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What methods are used for automatically measuring the dimensions (width/height/distance/etc.) of a subject without having to get in there with a measuring tape?

I am just starting to get into photography, but coming from a programming background it seems like this should be possible given the right tools. I'd like to be able to do this with any Canon DLSR camera.

Maybe there is a laser (separate component) that could be used to determine distance to the subject, which can then be used in an equation along with DOF and other optics variables for the specific lens you're using, to calculate the size of an object.

Or, is the only way manually?

Thanks for your help.

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I hate when someone asks a question and people don't answer and just ask them why would you want to do THAT? But I have to ask why? :) – MikeW Jun 9 '13 at 23:04
@MikeW - More likely question is "what's the Photography aspect of this?" – Esa Paulasto Jun 9 '13 at 23:11
possible duplicate of Can we measure size of an object using EXIF data from a photo? – mattdm Jun 9 '13 at 23:26
If you could share why you are interested in such an exercise, it would help to understand your question and possibly allow us to create answers that would tell you what you want to know. – Michael Clark Jun 10 '13 at 0:39
A I am also looking for an answer for same question... How to measure length without physically measuring it? My son has been asked this question in one of the science practical examinations.. Pl help. Sunil Pansare – user24531 Dec 5 '13 at 7:06

1 Answer 1

First off to aid in searching for more information, what you're referring to is called Photogrammetry.

From a single image you can't [reliably] measure anything - there's no way of being able to tell if you're looking at a photograph of a photograph (i.e. a flat object very close to the camera).

From 2 or more images you can recover some or all measurements up to a certain accuracy (except in some rare cases), provided you have correspondence information, i.e. you know what parts of the first image correspond to the same parts in the subsequent images. At least 7 different points are required, 8 or more strongly recommended.

There are algorithms to estimate correspondence and with enough images that contain strong distinctive features the results can be quite accurate. To get absolute (as opposed to relative) measurements the camera must be calibrated. There are many ways to do this, the easiest being shooting a calibration target of known size.

Since you're comfortable with programming there are various bits of code that implement these methods (as well as proprietary products with SDKs) search for wide baseline stereo, or structure from motion.

Finally if you know the distance to the object then you can obtain estimates of the size using trigonometry (provided you know the focal length), see this question for more details: How do I calculate the distance of an object in a photo?

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