Digital cameras can be confusing. With their multitude of options and abbreviations, it is not surprising that many camera functions go unused by the average amateur photographer. The "Fn" button is one such button that is unlikely to be used, unless you know what it does. However, once you know what the Fn button is capable of doing, it is likely you will be using it very often.
The Fn button allows for greater customization of your camera by allowing you to assign a specific function to it. Depending on the camera model, the Fn button may bring up a menu of other options, or it may perform an action. Some actions may require other buttons to be pressed in conjunction with the Fn button. For many cameras, the Fn button has presets, but it is also customizable. The Fn button's functions are also customizable between modes. For example, the Fn button can be set for certain functions during the use of portrait mode, but then other functions can be set when you are using action mode.
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Often, the Fn button has a preset menu of options. According to Olympus (olympusamerica.com), some common presets are Raw, Preview, Test Picture and My Mode. These presets allow you to easily perform commonly used tasks without having to take multiple steps to do so. For example, clicking the Fn button toggles the camera between Raw mode and your previous image settings.
Some cameras allow you to customize the function of your Fn button. The RICOH R10, for example, allows the user to change modes quickly by pressing the Fn button, but it also allows users to set the camera to compensate for backlighting. This will cause the camera to automatically adjust to current lighting conditions. Another customizable option for the RICOH R10 is Step Zoom, which allows you to quickly shift to a zoom setting you commonly use. This feature is ideal for quickly switching from no zoom to full zoom to capture an image that may not be there by the time you zoomed manually.
Some functions of the Fn button require you to press other buttons while pressing the Fn button. By pressing these buttons at the same time, you can perform other, more specific actions. With Olympus' example of the Test Picture preset, if you press the Fn button and the shutter at the same time, you can generate a test picture. This feature forces you to verify each image before it is saved to your camera's memory card.