the lens is very important. A lower F-stop rating means that you can image an object in darker conditions.

The most light sensitive lenses commonly available are about F/1.4. The following table describes how much light you will need to operate with various lenses as compared to the light needed if you are using a F/1.4 lens.

F-stop	Sample lens	Light required compared to F/1.4 lens
F/1.4	Canon f=50mm lens, many C-Mount lenses	1x
F/2.7	FrogEye FL-200 f=200mm free-flooded/collapsible lens	3.6x
F/3.5	Canon f=28mm-105mm zoom at 28mm setting	6.1x
F/4.5	Canon f=28mm-105mm zoom lens at 105mm setting	10.1x
F/5.3	FrogEye FL-400 f=400mm free-flooded/collapsible lens	14x

Use this table for example to consider the situation of trying to monitor a darkened site with people, vehicles and building at night from a distance. Perhaps initially you are using the sensitive Canon f=50mm F/1.4 lens. You can see the buildings, but there just isn't enough detail to make out personnel and you can hardly see the vehicles. In an effort to get better detail, you switch to the FL-400 lens. You know it will require 14 times as much light as the Canon 50mm lens. So, you use a tripod, a long exposure time approaching one second and you probably enable the dark enhancement mechanisms of FrogEye including dark frame subtraction (DFS) and sensor cooling.

The potential detail you might get with the FL-400 lens is great due to its long focal length, but you find that it is really hard to make out any targets other than the bright areas of the scene. So, your next choice might be a switch to the FL-200 lens. Sure, its shorter focal length means less detail to begin with, but still much more than with the Canon 50mm lens. Yet, the FL-200 requires only 1/4th the light needed by the FL-400 lens. You may have found the happy medium between focal length and an acceptable lens sensitivity.