IE 
23071- 
ange 
aeology, 
for this 
lution is 
mm film 
nat. This 
racy and 
vith own 
portional 
ith field 
t was for 
y sensor 
the final 
lications 
1, Canon 
ogy and 
jore the 
nd fast 
ks with 
ters). 
  
ery grip 
a 3,25 
ynductor 
ixels are 
e sensor 
' smaller 
2). Files 
pression 
:t Flash) 
90-1600. 
several 
systems, 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004 
  
^ 
continuous shooting at 3 high quality JPEG images/second, 
seven white balance modes, built-in flash, etc.) of present SLR 
digital cameras. An important aspect is that the camera has a 
Canon EF mount, so normal Canon EF lenses (and compatible 
ones) can be used. The focal lenght/angle of view conversion 
factor is approximately 1.6x compared to full-frame 35mm film 
format. Finally, it is necessary indicate that at the time the 
abstract of this paper was submitted (September. 2003), the 
camera has been updated by some higher models with a similar 
CMOS sensor but at 6 Mp (Canon, 2004). 
  
Figure 2. Sensor size comparison. A: 35mm film format and 
present high quality digital reflex cameras. B: 
Canon D30 CMOS sensor size. C: Three typical 
CCD sensor sizes in conventional “off the shelf" 
digital cameras (1/2.7", 1/1.8" and 2/3") 
3. CALIBRATION 
3.1 Lenses used 
Two lenses were available for the camera: Sigma 20 mm 1:1.8 
EX DG aspherical lens; and a Canon EF 35 mm 1:2 lens. 
Because the conversion factor to full-frame 35mm film format 
(1.6x), the equivalent focal lengths are 32 mm (wide angle) and 
56 mm (normal), respectively. Both lenses were calibrated at 
laboratory conditions. Because workspace at laboratory was 
limited, exposure mode was manual in order to select an f-stop 
setting suitable for a good depth of field with the camera 
focused at infinity. Focus rings were fixed with adhesive tape to 
maintain the inner orientation as stable as possible. 
32 Calibration procedure 
The camera with the two lenses was calibrated by means of self 
calibration (Fryer, 1992) by adjusting blocks of convergent 
photographs of a targeted test range (Figure 3). The test range 
consisted in 35 white circular retro-targets fixed at a wall and 
10 additional targets at different depths. 
Each lens was calibrated with two epochs of 6 convergent 
photographs (12 photographs per lens). As usual in close range 
photogrammetry shots with 90? rolls were taken. 
Targets were illuminated with the built-in flash in the 35 mm 
lens, but it was necessary an external flash unit for the 20 mm 
lens to avoid vignetting because the large lens size. 
The target photo-coordinate measurements were made by 
means of a routine programmed by the authors under I.D.L.® 
32. This program locates and computes the centroids of the 
targets and it is based on the optimum binarization threshold of 
the elliptical targets (after Trinder et al, 1995). 
  
Figure 3. Retro-targets in the test range. Block of 6 
convergent photographs taken with the Canon D30 
and Canon 35 mm lens. 
Self calibration was solved by means of a routine programmed 
under LD.L.& 5.2. It was a free net adjustment by minimal 
inner constraints, without any external surveyed control point. 
The mathematical model is shown in equation 1: 
B'WB  B'wB B'wB olA |B'we 
B'wB B'wB c'A B'we QD 
B'WB oj|À B'Wz 
| symmetric 0 Ik 0 
where: B: design matrices (after lincarization of 
collinearity equations) 
A: unknown corrections 
W: the photocoordinate weight matrix 
€: discrepancy vector. 
G: Helmert matrix 
k: 7x1 vector of lagrangian multipliers 
Matrices quoted with one dot (-) are related with outer 
parameters, (^) object point coordinates and (-) inner 
parameters. Since network is free, the rank deficiency of 
normal equation matrix is overcome by the use of the seven 
constrained equations grouped in the G matrix (Atkinson, 
1996). 
3.3 Calibration results 
Self calibration was applied using a block invariant model 
(Fryer, 1992). The adjusted inner parameters were the principal 
distance (c), principal point offset (xo, yo) and the first and 
second radial symmetric distortion coefficients (Kj, K5). The 
first one, Ki, was enough for the Canon 35 mm lens, while 
distortion in the Sigma 20 mm lens was better reproduced by 
both K, and K; coefficients. Our previous experiences have 
shown that others higher order and decentering distortion 
coefficients were not significant for the tested lenses. Affine 
parameters or other additional parameters were not taken into 
account. Table 1 shows the result of the self calibration for both 
lenses.