When calculating magnification ratios, do you use square footage of film or the size of the aspect ratio? In other words, if using a 4x5 camera to photograph something 8x10" in size, is the ratio 1:2 (8 inches is twice as many as 4 inches) or would it be 1:4 (80 square inches of subject being documented on 20 square inches of film)? You'd think I'd know this after this much time taking pics, but I blame this on TTL metering
the ratio would be 1:2. One inch on film equals two inches on the subject.
As Jay said, the magnification is the ratio of linear dimensions: that in the image to that in the subject.
Keep in mind though that the 4 x 5 frame is actually slightly less than 4 x 5 inches. For example, my 4 x 5 negatives actually have about dimensions 95 mm = 3.74 in by 120 mm = 4.72 in. Similarly an 8 x 10 print will often have a margin reducing its size somewhat. In practice, this may not make much difference but in certain critical applications it could.
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