Chuck Norcutt wrote:
> The lens resolution is not tested with the sensor.
"In practice this is an oversimplification; our tests measure system MTF
rather than purely lens MTF, and at frequencies close to Nyquist the
camera's anti-aliasing filter will have a significant effect in
attenuating the measured MTF50. In addition, our testing procedure
involves shooting a chart of fixed size, which therefore requires a
closer shooting distance on full frame, and this will also have some
influence on the MTF50 data."
> It's tested with DXO's measuring gear and the results are plotted against the
> Nyquist frequency of the sensor. It appears that you did not actually look
> at the test data I referenced
Well I did. The above quote is from the test page you referred to and
the following is from the methodology discussion page for their tests in
general:
-------------------------------------------------
"How The Test Data is Shot
By Andy Westlake
Sharpness
Test images for are shot in RAW at the camera's base ISO, and processed
using a common converter (Adobe Camera Raw) with all sharpening disabled
– this eliminates any differences from in-camera processing. Using RAW
is crucial, as many current cameras can apply lens aberration correction
to JPEGs - most notably Nikon DSLRS such as the D3, D300, D70 and D90
feature automatic correction of lateral chromatic aberration, and the
Canon 50D features automatic peripheral illumination (falloff)
correction. Using JPEGs from these cameras for testing would therefore
not provide a true description of the lens itself.
Sharpness is calculated from the slanted-edge patterns arranged across
the frame; the test chart has four ‘arms’ extending from the centre to
each corner of the frame, and the sharpness data we display is obtained
by averaging the results from each, to give the most accurate
representation of the lens’s performance."
-------------------------------------------------
> or you would have seen that lens resolution did indeed exceed the sensor at
> some points. That would not have been possible to measure using the sensor.
>
Exactly! And that's why I commented on how weird it was to publish this
"maximum theoretical resolution of the camera body used for testing"
along with MTF data showing higher values.
What you suggest would indeed make sense if they are in fact using bench
testing equipment, but that's not what they say they are doing.
Have I missed something on their site?
Moose
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