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There is no such thing as conversion factor.
50mm lens is ALWAYS 50 mm lens.
However, because CCD/CMOS sensor in most digital cameras is sometimes smaller, the FOV for that particular camera will be smaller (in case of Canon 400D, this is 1.6 times smaller, compared to standard 35mm film camera )
In astro photography, the important parameter is camera scale, which is expressed as pixles/arcsec, or arcsec/pixel. This parameter depends on lens focal length and pixel size.
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