Focal Length and Depth-of-Field
Technology - Technology
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 lens 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 diameter 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 equivalent.

Here's an overview for the various image sensor formats:

Sensor Magnification factor Example
35mm full format 1 Canon EOS 1Ds
4/3 System 1.97x Olympus E-1
2/3" 4x Sony F828
1/1.7" 7.5x Fuji F700

Depth-of-Field

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 the 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.

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