March 29, 2013

How to measure the distance to an object in a photo?

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How to measure the distance to an object in a photo?

I recently took this video of a White-bellied parrot eating brazil nuts on top of a tree.  Its very difficult to see this sort of thing since the brazil nuts they are eating are so high up in the tree. 

To get the shot I climbed to the top of the 33m tower at Posada Amazonas then used a Canon 600mm f/4 with a 2x teleconverter and a Canon 7D for a total of 1920mm to take the video. Even though the camera was on a tripod I used image stabilization to remove any vibration that was induced by people moving on the tower. I then moved the video into iMovie and stabilized it even more just to be sure that all the shake was out:






A friend of mine asked me "How far away were you?"  I really didn't know so I set off to find out.

I have some experience with macro photography or photomicrography as it is sometimes called.  I use a high power lens to magnify subjects up to 7.5x.  At this magnification one can see all sorts of cool things..butterfly wing scales, compound eye structure of ants etc.

To find the size on a photo taken with a microscope you use the following technique:
  1. Take a photo of the object.
  2. Without adjusting magnification take a picture of a ruler
  3. Import both photos to photoshop
  4. Superimpose the photo of the ruler over the photo of the object using a opacity of around 50%
  5. You can now measure the length of the object.

I decided to use a similar technique for the bird.  Here is what I did.

I started with what was known:

  • Focal length 1200mm (1920mm with the 1.6 crop factor of the Canon 7D)
  • The average size of a White-bellied parrot is 23cm

I then made a scale on a whiteboard that was divided into 10 cm increments.


Using the exact camera setup that I used to take the parrot photo, I placed the scale along the street at known distances and took photos:

Scale at ~145ft

Scale at ~405ft


I then superimposed the scale over the photo of the parrot:



When the height of the parrot (23cm) was equal to 23cm of the scale I would check the distance that I took the scale photo at and then know the distance to the macaw.


It turned out to be that the photo was taken at a distance of around 150 meters.

Thats basically how its done. Hope that you liked the blog entry.

To learn more cool stuff about photography you can take a photo tour with me:

http://www.perunature.com/rainforest-nature-photography-tour.html

Jeff




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