I just upgraded from a point-and-shoot camera to my very first D SLR and I need help. Sometimes, I get a blinking green circle in the viewfinder and the camera refuses to take a photo. This happened several times when shooting at night in Las Vegas but also once in daylight when I was taking a photo of a flower. How do I solve this problem?
The blinking signal in the viewfinder data panel is an indication that autofocus is not possible in this situation. No electronic feature is 100% reliable and sometimes the conditions cause the AF system to fail. When that happens in Single Shot AF (the default mode) the camera will not take a photo. That’s just as well because you would not want a picture where the subject is out of focus.
Any Autofocus system can struggle in very dark conditions because it requires a certain amount of light to function. This is less of a problem when the subject is close to the camera, roughly three meters or closer. In that case, the camera—or the flash—will fire a beam of light or a burst of light. That should allow the AF system to set focus (on the bright area). Setting the autofocus sensor so only the single central AF point is active can also be useful in low-light photography.
But when the camera/lens simply cannot focus automatically, switch AF off and focus manually. Rotate the narrow ring on the lens while looking through the viewfinder. When the subject is sharp, stop rotating. (It is now in focus.) Of course you may find it difficult to see the subject adequately well to know when it’s sharp in a dark location. After a few minutes however, your pupils will dilate (open wider) and you’ll be able to see whether the subject is in focus.
Note: Both autofocus and manual focus are impossible when you are too close to the subject. (Cameras with a built-in lens can focus much closer when set to the Macro focus mode.) Move further back from the subject and try again.