Most EOS digital cameras offer Custom Functions. These allow you to alter the way the camera operates to suit your own personal needs. Exposure, focusing and flash are just some of the areas covered by the functions. Each function has a factory-defined default setting, plus one or more alternative settings.
Custom Functions are set through the camera menu system. Find and select ‘Custom Functions’ and you will bring up a screen with each Custom Function number displayed. Select the appropriate number and you will be taken to a screen where you can select from the various options for that custom function.
The display will usually indicate which functions are no longer at their default setting. You can either go to each function to change the setting, or go to the ‘Clear Setting: menu option’ to reset all the custom functions to the default settings.
On some models, it is possible to register up to three groups of Custom Function settings. This means you can change a number of different Custom Functions and save all these change under one group registration. This saves you the time and trouble of going through and changing each custom setting individually the next time you want to use the same set. You could, for example, have one group of settings for landscape photography, and another for sports shooting.
For previous generation cameras, a specific Custom Function is usually written as C.Fn.-XX-Y, where the first number (XX) is the Custom Function reference, and the second number (Y) is the selected option within that function. The default setting for each Custom Function is usually ‘0’. So custom function 12 (mirror lockup) is written as either C.Fn.-12-0 (lockup disabled) or C.Fn.-12-1 (lockup enabled). On the EOS-1D/Ds Mark III models as well as the latest generation of consumer level cameras like the EOS 40D, EOS 50D, EOS 5D Mark II, EOS 7D and EOS-1D Mark IV, Custom Functions have been separated into four groups: Group I for Exposure, Group II for Image/Flash exp/Display, Group III for Auto focus/Drive and Group IV for Operation/Others.
These new settings are written as C.Fn XX-Y-Z where XX is the group, Y is the Custom Function and Z is the setting, for example C.Fn IV-3-1.
The first table below shows how many Custom Functions some of the most recent EOS cameras have and how they are grouped. The number of Custom Functions varies from camera to camera as does the reference number for a particular function. Since Custom Functions are a personal choice, there are no settings you have to make. However, it is wise to spend time acquainting yourself with the various Custom Functions on your particular EOS camera to then decide if they are useful for your photography and which functions you prefer to use. The second table below shows the Custom Functions on the EOS-1D X and EOS 5D Mark III DSLRs.
Check out the entire article on Canon's website.