Take calibration pictures
Creating Calibration-Pictures for shift-lenses
In order to test the function of ImageIron, the photographs can certainly also be made in the quick-and-dirty style, either with a DIN-A4 Calibration-Map printout lying on the floor, or a full screen view of the Calibration-Map on a flat screen (not recommended if good chromatic aberration correction results are needed). Thus astonishing good results can be achieved also.
Generally the lens aberration is different for each combination of camera and lens. Due to this you must create one Calibration-Set per camera/lens combination for e.g. a SLR camera with exchangeable lens.
In this chapter you will find a step-by-step description on how to create a set of Calibration-Pictures for a camera/lens with variable focal length, which can be automatically processed by ImageIron later. The procedure accordingly simplifies with fixed focal lengths.
You can lie the Calibration-Map onto a solid surface or mount it on a wall. Especially for photographs with small zoom it is favorable with a paper printout to let it lie planar on the floor. This way the formation of wrinkles is minimal, which particularly have an effect with photographs in the wide-angle range.
For optimal vignetting parameter results it is important that the Calibration-Map is illuminated as uniformly as possible. It is recommended to take the photographs out of doors under moderately bright daylight. Alternatively a continuous source of light, for example a tungsten-halogen light, can be used. Ensure that the light incidences laterally, in order to avoid direct reflections.
If however the vignetting parameter quality is not important, then the lighting can be neglected. For the distortion parameter calculation it is absolutely sufficient, that the black points sufficiently contrast with the brighter background.
The best results can be reached, if the exposure measurement is performed by the camera automatically (so that the white Calibration-Set background ranges from a middle to a bright gray on the picture).
One advantage of ImageIron is that any number of photos can be calibrated automatically, if a suitable Calibration-Set exists for the used camera.
ImageIron extracts the focal-length and f-stop values out of the EXIF-data automatically, therefore it is not needed to make notes during the take.
It is recommended to proceed as follows (pdf-version to print):
Indeed following Calibration-Pictures are not ideal, but can still be processed due to the tolerant recognition engine:
Calibration-Map edge visible.
Can be processed in all probability.
Small number of Calibration-Points.
Causes less precision, but is needed for pictures with an image resolution lower than 1024x768 pixels.
To a certain level decreases the vignetting-parameter quality.
Slightly angular exposure.
Can be processed.
ImageIron supports the calibration of pictures, which were taken with shift-lenses. A shift-lens is shifted parallel to the plane of projection, whereby also the optical axis shifts the same amount along the CCD-sensor, making a new cutout of the lens projection visible in the photograph. The Calibration-Set, which must be created for the correction, therefore additionally contains a measure for the distortion center shift as a function of the shift-value.
If you use a tilt/shift-lens, then please note that ImageIron does not provide a full support for the tilt feature. Thus a separate calibration set must be created for each tilt-angle, since an adjustment of this value also causes a nontrivial distortion change (with tangential portion).
When building a Calibration-Set for shift-lenses, you must note, which pictures were taken with shift, and which were taken without shift, because this setting is not contained in the EXIF-data.
With the aid of this checklist (pdf) it is recommended to proceed as follows:
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