Take calibration pictures

Abstract
Assembly
Illumination
Taking pictures
Examples
Creating Calibration-Pictures for shift-lenses

Important note:
Please read especially this section carefully, so that the Calibration-Picture creation succeeds with the first approach. If you go in for optimal results, then you should take a few minutes for the photographs. This is a one-time effort, which will be worthwhile for you!

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.


Abstract

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.


Assembly

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.


Illumination

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).


Taking pictures

One advantage of ImageIron is that any number of photos can be calibrated automatically, if a suitable Calibration-Set exists for the used camera.
As the distortion parameters depend on the focal length and the vignetting parameters additionally depend on the aperture, the Calibration-Map photographs must cover all lens settings as far as possible.
Though ImageIron is able to interpolate or extrapolate missing focal lengths and f-stops, the results gained with missing values will always be qualitatively inferior at not measured focal lengths respectively f-stops.

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):

  1. Adjust the camera to optimal picture settings, therefore: maximal resolution, highest image quality (jpeg image quality is sufficient).
  2. Adjust the camera to the lowest focal length / zoom level.
  3. Align the camera in such a way that a sufficient number of calibration points can be captured. Take care that the Calibration-Map detail should be kept rather smaller than too large, since the Calibration-Map edges shall not be captured if possible, and not all points need to be visible in the picture (attention: the viewfinder of some digital SLRs display a smaller cut than is actually taken). The absolute minimum quantity of needed Calibration-Points is approximately 15x12.
    For cameras with a resolution lower than 1024x768 pixels do not photograph the complete Calibration-Map. In this case a size of approximately 24x18 points is recommended.
  4. In order to get all vignetting correction parameters, take about 3-4 pictures with different f-stops for the adjusted focal length (the range of all possible f-stops should be covered as evenly as possible).
  5. Repeat steps 3 and 4 several times, while increasing the focal length up to the maximum value in 4-8 steps.

Calibration-Picture examples

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.

Dark exposure.
To a certain level decreases the vignetting-parameter quality.

Slightly angular exposure.
Can be processed.

Creating Calibration-Pictures especially for shift-lenses

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:

  1. Adjust the camera to optimal picture settings, therefore: maximal resolution, highest image quality (jpeg image quality is sufficient).
  2. Adjust the camera to the lowest focal length / zoom level.
  3. Adjust the shift-lens to shift = 0.
  4. For camera-alignment see step 3 of the previous section.
  5. With this setting take 2 or 3 pictures of the Calibration-Map. For shift = 0 it is not necessary to include multiple different f-stops.
  6. Shift the lens to the maximum right position (resp. left position, viewed from the lens front).
  7. Align the camera to the Calibration-Map analog to step 4.
  8. In order to get all vignetting correction parameters, take about 4 pictures with different f-stops for this shifted setting (the range of all possible f-stops should be covered as evenly as possible).
  9. Repeat steps 3 to 8 several times, while increasing the focal length up to the maximum value in 4-8 steps.
Continue with "Calculate calibration parameters"