Not Your Everyday Banana

by Bart Arondson

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I have to photograph 15cm high printed text from 500 meters away. The text has to be human readable in the result.

  1. What kind of focal length do I need?

  2. Can i compromise, and get a camera with less optical and more digital zoom?

  3. Can you guys give me the process for working this out as well as the answer?

  4. What is the minimum price that I can do this for?

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marked as duplicate by AJ Henderson, Paul Cezanne, mattdm, Michael Clark, John Cavan Nov 17 '13 at 23:49

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

This question appears to be off-topic because it is about product recommendations and computer vision. I'm also pretty sure the basic question of how big of a focal length is needed for a given magnification is also already answered in multiple questions. –  AJ Henderson Nov 13 '13 at 22:09
It's not really about computer vision. I just want to know how I can get 150mm text from 500 meters away to appear as 2.5 mm text –  RRs_Ghost Nov 13 '13 at 22:19
In addition I wish to know how digital zoom will effect my ability to read the text. I do not see how this is off topic. –  RRs_Ghost Nov 13 '13 at 22:21
What does "2.5mm in height" mean? That big on the sensor? On a print? What size of print? How big is a piece of string? –  mattdm Nov 13 '13 at 22:43
Digital zoom won't be helpful, because digital zoom is just cropping and expanding the result. For your purposes, you can do that after the fact. –  mattdm Nov 13 '13 at 22:52
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1 Answer 1

Putting aside some of the details of this and strictly speaking, shooting something that is 0.15 m tall and 500 meters away on a full-frame camera will require a nearly 100,000mm long lens (which doesn't exist, or at least isn't generally available). Two lines of text tall might be 0.3 m and needs a 53,000mm focal length (which doesn't exist). Ten lines of text (1.5 m tall) will require an 11,000mm long lens. 100 lines of text tall (15 m tall) will require a 1200mm focal length. Finally, we're down to a focal length that is available (not cheap or even affordable for most, but actually available). Useful: a lens focal length calculator.

The D800 has 36 megapixels, making an image 7360x4912 pixels. With 100 lines of text that means there's about 49 pixels of height per character. Viewed on screen at 100%, that's completely readable, perhaps 1-1/2 to 2 times larger than the text on this site appears for most. I don't understand the meaning of the "human readable (2.5mm height)" requirement and the specifics of that could negate the readability of this size, of course.

So, as I've calculated it, you'd need a D800, a 600mm lens, 2x teleconverter, and a beastly tripod to hold that. Shouldn't cost you more than US$15,000. You may want to figure a little extra in there to take some classes to learn to use such beastly gear, too. :)

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