Hi all
Can somebody explain me why there is (AFAIK) still no Camera available with AF points in the corners of the picture?
Is it not possible at the moment, what is the technical challenge?
Hi Ben,
It depends on the type of AF.
For Contrast detection based AF there is not much of a restriction, just that there is enough light and some subject contrast at the region where one wants to measure it. It's strictly a local contrast operation. Of course, due to field curvature, the edges and corners may focus at a different distance than in the optical center.
For Phase detection based AF, it is necessary to have at least two optical paths that can be compared (for perfect alignment if focused) and that requires to use the edges of the lens for the maximum path angle difference. That also means that closing down the aperture, or focusing closer than infinity, will cut off the image forming rays from those edges. In addition, the additional optical system that focuses those edge rays from opposite sides of the optical axis on the AF sensor has a limited diameter, a max aperture of itself.
So theoretically one could make a phase detect AF that could focus closer to the edges when the aperture is wide open but that would be limited by the widest aperture of the lens. That would be confusing with interchangeable lens cameras, or zoom lenses that have a variable widest aperture, and still vary with focus distance. The field curvature issue also plays a bad role.
The current compromise will usually allow to use Phase detect AF with lenses that have a widest aperture of f/5.6 or sometimes f/8, and that limits the maximum diameter for the AF system.
There is a fundamental expression that relates the half-angle θ of the light cone in image space to the F-number N of a well-corrected lens:
N = 1 / (2 * sin(θ))That also explains why smaller image sensors will allow to have Phase detect sensors closer to the edges than larger image sensors, it's an angle from the optical axis limitation.
Cheers,
Bart
