Originally posted by sophotec First thanks to all of you for the answers.
I know that a given focal length lense produces the same image on both cameras, the problem here is that the measuring unit: focal length on the DA lenses are expressed in their 35mm eqivalent ?
Yes, else there's no way to explain the behavior.
Or Am I wrong ?
There's no differences in 35 mm on either of the lenses. They focus the light the same. On a lens for a film frame the resulting image circle has been made to be big enough to cover the sensor, on some lenses for digital systems (rather read smaller sensors) they don't care so much for the edges since the sensor isn't that large anyway and everything outside will be cropped away anyway.
If you had mounted your Pentax FA 20-35 AL on a full frame body the resulting image circle would had been bigger than your current sensor and covered the whole film frame, and since that film frame is wider than the aps-c sensor in your current camera you would had got wider coverage on the garage of whatever it is. And you'd see more along the sides of the building.
But now since your aps-c sensor is smaller you only get the center of the image and regardless that your FA lens can draw a bigger image circle only the middle of it is used, and hence you only see the garage and not much out on the side.
Mount the DA lens and it still cover your whole sensor with light so it look the same.
I don't know how it look since I have no DSLR or designed for digital lenses but I assume that if you would mount the DA glass on a full frame body you'd get vignetting on the sides, or maybe not big enough image circle with totally dark edges, or maybe just less sharpness around the edges since it's not designed for good image quality so far out since there's not sensor catching that light anyway.
This would had been much easier to draw.
Even point'n'shoot cameras is speced with their real focal lengths, look at this Pentax Optio P70 for instance:
4.9-19.6 mm.
That would be very wide on a full format body but the sensor in that one is very small so it will only catch the center which would crop away lots of information along the sides so even if you'd be able to catch your whole garden with a full frame 4.9 mm on such a small sensor only your garage may be left.
(I hope I don't mess anything up now since the lens for the point'n'shoot isn't designed for the same lens to film/sensor distance, but the idea is the same anyway.)