On Aug 29, 2005, at 2:39 PM, Mark Roberts wrote:
It's not BS, it's just not as big an issue as many seemed hysterical
to present it as. It becomes an issue when working with short focal
length lenses and a large sensor, and contributes to chromatic
aberration, mosaicing, and blooming problems at the edge/corner
regions of the frame.
Why only with short focal length lenses? (Since on an SLR they're
retrofocus designs, they don't have a more severe angle of
incidence at
the edges of the sensor than normal lenses.)
The position of the nodal point is what's significant. Even inverted
telephoto designs move the primary nodal point closer to the focusing
plane. Lenses designed for digital sensors should have additional
elements at the rear of the lens that will help "straighten" the
light path.
Canon's EF-S lenses move the optics *closer* to the sensor, making an
EF-S lens reach the edges of an APS-C sensor at a greater angle than a
standard lens on a full-frame sensor.
EF-S lenses allow the rear element of the lens to be further inset
into the body, which means little regards the register or primary
nodal point of the lens but allows for more in way of that rear
element correction. It's a mount that allows some greater flexibility
in design presuming a shorter, smaller mirror flapping about, that's
all.
Yes, wide angle lenses are more difficult to design for sharpness
at the
edges than normal lenses and digital sensors may manifest problems
differently than film, but the angle business was invented on
Usenet and
isn't mentioned in any CCD data sheet I've ever seen (and, man, that
took some reading: These kinds of "data sheets" are 100-or-more-page
novels!)
I didn't hear about it from Usenet. I've discussed this at length
with my engineering colleagues at Agilent who were designing digital
capture sensors. The goal in their brief is to minimize this problem
as much as possible as it constrains lens development and sensor
applicability, but given the geometry and physics of sensor design
there's only so much that can be done without very expensive
manufacturing issues. This is one of the reasons why large sensors
for cameras remain so expensive.
Godfrey
