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Bird might want to comments on this, but for webcam imaging using barlows and powermates I had the impression/belief (perhaps misguided?) that it is the effective Focal Ratio (eg f20, F25 etc) that determines image scale not the effective focal length. I'm pretty sure my 2150mm FL Mewlon 180 with base Focal Ratio of F12 provides a slightly larger image than my 2350mm FL C9.25 with its focal ratio of F10 when imaging with the 4x power mate on each?
regards,
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