Calibration Targets |
If your camera supports both Raw and JPEG images I'll need images taken in both modes. This is easily accomplished by configuring your camera to shoot both Raw and JPEG at the same time. Distortion changes when optics move. This can happen when you zoom or focus a lens. These factors imply certain restrictions when photographing a target:
Zoom Lenses. If you have a zoom lens then take about 10-12 photographs. Each image should be at a unique focal length. A separate calibration will be done on each image and PTLens will select the appropriate calibration or interpolate if necessary.
Distortion often changes a lot when zooming at the wide end. Examine your images. If there's a big change in distortion in images taken at two focal lengths then you'll need to take some filler shots at in-between focal lengths to cover this change.
Zoom lenses report approximate focal length. For example an 18-200mm lens may report 18mm, then 24mm, but no values in-between. It is important that I calibrate the endpoints. For example, if you zoomed from wide angle to telephoto and had several 18mm images, you'll know that the first one was at full wide angle and is the one to submit for calibration.
Prime Lenses. Since a prime lens only has one focal length I'll only need one image.
Focus. Calibrations in PTLens are valid for distant architectural subjects. Therefore calibration images should be taken from a distance of at least 25 feet. The resulting calibration will be reasonably accurate from 10 feet to infinity. If you're shooting with a zoom lens it may be necessary to get closer to the building for wide angle, and further away for telephoto, so that horizontal features are correctly positioned.
Target features. The target should have a flat surface with continuous horizontal feature near the top that runs end to end.
Landscape orientation. Calibration features are longer in landscape orientation than in portrait orientation. For best results photograph the target in landscape orientation.
The following features make for poor calibration targets:
No bricks. No brick buildings or brick walls as they have minor imperfections.
No top. Do not include the top of the building. This feature is often not straight.
No sides. Do not include the left and right edges of the building. Horizontal features must run from end to end.
No tilt. Do not tilt the camera. The sensor should be positioned so it is parallel to the target.
Good targets have been found with modern buildings at industrial parks, hospitals, and college campus locations.
Horizontal Features
For an accurate calibration a straight continuous feature near the top that runs end-to-end is required. This is indicated by the small red dots in the following illustration. For zoom lenses you may shoot at different distances so a horizontal feature appears at the top of each frame. Just be sure maintain a distance of at least 25 feet (8 meters) from the target.
Aim your camera so that the sensor is parallel to the target. Usually this means that the first floor of the building is in the center of the frame. Imagine a friend is standing in front of the target. Their head should be in the center of the image. If the building has glass windows you should see your reflection in the center of the frame (see above image). Although this person used a tripod, it's not necessary. It doesn't matter what is in the lower portion of the frame.
I have calibrated hundreds of lenses and often see the photographer's reflection or shadow. One of the classier examples is illustrated in the crop shown below.
