[Digikam-users] Don't get the 35-mm-equivalent working

[Jean-François Rabasse]

> Hello.
>
> When I use digikam with my DSLR, than there isn't any problem with the 
focal length in the meta data. I see something like "50 mm (35 mm: 75 mm)".
>
> However, now I have also a small compact camera. And when I use it, 
> digikam shows me "5 mm (35 mm: unknown)". How can I get this working?
> Is this a problem of the EXIF data that the camera produces? If so, can
> I fix this using digikam? If not: is it a problem of digikam? The camera
> is Fujifilm Finepix XP 30. The manual claims the the focal length is
> 5mm-25mm, which according to the manual corresponds to 28mm-140mm for
> a 35-mm-film.
>
> Regards

Hi Urwald,

The 35mm film equivalent focal length is not a digital camera property
but only an information that will help human minds (and photographers
minds) to evaluate a focal length value, wide angle lens, normal angle,
narrow angle lens (telelens) etc.
Some digital cameras manufacturers write this info in the EXIF section
of images files. Some don't write it as is but provide values about the
CCD sensor of the camera, that will permit recomputation. And some
metadata processing software use that.
What is needed is the number of pixels of the sensor in both dimensions,
usually "Focal plane image width" and ".. height", and the sensor
resolution (the pixels density), usually "Focal plane X resolution"
and "... Y resolution". Plus the geometrical units used for that
resolution, property "Focal plane resolution units".
(It can be centimetres or inches.)
Given that, it's possible to recompute the geometrical dimensions of
the image and compare to a standard 35mm film image, i.e. 24mm x 36mm.
And this gives the focal length ratio.

But when these informations are not available, and software just display
"35mm equivalent: unknown", all what you need for a specific camera,
i.e. a specific sensor, is the ratio information. On most cases it can
be found in the technical data sheet of the camera, or the manufacturer
web site, given the camera model.
And you have this information, your "5mm-25mm" zoom corresponds to
"28mm-140mm". This gives you a dimension ratio of 5.6 (5.6 is 25/5 or
140/25). And this is a fixed value for your camera, in all cases,
whatever your zoom setup is; you just have to keep it in mind.
So, if you take a shoot with your zoom at, say, 10mm, the equivalent
focal length will be 56mm for a 35mm film; if your zoom is at 18mm,
the 35mm equivalent length is 18*5.6 = 100mm. Etc.

Regards,
Jean-François