Back when I was just starting out I worked on a project where my boss used to infer the focal length of a complicated lens snorkel system by photographing a 1 foot plastic cube. He did this so that we could match move CG and motion control robotic arm camera live action. (Nowadays there are better match moving solutions, but this was a while ago)
It occurs to me that this cube thing would be handy for matching CG to still photos, especially in a zoom lens where the focal length is not what the lens claims it is (as in the Nikkor 70 - 200 VRII)
Question:
Someone hands you a photo of a glass cube centered in frame and oriented with its face flat to camera.
The front face takes up 80% of the width of a Nikon FF image. The back face takes up 70% of the width of the image. The actual cube is 1 foot on each edge. The image is in 'landscape' aspect so the longest side is the width of the frame.
You have no more information.
Q1: How far is it from camera to cube?
q2: What is the focal length in mm of the lens?
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Various answers are here. So far I have:
Distance to camera = 7 feet
The 133.35 mm comes from the fact that the front face is 0.8 of the frame width and the back face is 0.7 of the frame width so the back face is (0.7 / 0.8 = 0.875) of the front face. 0.875 times half a foot in millimeters is 133.35 mm
Front face projection is 80% of 36 mm = 28.8 mm
f = ?
Front face is 1 foot
D = 7 feet
1 : 7 :: 28.8 : f
Lens focal length f = 28.8 * 7 = 201.6 mm
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