Getting To Know the F-Stop

What is the Focal Length?

Focal length is a staple when it comes to cameras and video equipment. It means the distance from the optical center point of the lens towards the film plane (if you’re using film cameras and the CCD, if you’re using camcorders.) The more you lengthen the focal length, the more it helps magnify your subject.


The F-Stop serves as a measure for the aperture of the lens. It indicates how wide open your iris lens is at any one point. Essentially, the smaller the number means the wider the aperture, which in essence admits more light into your lens. This is why an F2.0 means it’s a wider aperture compared to that of the F11.0

When you reduce the F-Stop of a lens, effectively increasing the aperture, you get:

  1. Increased exposure because there’s more light that enters the lens
  2. The background becomes blurry, creating the popular “bokeh” effect as a result of the depth of field decreasing
  3. For film, the general sharpness decreases while chromatic aberration becomes apparent. This is the natural result of having a wider aperture. For shooting videos on 1/3 CCD cameras, however, you want to consistently use a wider aperture for a good depth of field while pairing it with a neutral density filter (NDFs) every time it’s needed.

A quick formula to guide you is below:

F-Stop = Focal Length / Diameter of the Lens Opening

Example: 50 mm Lens / 25 mm Iris diameter  = F2.0

Standard F-Stops available are:

1.0, 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22

Exposure and F-Stop don’t have a linear relationship. Every time you open the iris lens up to one stop, you are doubling the light that goes into your lens.

Knowing the basics is always a good idea if you’re a beginner. Learning how to manipulate the F-Stop to your advantage can help you capture the shots you need.