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-   -   How to calculate the size of an object (https://forums.steves-digicams.com/general-discussion-11/how-calculate-size-object-26691/)

PJWilson Jun 3, 2004 12:35 PM

I have the feeling this may not be possible, but here goes...

I want to precisely record the scale of objects that I photograph, for pathology purposes. Since the camera 'knows' the distance to the object and the zoom employed, this should be fairly easy to work out. Any idea how to access this information?

Is there any camera out there which records distance & zoom for every photo? Or maybe I will just have to put a scale (e.g. ruler) into the picture. Suggestions appreciated. Thanks,

Peter.

Klaus DK Jun 3, 2004 12:42 PM

I have never heard of such camera, but it could exist though. I think it will be pretty pricey. The right way could be to buy a small laser. Lasers meassures very precise and are very easy to use too. This will cost you however.

Good luck on your quest!





slipe Jun 3, 2004 1:42 PM


There are quite a few cameras that read out the distance you are focused at in manual focus. Focus automatically and switch to manual – it should stay the same and give you the distance.

You can get the focal length from the EXIF. This has a formula and some information:
http://www.dslreports.com/faq/9712

You might just want to take some photos of a yardstick and relate the image size to different focal lengths yourself.

Mikefellh Jun 3, 2004 7:53 PM

There was actually a story a few years ago where a picture taken by a tourist with a FILM camera was used to determine the altitude of the helicopter they were in, before it collided with another helicopter over Niagara Falls. Since it was a fixed focal length camera, it was easy using that information and then measuring the distance and angle of the buildings with the camera source; if it can be done with a film camera, it can certainly be done with digital (especially since the focal length is recorded).

BillDrew Jun 3, 2004 9:22 PM

How acurate do you need the results? Many cameras record focal length and focal distance. I think the focal length is pretty good: 2-5% errors when meansured by doing a 360 degree pano seem typical. Distance is another issue. First, the distance can only be correct if the camera has focused on the subject of interest.

Second, it seems that many cameras have step the distance. The size of that step will be a source of error.

As a pure guess, I'd say you could come within a factor of two with just about any camera that recorded the distance. Worse at a long distance (big objects) than for close.

PJWilson Jun 4, 2004 4:18 AM

Thanks to all the helpful replies posted.

I was particularly interested in Bill's answer - "Many cameras record focal length and focal distance." I had phoned round a few shops and was told that no cameras could do this. Would you be able to give me some examples?

2-5% errors is acceptable for the purposes of this project. As the objects will typically be quite close (30-60cm), focal distance error should not be too great either.

BillDrew Jun 4, 2004 10:04 PM

My 3 year old Casio 3000 records focus distance so I figured many others made since must as well - could be wrong. The EXIF viewer I use (http://home.pacbell.net/michal_k/ -freeware) has subject distance listed as one of the parameters - though it doesn't work with my camera, a specialized one does.

I'd suggest looking at a bunch of EXIF viewers. They should/might have a list of cameras.

When I mentioned a 2-5% error, that was only the error in focal length. I expect that the distance error will be much larger.

NHL Jun 4, 2004 11:42 PM

1 Attachment(s)
PJWilson wrote:
Quote:

I was particularly interested in Bill's answer - "Many cameras record focal length and focal distance." I had phoned round a few shops and was told that no cameras could do this. Would you be able to give me some examples?
Bill is correct: my 3 years old Minolta D7 also record distance and focal lenght info in the exif as well (it's also displayed in the EVF on manual focus) ;)

NHL Jun 5, 2004 10:45 AM

Also FYI my newer and more recent Canon 10D, and it's a dSLR, does not record the distance info! (go and figure) :?:

The EXIF reader that I use is Exifier (free): http://www.exifer.friedemann.info/

Effen Jun 5, 2004 11:10 AM

Although everyone here is correct, for the purpose of presenting evidence to others, simplier is better. I would place a scale in the same view as the subject.

I work in engineering and have often had to convince people the reason behind a certain phenomena. Sometimes I need to build a case of evidence to make my point. Even with a solid math background, explaining things like focal length, EXIF headers & viewers, etc. to a jury would lose a few people and weaken my case.

I would opt for the 'scale' approach. Its basically irrefutable without claiming the scale or picture was faked.


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