focalLength1.jpg

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?

---------------
Various answers are here. So far I have:

Distance.jpg

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

Recent Comments

  • Tommy Francis: Where are you ??????????????????????????????? read more
  • Joseph Francis: I'm afraid this is just a one-off. I'm glad you read more
  • Mickie: I am interested in contacting whoever put together this example read more
  • Joseph Francis: Rene - Thanks for the comment. I'm not sure it's read more
  • Ál: Great colors and a very beautiful girl. :) read more
  • Joseph Francis: It's along this trail in Runyon Canyon: http://maps.google.com/maps?hl=en&source=hp&q=runyon+canyon&ie=UTF8&hq=runyon+canyon&hnear=Los+Angeles,+CA&ll=34.108582,-118.347744&spn=0.001899,0.002323&t=h&z=19 read more
  • Are: Can someone give us the exact location on Google earth read more
  • Adam Bradley: Thankyou - going to play with it in a couple read more
  • Joseph Francis: Adam, I uploaded an OBJ format lens for you. read more
  • Adam Bradley: Would it be possible to get a hold of this read more

Recent Assets

  • Distance.jpg
  • focalLength1.jpg
  • longLens.gif
  • grundy.jpg
  • RingFlashAnim.gif
  • RINGcombo.jpg
  • colorDecon01.jpg
  • shadowOnly.jpg
  • Skullphone_logo.jpg
  • wildthings.jpg

Find recent content on the main index or look in the archives to find all content.