Canon Camera, Руководство хакера

Страница: 225 / 257

216
C H A P T E R 7 The Stereo Data Maker (SDM)
f
You can now use the Edge Overlay to register the nearest objects. The
Deviation Line at the right-hand side of the grid’s centerline shows the
maximum deviation that is allowed for far objects. If this distance is
exceeded, you should reduce the camera spacing and redo the above
steps.
Unfortunately, this method does not work in all cases. Especially in macro
mode, your camera might not be able to focus at close distance while the
lens is in telephoto position. The SDM, on the other hand, can only capture
distances in telephoto position.
In this case, you are thrown back to using some old, well-known rules
for computing Camera Spacing and Deviation factor:
f For close-up work, the Camera Spacing should be lower than the dis-
tance between the lens and subject divided by 20. So, when your sub-
ject matter is 30 cm away from the lens, you should use a
Camera
Spacing of less than 15 mm.
f For “normal” subject distances, the Camera Spacing should be lower
than the distance to the nearest point of the subject divided by the EFL
(35mm equivalent focal length) plus 20%. For example, if the distance
to the nearest point is 3 m, and the current EFL is set to 50 mm, an ac-
ceptable
Camera Spacing would be 3000 / 50 + 20% = 72 mm.
f
The Deviation factor should be, in most cases, between 25 and 30. The
lower the value is, the larger the maximum acceptable deviation dmax
will be, and the more difficult the viewing experience will be. The max-
imum acceptable deviation is computed by dividing the image width by
the deviation factor. After entering this factor, you can control the ac-
tual deviation with the grid
Deviation_H.GRD and the Deviation Line as
described above.
If you want to play safe, you can “bracket” the Camera Spacing, e.g., by
following the “thirds” method ( Figure 7-9 ). Later, when combining the im-
ages, you can select the best pair for achieving the desired 3D effect.
c c/3 2*c/3
1 2 3 4
Figure 7-9
Given the computed camera spacing c ,
four shots are sufficient for coverin g a
stereo basis between c/3 and 2*c. By
selecting two shots from this series,
you can decide between c/3 (1+2),
2*c/3 (3+4), c (2+3), 4*c/3 (1+3), 5*c/3
(2+4), and 2*c (1+4).