The shutter button half press out of the box locks the auto focus on the subject closest to the camera. Depending on the camera set-up, and this can vary from camera make and model to camera make and model, it should lock the correct exposure on half press as well. In short the half press of the shutter button – locks the auto focus and the cameras exposure system.

When starting out with auto focus, and for day to day shooting I recommend changing from multiple focus points to single centre point by turning off all focus points but the centre point – then half press the shutter button to focus on the subject of your choice and as needed recompose the image in the viewfinder keeping the shutter button half pressed.

If the camera is set-up correctly on full press of the shutter button the exposure is adjusted to take into account any light changes in the recomposed image. 

Simple enough, right? If you chose to leave the camera’s AF system to select the point of focus it should lock focus on the area closest to the camera lens but this is not always the case and can be unpredictable – I reiterate for best results use single centre point half press and recompose as needed (or not). This method is likely your best approach as you get familiar with your camera’s auto focus system and go from there.

Most cameras have an AEL/AFL button which can be configured to act as a focus lock and some people find it a better approach than the half press shutter button to lock focus. In one scenario you can use the AEL/AFL button to lock focus and the shutter button full press to set the correct exposure.

I should stop here and point out that the AEL/AFL button is usually located on the back of most digital cameras and thus the back button auto focus term.

Yet another scenario is to use the AEL/AFL button to control exposure. An example might be to use the shutter button half press lock focus only and then use the AEL/AFL button to set exposure using the cameras internal exposure meter.

It is becoming popular to use the AEL/AFL button as the so called back button to auto focus the camera lens. It might be worth exploring this method to enhance your shooting experience.

The actual set-up for your camera brand will be very specific to the model and covered in your manual. For example the Olympus PEN E-P5 doesn’t have an AEL/AFL button and it has to be configured to one of the function buttons available on the camera, the Olympus OMD E-M1 has an AEL/AFL button – I wish there were some standards but that is wishful thinking on my part.

Terry Cioni – www.flickr.com/photos/streetfusion