UNDERSTANDING F-STOPS ON PHOTOGRAPHY

First of all, before I begin, I will define some terms which I will be using in this article, to give you a better understanding of their meaning:

Diaphragm: The device in the lens which controls the amount of light entering the lens.
F-stop: The size of the opening in the diaphragm of the lens, each different f-stop allows a certain amount of light to enter the lens.
Depth of Field (DOF): The amount of area in or out of focus in an image.
Aperture: The opening in the diaphragm which allows the light to enter.
Shutter Speed: The length of time the shutter remains open in your camera.
Exposure: The aperture and shutter speed combined.

So now, once you understand the different terms, on your camera each f-stop will be represented as a number, depending on your lens, and they can vary greatly.
The smaller the number the larger the aperture, or opening. Each f-stop increases or decreases the amount of light entering the lens by a magnitude two times. These f numbers increase by the following increments:

f1.0, f1.4, f2.0, f2.8, f4.0, f5.6, f8, f11, f16, f22, f32, f45, f64, f90

For this tutorial, let’s begin at f5.6 and go up as far as f45. Remember that as the size of the number increases the smaller the aperture in the diaphragm is, and therefor, the less light is being let into the camera. So, f5.6 allows x2 as much light enter the lens as f8, this means that f8 allows x2 as much light in as f11, and so on.
Each f-stop (if your camera/lens allows) can be divided into 1/3 or ½ stop increments, which are as follows:

(1/3 stops): f5.6, f6.3, f7.1, f8, f9, f10, f11, f13, f14, f16, f18, f19, f22

(1/2 stops) f5.6, f6.7, f8.0, f9.5, f11, f13, f16, f19, f22

The largest f-stop on your lens (in this example) f5.6, will be allowing the most light into the camera (since the apateure opening will be larger), and you will also have the shallowest depth of field. This means that when you are focused on the main subject you want to photograph, the objects on front or behind it will remain out of focus, depending on how far they are from the main subject. As the f-stops decrease (remember, the f numbers get bigger but the aperture size decreases), the surrounding objects become sharper.

To get the full benefit of f-stops and DOF (Depth of Field) you will need to understand the basics of the built in light meter in your camera:
In most digital and film SLR’s, the light meter is represented as the following on both the LCD screen and when you look into the viewfinder:

-2..-1..{ }..+1..+2
•

The ‘•’ represents the current light reading. If it is positioned in the centre ‘{ }’ it means that the camera is set to the correct exposure for the given amount of light. Should the ‘•’ be positioned under the -2, -1, +1, +2 this means that the camera is set one or two f-stops under or over, for the ideal exposure. So when you are out photographing, for instance if you have your camera set to f11 and the light meter marker is at -1, change the f-stop to f8 to open up the aperture, likewise if the marker is at +1, change the f-stop to f16.

Additionally, it is also good practice to take photographs one stop under, one stop over, and one at the correct exposure This way, you can view the results and see which image has the best exposure on screen or when printed; this is called bracketing. Bracketing gives you some lee-way if the lighting conditions are tricky or you are using the flash. Some camera models have an “auto-bracketing” mode, which automatically takes 3 (or sometimes even 5) photos at various f-stop intervals, so that you can later choose the photo with the most pleasing exposure.

It should also be noted that the built in light meter in your SLR is not accurate 100% of the time, as it generally just takes an average light reading. Hand held light meters are much more accurate as you can do readings on various areas on your subject.

To get more in depth on the subject of light readings, the two small dots between each number ( -2..-1..{}..+1..+2) represent 1/3 stops. If there was only one dot between each number, this would mean that they were representing ½ stops.

I hope that this article has shed some light (no pun intended!) on the subject of f-stops and exposures. It takes time and practice to fully come to grips with lighting, shutter speeds, depth of field etc... Many people might think that it is not very important to know these things (since your camera may have a full-auto setting). However, in the long run, as you get more and more accustomed to using the manual settings in your camera, the more you will enjoy the wonderful art of photography. The results won’t come immediately, but keep your mind on the task at hand and you might just be impressed with what you come up with!

by: Kenneth Fagan