Reason for so many focus errors we see today?

RichA wrote:
Plastic? Thermal expansion of plastic is much greater than metal and
Bzzt, wrong.
Coefficients of Linear thermal expansion (10^-6 m/m/C):
Aluminium 23.1
Magnesium 26
Brass 19
Stainless Steel 17
Steel 11-13
Glass reinforced Polycarbonate 22
So there's not a lot in it.

it could very well be why we are seeing focus issues that need "lens
re-calibration" at service depots or that we see the need for in-
camera focus fine-tuning. Even cameras and lenses that appear to be
metal today may have plastic cells holding lenses, components in
cameras. The cameras are produced in a control temp environment but
that isn't real life use where temps can vary by 10's of degrees. I
To put expansion in perspective, a 30 degree C change in
temp on a 300mm lens made of Magnesium would be a mere
0.2mm. Every lens I've ever seen has that much play in it's
movement if not more.

don't remember all metal AF SLRs needing focus fine-tuning (or having
that facility) in the film days.
Because of the lower resolution of the sensor it wasn't as
critical. Consider that a typical 35mm frame of film can
resolve the equivalent of maybe 12-15MP. Modern DSLR's will
cram 12-15MP on a sensor that has half the surface area, or
are putting 24MP onto the 35mm frame. Because of this higher
resolution they require more critical focus from the lens,
and so errors that have always existed are now noticed.




--
Don't blame me - I didn't vote for Kevin Rudd or Anna Bligh!

"Doug Jewell" wrote in message
...
don't remember all metal AF SLRs needing focus fine-tuning (or having
that facility) in the film days.
Because of the lower resolution of the sensor it wasn't as critical.
Consider that a typical 35mm frame of film can resolve the equivalent of
maybe 12-15MP. Modern DSLR's will cram 12-15MP on a sensor that has half
the surface area, or are putting 24MP onto the 35mm frame. Because of this
higher resolution they require more critical focus from the lens, and so
errors that have always existed are now noticed.

How thick is a film emulsion, versus the sensor plane of a chip. I would
hazard to suggest a CCD/CMOS is more critical than film.

["Followup-To:" header set to rec.photo.digital.slr-systems.]
Doug Jewell wrote:
RichA wrote:
Plastic? Thermal expansion of plastic is much greater than metal and
Bzzt, wrong.

Kindly do not confuse the RichA with facts. It doesn't understand
the concept.

Modern DSLR's will
cram 12-15MP on a sensor that has half the surface area, or
are putting 24MP onto the 35mm frame.

24 MPix on the 35mm frame is equivalent to 9.4 MPix on a 1.6x
crop (10.7 MPix on a 1.5x crop). That's not so much.

Because of this higher
resolution they require more critical focus from the lens,
and so errors that have always existed are now noticed.

On the other hand, a '100%' pixel-for-pixel view on a 80-100 PPI
monitor has nothing to do with it. :-)

-Wolfgang