Candid photos are not difficult but they do require some practice, patience and preparation. Timing is critical to candid photography. With modern day DSLR cameras pre-setting the focus point is not as necessary as it once was. Having said that, getting it close should be part of your practice, especially if you are indoors or in low light conditions. If you are outdoors I recommend using both a high shutter speed of 1/250th or faster f-stop (f/8 or smaller). By setting your camera to these settings you increase the chance of getting that perfect candid shot. Using a small f-stop gives you a large plane of focus. Using a high ISO gives you greater latitude for different lighting conditions.
Good Etiquette Tip: If you are in unfamiliar territory and your subject notices you taking their photo, stop and ask their permission before shooting. If they don’t want their photo taken, thank them and move on.
Another key factor in candid photography is patience. Be patient and wait for your subject to relax and start acting natural.
A fun way to shoot candid is to try to look like you are not taking photos. Wear your camera in with the neck strap around your neck, hold the camera so that it is facing forward and press the shutter release without raising it to your eye. One of my favorite ways to capture candid photos is to hold the camera on my shoulder, pointing the camera lens forward and use my body as a pivot point. Just walk around snapping away. Looking in a different direction from where the camera is pointed can help with the ruse.