Image Conversion Factor
Since image size is smaller than 35mm film format , It will look like the lens focal length is increased by Conversion Factor.
For example , A canon 28-135mm f/3.5-5.6 IS USM Lens And With 40D Body, The Conversion Factor Is 1.6 .
The Focal Length will increased by 1.6. 28mm X1.6 - 135 mm X 1.6=44.8mm - 216mm

Camera
Conversion Factor
Canon EOS-1Ds Mark III          1
Canon EOS-1D Mark III         1.3
Canon EOS  5D                        1
Canon DSLR                           1.6
Nikon D3                                    1
Base Nikon DSLR                 1.5
Base Pentax DSLR               1.5
Base Panasonic DSLR         1.5
Base Sony DSLR                   1.5