Enhanced low-light auto-focus system model in digital still and cell-phone cameras

In low-light conditions, the focus value curve or sharpness function that is used in passive auto-focusing becomes flat due to the increase in the noise level. This paper presents an enhanced low-light auto-focusing system model which provides performance improvements over our previously introduced low-light auto-focusing model. This is achieved by utilizing an indexed family of focus value curves via a digital band-pass filterbank. A new sharpness ratio metric is presented for evaluation of auto-focusing in low-light conditions. Experimental results obtained from three different prototype camera platforms demonstrate the improved low-light performance over our previous model.