[OSM-dev] OpenStreetView: Digital Photos: Getting the field of view angle

[Nick Whitelegg]

Vincent MEURISSE <osm-dev at meurisse.org> 
Sent by: dev-bounces at openstreetmap.org
16/08/2009 14:47
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Re: [OSM-dev] OpenStreetView: Digital Photos: Getting the field of view 
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On Sunday 16 August 2009 14:42:26 Nick Whitelegg wrote:
>> The thing I don't really know much about is: how can you get the
>> field-of-view angle of a photo taken from a digital camera (either a
>> standalone camera or within a phone)? 
>The formula is :
>Tan(angle/2)=size of film/(2*focal)

>for a standard film you get and a 50mm objective:
>size of film = 24x36 (diagonal=43.3)
>so angles are:
>vertical 27°
>horizontal 39.6°
>diagonal 46.8°

Thanks for the replies on this. What I'll probably do is put up my 
thoughts for how this app should work on the blog 
http://www.openstreetview.org.uk/wordpress/.

One question about something which I *think* should be possible: is it 
possible to overlay a ground (not aerial) photo on a DEM? In, say, OpenGL, 
if you set the field of view and aspect ratio to the same as the photo, 
I'd imagine you could overlay a ground photo on a DEM generated from, say, 
SRTM. There would be a number of issues, principally the inaccuracy of the 
SRTM data, plus the presence of trees, etc, but I'd imagine it would be 
theoretically possible. 

Nick

> I've seen a few explanations on the
> web on how you get it from the focal length (which appears to be quoted 
in
> e.g. the N95 specs) but they also require another parameter (the "size 
of
> the film"). Where might I get this from?
In fact you don't really need the size of film (captor size). Most of the 
camera store the focal in both real and equivalent distance.
If I take a random picture from a nikon D300 the exif information contain 
this:
Focal Length: 70mm
Focal Length In 35mm Film: 105mm
Digital zoom ratio: 1.0

So with the equivalent focal and the digital zoom, you can get your angle.

-- 
Vincent MEURISSE

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