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The distance of the reflected image from the mirror depends on the distance of the object from the mirror . For an image at great distance (approximately infinity) the image distance will be 750 mm from the mirror which is the focal length .In your test the object was relatively close to the mirror giving an image distance of 2100mm. Since focal ratio is focal length divided by aperture your. focal ratio will always be 750/150 = 5. When determining focal length by projection of an image the object must always be effectively at infinity(a very large distance compared to the focal length).
Hope this answers your question regards Philip
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