understanding camera lenses – what is F stop

Understanding camera lenses

Understanding camera lenses is critical for the easy learning on how to control your results.What is focal length and what does it tell us? First we need to have a basic understadning of optics. I will try to explain as easy os possible, as I know how much difficulty I had understanding the basic of terminology of such F Stop, focal distance, aperture and their relationships

I hope we all know what reflection and refraction is. How lenses work is basically understanding the refraction.

Refraction is the change of direction when light hits the surface and enters a new transparent material such as glass or water.

Lenses are basically curved surfaces made of a transparent material which due to curvature, through refraction, are converging and diverging the light beam. See here the most detailed explanation what lenses are and how they work. In terms of photography we need to know that several lenses are working like one with different properties and their main area of interest for us as photographers,  is the focal point (or plan) where they are converging the lights beams going through the lens, and focal distance distance which represents the distance from the lens plan and where the focal point is.

Focal length

Focal length is the distance between the lens and where the focus point or plan is – and that is described on a lens in mm. To be noted the a series of lenses act like one (see image below)


Also this is a schematic view on how that translates inside a camera.

For the easiest understanding of the focal length terminology and the width of the image you will going to get from a specific lens on the market see the image below with a very good representation

F- Stop – Aperture size

F stop represents the diameter of the hole and it is expressed in a ratio as a part of an inch 25.4 mm.

F Stop 1.4 means 1/1.4 of an inch and the bigger the F -stop number is the smaller the hole is. It also represents the “speed of a lens.

The speed of a lens is in fact the an expression as the lens does not have any speed and it does not move during an exposure. What this means is that a wider hole, or aperture allows a faster shutter speed to get a properly exposed photograph. The wider the aperture, the smaller the F Stop number. In order to understand the speed of the lens, please see the relationship between the shutter speed and the aperture.

So the best way to understand the camera lens terminology  is to go though an example:

this is a Canon lens described as 70-200 mm FStop 2.8, and that means the zoom is able to vary the width of the image from 70 mm the widest to the 200 mm focal length – the most narrow.The F stop is 2.8 which means 1/2.8 =25.4mm/2.8=9.07 mm.

I hope this short tutorial about understanding camera lenses and their terminology as part of our short course in how to become a photographer has been helpful and feel free to leave any comments or ask any questions below


About Christian Tudor

professional photographer, main editor at Academy of Photography and