What's happening to the E system..?

Started Jan 12, 2011 | Discussions thread
Rriley Forum Pro • Posts: 21,846
Re: Rriley, rriley ..

bobn2 wrote:

mcabato wrote:

bobn2 wrote:

mcabato wrote:

  • oly equipment delivers excellent IQ the whole range from iso 100 to 800 or on E-5 iso 3200

Also would appear to be the case. IQ wise, I would have a concern about the artifacts produced by the alias busting firmware on the E-5, but that's a matter of taste.

don't you think it would be easy for me to pick out one camera model of canon or nikon where i can make negative comments about one aspect of IQ?

You should do so where people are spreading misinformation, and indeed have done so.

and that contest should begin with people like you

what you describe as bunk and misinformation actually works and is used in most lenses designed for digital cameras.

more fully the supporting text by Joseph S. Wisniewski, lens designer
1.1) Longer "throw"

Film is sensitive to light that strikes its surface at virtually angle, even angles far from perpendicular. For example, some ultra-wide-angle lenses such as the Voiglander Heliar (for 35mm rangefinder cameras) and the Schneider Super Angulon (for large format cameras) actually reach an angle of 60 degrees from perpendicular in the corners of the image. These lenses are symmetric in construction (which is wonderful for controlling lens aberrations) and their "exit pupil" (the point in space from which light seems to originate) is very near the "optical center" of the lens.

Digital sensors are more selective, being most sensitive to light that arrives perpendicular to the sensor. Most sensors use "microlenses", a tiny lens in front of each pixel of the sensor, to increase their sensitivity. Such sensors normally lose 50% of their sensitivity when the light diverges just 15 degrees from perpendicular horizontally (from the data sheets for the Kodak KAI-11000CM "full frame" and KAF-5101CE "four thirds" sensors). To avoid noticeable vignetting, light should be within 12 degrees of perpendicular, horizontally. Even sensors that do not have microlenses have a noticeable decrease in sensitivity by the time light diverges 20 degrees from perpendicular.

This means that any conventional symmetrical wide-angle lens would not work on a digital camera. Fortunately, for the last 50 years, the popularity of SLR cameras caused the creation of many "retrofocus" lenses, lenses that have "virtual" exit pupils much farther away from the film than you would expect from their focal lengths. This was done to increase "back focus" (the physical distance between the rear lens element and the film or sensor) so that the rear elements of the lens would physically clear the swinging mirror in the SLR cameras. It has the benefit of moving the exit pupil to a point around 50 mm from the film plane. Light from a 50mm exit pupil will strike the film within 19 degrees of perpendicular, a big improvement from 60 degrees for the Heliar. So digital wide-angle photography is possible.

Ordinary retrofocus lenses move the exit pupil just far enough to clear an SLR mirror. A lens can be made more "digital friendly" if the exit pupil is moved farther than this. For example, the Nikon 17-35mm f2.8 lens has an exit pupil that ranges from 98mm (at the wide 17mm setting) to 78mm at the 35mm telephoto setting. The Sigma DG lenses also feature such "extreme" 80mm exit pupils. Camera and lens companies refer to such lenses as "telecentric". This is a slight exaggeration: a true telecentric lens has an exit pupil an infinite (mathematically, anyway) distance from the sensor. This causes the light to arrive perpendicular to the sensor. It is also costly, and creates problems, so 80-90mm "near telecentric" lenses are considered the norm.

This table shows that a lens with an exit pupil 52mm from the film plane (typical of Nikon or Canon wide angles) has the potential for severe vignetting on a 1.0x or 1.3x crop camera, and noticeable vignetting on 1.5x or 1.6x cameras. (1.7x and 2.0x cameras are essentially immune to such vignetting). Increasing the exit pupil to 80mm means that the 1.0x camera may have objectionable vignetting, but 1.3x to 2x cameras will have no vignetting at all.


diagram attributed to Joseph S. Wisniewski

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