Professionals thrive on feedback. An amateur can shoot at a deer and know he hit the right spot if that deer falls down. If the scope is zeroed right and the amount of compensation was correct, if the rifle has adequate accuracy for the range, the deer will fall. If not, the hunter can try again or go to lunch to a restaurant.
A professional dealing with a variety of targets, from deer to enemy soldiers to dinosaurs escaped from secret labs would more likely use a machine gun with tracers to adjust aim by observation of previous shots. That way the exact sight adjustment can be imperfect but the results would still be good enough and more reliably obtained.
By that logic, studio professionals have used Polaroids for decades. If, despite their best efforts, the lighting or other factors were not perfect, the pros could refine their aim with successive “instant” captures before proceeding onto regular film. Think “ranging machine gun coaxial to tank cannon” approach.
With cameras, we see a curious inversion of this principle. Currently, professional cameras use phase detection auto focus and amateur cameras use contrast detection auto focus. Phase detection is faster and a bit more reliable in low light, while contrast detection is more precise. That’s because DSLR focusing is conducted by a separate system which is calibrated at the time of construction but generally not updated once the camera is in use. The imperfections and eventual changes in calibration wouldn’t even be noticed until the photos are reviewed under higher magnification, at least with conventional lenses. With pro grade large aperture lenses for low light and nature photography, the margin for error is tiny due to the minimal depth of field. This is where contrast detection from the sensor could theoretically shine — that system provided incremental feedback directly from the final image.
At this time, contrast detection isn’t perfect or even good enough under certain circumstances (such as low light or fast subject movement), but the on-going improvements make me wonder if video viewfinders, the traditionally amateur feature, wouldn’t become common on pro cameras before long. After all, they are standard on the extremely expensive video cameras and that doesn’t surprise anybody.
If phase detection is superior in principle, I wonder if AF calibration can be incorporated into the start-up routine for every serious camera. Somehow, the concept of serious equipment with one of its most important functions on an open loop without internal feedback from the result seems like a stop-gap solution to me. Am I missing some technical reason why phase detection AF is here to stay?
