Possible Duplicate:
How does one focus for landscape photos in very dark conditions?
Recently I went on a night forest safari trip with my Canon 1100D. I had put the camera in manual mode and set focus to AF. When I saw a elephant I pressed the shutter, it was always searching for the focus, and couldn't get the focus.
How should I take photos at night and focus correctly?
You must provide a source of illumination. This can be a torch or spot or similar. The light can be colored (eg some cameras use a red light) and some cameras use a low energy burst from the flash to perform AF.
In marginal light conditions try and find a high contrast edge in the scene to assist focusing.
In conditions when AF is not practical and use of a flash may be life threatening ( eg often with elephants and most times with leopards :-) ) you can work out the approximate range in advance and then find some object off the line of site that is better illuminated and which is at the same distance. Or you can refocus with test shots and note the range to various points then manual focus as required.
If using flash then you may be able to use a small aperture (higher f number) and thus increase your depth of field and so the ranges that are in focus.
If you have enough aperture lee way you may be able to set the camera to the hyperfocal distance so that ALL subjects from beyond a certain distance all the way out to infinity are in focus. Then you can "point and shoot" without needing to refocus. As above, this is liable to be successful only of a flash is used as otherwise the shutter speed will be too low. For more information on hyperfocal distance search for some of the many useful discussions on this site.
Were you using flash?
Without sufficient light, the phase detect AF in most DSLRs really won't function. Essentially, the camera takes light and splits it to two different sensors. It uses the differences in the scene to determine focus. If there is not enough light to determine differences, you won't get autofocus.
In the field, I handle this in one of two ways:
1) I always carry a flashlight, that I use to light the scene to establish focus: Point your flashlight at the subject (or area), allow your camera to establish focus, then switch to Manual focus. (I am assuming you are using a tripod). This will maintain focus on your area of interest. (turn off the flashlight when done of course)
2)Turn on LiveView, then switch to Manual Focus. Zoom in with LiveView, and using the view on the LCD, adjust focus until you achieve a sharp image. Leave camera in Manual focus, but turn off Live view.
When a subject enters your pre-focused area, shoot away. Note that moving your camera will of course, likely result in images out of focus.
