Απάντηση: Re: Νέες Sony A7 & A7r Full Frame Mirrorless
Ηλία, ξαναβάζω το link:
http://theonlinephotographer.typepad.com/the_online_photographer/2013/10/two-reasons.html
Για κάθε ενδεχόμενο όμως παραθέτω το παρσακάτω κείμενο από τον σύνδεσμο αυτό.
The Leica M9 sensor's cover glass thickness is 0.8mm; this is much thicker than the M8's 0.5mm, but presumably offers much better infrared filtering, avoiding the "black synthetic fabric being imaged as dark purple" syndrome that plagued the M8.
There's little reason to expect that the optical stack (OLPF, IR filter etc.) in front of the Sony A7's sensor is thinner than the Leica M9's.
In the film era, lenses were designed to be stigmatic in air. In other words, back in the days when digital sensors didn't exist, lens designers expected the medium present between the lens' last vertex (the rearmost glass surface) and the imaging plane (the film) to have a homogeneous refractive index (RI) of one.
Leica's (and other manufacturers') wide-angle rangefinder lenses (those with a focal length shorter than approximately 35mm) tend to have a quite short exit-pupil-to-imaging-plane distance. The exit pupil is the apparent position of the diaphragm, looking at the lens from behind. Obviously, the exit pupil is where the light cone appears to emanate from, and thus determines the tilt of the light cone's principal ray—the cone's "axis," if you will.
In the center of the image, the principal ray will be perpendicular to the imaging plane. In the corners of the image, if the distance of the exit pupil to the image plane is short, the light cone (and its principal ray) will be quite tilted.
A slab of optical material—a sensor's cover glass, or an optical low-pass filter stack—has a refractive index higher than one, and will spatially shift the light rays impinging upon it—the amount of this spatial shift being dependent on the ray's incidence angle.
As a light cone is composed of rays that have varying angles, each of these rays is going to be spatially shifted by a variable amount.
The net result is that a light cone that was designed to converge to a single point—i.e., be stigmatic—in air, can have its light rays dispersed all over the place after it passes through a slab of material with refractive index larger than one.
The cone's dispersion is negligible when the incidence angle is perpendicular, at the center of the image.
With a sensor cover glass thickness of 0.8mm (800 microns) and a RI of 1.5, an exit-pupil-to-imaging-plane distance of about 30mm (a fairly typical value for wide-angle rangefinder lenses with focal lengths less than or equal to 35mm), and a cone aperture of ƒ/4 (that is, an exit pupil diameter of 30mm/4.0 = 7.5mm), one can easily calculate that in the image corners, the light cone's rays, instead of converging to a point, will be dispersed over an area of about 35 microns.
On a Leica M9, 35 microns corresponds to about five pixels!
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