Focal Length and Depth-of-Field
The focal length doesn't really change on digital cameras but humans
tend to be
confused by the given focal length which simply doesn't fit to our
experience based on the classic film format. Things get even more worse
because of the various image sensor formats (and therefore different
designs). Therefore most manufacturers or magazines provide the "35mm
format equivalent" for the lens system in order to allow a comparison
between the different formats.
Calculating this equivalent is fairly easy. The 35mm format has a
of 44.3mm (36x24mm) - remember good ol' Pythagoras. e.g A 2/3" sensor
has a diameter of 11mm (8.8x6.6mm). Therefore the "magnification
factor" is 44.3/11 ~ 4x.
A Sony F828 features such a 2/3" sensor combined with a 7.1-51mm
lens. So 7.1mm*4 and 51mm*4 translates to 28.4-204mm as the 35mm
Here's an overview for the various image sensor formats:
| 35mm full
|| Canon EOS 1Ds
| 4/3 System
|| Olympus E-1
|| Sony F828
|| Fuji F700
Things are easier regarding the depth-of-field - there's no
magnification factor here so e.g. a 7.1-51mm f/2-2.8 (Sony F828) has
same DOF like a film format lens with these specifications.
Consequence: the "200mm" f/2.8 setting of a Sony F828 does NOT provide
the same depth-of-field potential like a native 200mm f/2.8 film format
lens! Therefore claims that such (current) pseudo D-SLRs may be a full
substitution for "classic" D-SLRs (with a APS-C or 35mm format sensor)
are a little optimistic as of now.