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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! |
#2
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Reason for so many focus errors we see today?
"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. |
#3
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Reason for so many focus errors we see today?
["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 |
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