MULTIPLE FOCUS CALIBRATION OF A STILL VIDEO CAMERA 
Mark R. Shortis Stuart Robson and Tim Short 
Department of Geomatics Department of Civil Engineering 
The University of Melbourne City University, Northampton Square 
Parkville, Victoria 3052, AUSTRALIA London ECIV OHB, ENGLAND 
Telephone : +61 3 9344 6806 Telephone : +44 171 477 8000 
Facsimile : +61 3 9347 2916 Facsimile : +44 171 477 8570 
Email : M.Shortis @unimelb.edu.au Email : S.Robson/T.M.Short@city.ac.uk 
ISPRS Commission V, Working Group 2 
KEY WORDS : Photogrammetry, calibration, CCD camera, close-range networks, extended lens model 
ABSTRACT 
Still video cameras have been widely adopted for close range photogrammetry and machine vision applications. Due to 
the advantages of onboard storage of digital images, portability and rapid data processing, still video cameras are 
replacing medium format film cameras for measurement tasks such as heritage recording and industrial metrology. As 
for any photogrammetric application, the accuracy of the derived object data is dependent on the accuracy of the 
camera calibration, amongst many other factors. For the vast majority of photogrammetric applications, use of a simple 
model of lens distortion in conjunction with the collinearity equations is sufficient. However, the combination of very 
close ranges and the large distortions typically associated with the lenses used with still video cameras requires an 
extended lens model to account for variation of distortion within the object space. The fidelity of the calibration model 
becomes particularly important where stringent tolerances are set, for example in aerospace inspection tasks. This 
paper reviews previous research into distortion variation and outlines an initial investigation of the extent of distortion 
variation for two lenses used with a Kodak DCS420 still video camera. 
1. INTRODUCTION and backs such as the Kodak DCS460 and Rollei 
ChipPack have CCD arrays up to 2000 by 3000 pixels in 
Close range photogrammetry has rapidly embraced the size. Target image location algorithms, such as the 
new technology of still video cameras. Until recently, weighted centroid, have theoretical accuracies of 0.01 
videometric applications were limited by low resolution pixels, and internal precisions of 0.02 pixels have been 
CCTV systems which require a dedicated computer or achieved for many videometric applications using self- 
video recorder for image capture. Still video cameras are calibrating networks of many camera stations. In theory, 
now available with medium to high resolution sensors, the current limit of object space precision for networks 
substantial on board image storage capability and of ^ captured with still video cameras is approximately 1 : 
course complete independence of recording devices. 300,000, which is approaching the 1 : 500,000 image 
These cameras are as portable and reliable as the film space precision which can be readily realised by the 
cameras they are replacing, with the added advantages of ^ combination of large format film cameras and precise 
automated image measurement and rapid data processing image comparators. 
(Fraser and Shortis, 1995). 
High resolution still video cameras are not yet 
Reported videometric applications of these cameras competitors for large format film cameras because the 
include architectural recording, large scale engineering discrepancy between the theoretical and actual accuracy 
metrology, low altitude mapping and tool inspection for is wider than the discussion above would suggest. 
the aerospace industry. This catalogue of uses will Whilst internal precisions of 0.02 pixels are routinely 
certainly increase as the cameras become more widely reported, independent checking of photogrammetric 
available, increase in resolution and become more cost networks utilising still video cameras indicates that the 
effective. It is clear that in the near future only those internal precision is not always a reliable indicator of 
aerospace applications with the most stringent tolerances external accuracy (Shortis et al, 1995). Relatively few 
are likely to remain the province of large format film applications have incorporated an independent 
cameras. measurement of the target object space coordinates, 
generally supplied using film cameras, theodolite 
However, even the high accuracy domain is under threat systems or coordinate measuring machines. Although 
due to advances in sensor technology and target image there is always an element of doubt associated with such 
location algorithms. High resolution still video cameras comparisons due to the implicit assumptions of object 
534 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
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