Ideally I wanted to stop the lens down to about f2.8 but of course that isn't possible because focusing is always done at full aperture.
Like all wide aperture lenses my 50mm has a fair amount of flare causing low contrast at full aperture so I needed a way of reducing the flare during the test. I reasoned that Focal must work by comparing each shot by contrast and the one with the best contrast is the best focused. I know that most Canon 50mm lenses have inconsistent focus but this wasn't responsible for the strange results, neither does the lens show any focus shift at different apertures. I have made many shots at -2 and I have proven that is correct at all apertures so it bugged me as to why Focal calculated it incorrectly. Manual attempts at MFA would give me results of about -1 to -2, Focal gives me -11 to -14. I have tried to MFA my old Canon 50mm 1.8 Mk 1 various times using Focal and every time it gave me silly results.
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