I had not read this before and you raise some interesting and perplexing
questions. I knew they were using DXO something and, since they show
resolution in excess of the sensor resolution, I had assumed (bad) that
they were measuring the lens independent of the sensor and also with the
sensor since the tests clearly use a particular camera. You are
right... where does the lens resolution beyond the sensor Nyquist limit
come from?
Without a separate test of the lens independent of the camera, the only
thing I can think of is a backwards calculation from system MTF (their
measurment) and sensor MTF (defined by pixel count?) to derive the
presumed lens MTF. Whether such a thing is even possible I haven't the
slightest idea. It's an intriguing question.
Chuck Norcutt
Moose wrote:
> 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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