During a photography class, students are asked to calculate the distance at which a camera lens with a focal length of 50 mm should be placed from a subject to produce an image that is twice the size of the subject. If discrepancies arise in the students' calculations, provide a rationale for the differing results.

At 45 mm and 55 mm from the lens, the image is twice the subject's size but it is in different orientations.

At 45 mm and 55 mm from the lens, the image is twice the subject's size but one is for a single lens and the other is for a combined lens.

At 75 mm and 25 mm from the lens, the image is twice the subject's size but one is a real image and the other is a virtual image.

At 75 mm and 25 mm from the lens, the image is twice the subject's size but the lens is in different orientations.