What is that site that has Lens comparisons side by side?
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Canon Bob
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OP
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Unfortunately, at www.the-digital-picture.com they use a flat test target that is sometimes shot at very close distance (with wide angles) and the results they get are often, not to beat about the bush, rubbish.
The only reliable lens tests that I know of are the MTF graphs published by Canon USA. Nikon and Sigma also do similar.
www.dpreview.com also do good tests and although they also use a flat target, they openly discuss its shortcomings and support their lab test with relevant field tests. They also dwell on many other very important aspects of lens performance, not just sharpness.
Canon Bob
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Richard...one thing worth noting (if I'm right) is that Canon's MTF charts are calculated from computer models and not based on actual performance. I expect that they're very close to reality otherwise they'd change the model data used. I'm not sure if Nik' and Sigma do the same.
Bob
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Yes, I believe that is true - the graphs are computer models. I am pretty sure the same is true of Nikon and Sigma MTF graphs.
I used to be skeptical of this, but it cannot make the lens appear any better than it actually is. So if there is any doubt, it will only be about whether manufactuing standards are up to maxing out the potential optical performance. And unless the lens is actually faulty in some way, I think that they are.
To put it another way, I have never seen any credible evidence to suggest that the Canon graphs are not representative, or indeed are not the best tests available for consistent and relevant comparative data.