Re: Calculating 35mm focal length equivalent

[Felix Riemann <friemann svn gnome org>]

Am Montag, den 21.07.2008, 23:11 +0200 schrieb DigiSus: 
> Hello
> 
Hey!

> Thanks Felix for implementing variant B. :-)
> 
You're welcome! :-)

> As to calculation of the 35mm equivalent; it is not so bad/difficult. I 
> think the problem is rather to get the needed data (and it is not 14 
> parameters).
> 
Pretty much yes.
The 14 (actually 15) parameters I was talking about are the ones that
could be used by the implementation in ExifTool (in the worst case). But
I've taken a closer look at them and 5 of them are specific to Canon
cameras and one is available on Olympus cameras only (MakerNote).

I think my comment on that matter sounded harsher than I intended.

> Let me give a few points so you can think about what could make sense to 
> implement.
> 
> 1) The "standard" format for images on a film role from the ancient 
> analog world is 24x36mm, having a diagonal of d_full=43.27mm.
> 
Ah, yes that explains the 36 in the different formulas. For some reason
I applied the 35mm here as well all the time. :-)

> 2) Digital cameras have very differently sized CCD sensor chips, all of 
> them usually smaller than the full format.
> 
> 3) To calculate the crop factor/focal length multiplierFLM (different 
> names, same thing), one needs the diagonal d_ccd of the actual CCD 
> sensor chip.
> 
> 4) Then, it is quite easy: FLM = d_full / d_ccd
> 
> 5) With that the measured focal length (EXIF:FocalLength) times the MLF 
> gives the equivalent focal length in 35mm film camera.
> 
This is pretty much the technique behind ExifTool. The interesting part
is determining d_ccd. 

> Problems/Solutions:
> I did not find the CCD size in the EXIF spec. Am I wrong?
> 
Right. Exif currently has no tags for the sensor size.
Some Canon cameras seem to carry this information in their MakerNote and
Olympus has a tag for d_ccd directly.
Implementations normally try to calculate it by comparing the
FocalPlaneX/YResolution tags and the image size. But the Exif specs also
warn that the FocalPlaneX/YResolution tags aren't necessarily the pixel
resolution of the image sensor. So, I'm not sure how accurate this
really is.


Felix