 One reason is that there would be no other way to know when critical focus has been achieved, and the second reason is the huge file sizes, measured in hundreds of megabytes or even gigabytes.A 4 GB Compact Flash card can only store a handful of images.Let's say we have a 10,000 pixel array, and we want to take an image with a shutter speed of 1/50s, and 48 bit per pixel (16 bit per component) to achieve maximum quality.