Interpretation of focal length and film gate

[prima]

Hi everybody, I'm in trouble with camera match (doing it manually).

I always end up with misaligned perspective lines.
By misaligned I mean that they tend to match, quite
close, but never perfectly.
Now, the parameter I'm unsure of is "Film gate",
which, by vray manual, is this:

"Film gate - specifies the horizontal size of the film gate in milimeters.
Note that this setting takes into account the system units configuration
to produce the correct result."


I've set my system and file unit to centimeters.

From that I assume that to match my digital camera,I need to read
the specifications of the sensor and type its horizontal size (the x axis, to say).
It says 5x4 ratio. Ok, I now look at the pictures taken with the
camera and they are 5443x4008. They are quite close. But they are not the same. First question: why?

Anyway, by now I tried putting both values:

- "50" (as by the manual)
- and "54,43" which is the exact horizontal width of the images saved by the camera.

None of them is right, since I always have to adjust focal length to get closer to the original
perspective lines.

The focal length of the camera I'm talking about is 35mm, and it is fixed.

So when the images are matching the focal length goes up to 38. Which is not 35.

There must be something I ignore or misinterpret.


Any help or considerations?

Thanks

[anders]

let's say you have a canon 550D with a "22.3 x 14.9 mm CMOS sensor"
http://www.dpreview.com/reviews/canoneos550d/page2.asp

and a 50 mm lens
then you would set your film gate to 22.3mm and your focal length to 50mm

hope this helps!

[plastic_]

With lenses, the focal length is only valid at infinity position. As you focus closer it shifts.
For this reason the lens focal length specification is more or less useless for camera tracking. Besides, many manufactures "cheat" by rounding up or down to a nice number. It is also possible that in-camera processing corrects distortion and crops the image without telling you.