How do I take calibration images for a short focal length lens?¶
Short focal length lenses (24mm to 100mm) requires a process involving at least 12 images from six different locations to produce an accurate and stable calibration which can be used for future projects.
For any camera, the lens distortions are not uniform even with the same make due to imperfections in the manufacture process of both the camera and lens, the bundle adjustment results are usually worse when trying to use another calibration from a different camera and lens combination than just using distortions set to 0 for an uncalibrated camera. We therefore start with just the focal length of the lens to be calibrated and use calibration imagery to map the lens distortion characteristics. It’s important to have an accurate camera calibration because any errors in the calibration will flow through the whole project and will affect the generated DTM.
Zoom lens vs Fixed/prime lens¶
The zoom setting on a zoom lens has a significant impact on the calibration so you need to ensure that the zoom is consistent with that used for any future projects. Generally speaking, this means that only the minimum and maximum zoom can be used unless taping the focus ring using duct tape reliably fixes the zoom setting. (On modern lenses this is generally not the case as there is no mechanical connection between the zoom ring and the lens itself.)
A fixed or prime lens has a simpler optical path and generally produces sharper quality imagery over a zoom lens, with the added benefit that you don’t have to worry about setting the zoom.
Camera Settings for calibration and projects¶
Set aperture mode to Aperture Priority Av.
Select f/8 as aperture setting.
Set ISO to 100 (Canon cameras).
Set White Balance as AWB (Auto White Balance).
Set focus type in the lens as AF (Auto Focus), focus on a distant object and switch to MF (Manual Focus) to take the imagery.
Set Metering mode as Evaluate Metering.
Set Auto Image Rotation OFF.
Set image quality as fine.
Short range - Triangle method¶
You should calibrate for object distances that will be used for typically close range strip or convergent model projects.
Find a scene for the calibration that contains various different depths to aid the camera calibration process. Limit the amount of sky or terrain parallel to the field of view in the photos when looking through the viewfinder as no relative-only points will generate there. The object distance spreadsheet will compute the ideal object distance and resultant accuracy for the lens for planning purposes.
Twelve photos will be taken in total; 2 from each camera station position (a landscape image, then rotate the camera 90 degrees to take a portrait image covering roughly the same area). Use a camera tripod to take the imagery to avoid motion blur. The six camera stations will form an approximate triangle:
Object Distance¶
Use the Object Distance spreadsheet to calculate the hyperfocal distance for the lens and camera you wish to calibrate.
Station 1 should be at least that distance away from the scene. Before taking any imagery, switch the lens to AF (automatic focus), press the shutter half way down a few times to focus the scene. Once the view looks sharp through the viewfinder, switch the lens back to MF (manual focus) and do not bump the camera or lens. With the camera and lens set correctly and in focus, take a landscape image of the calibration scene using the tripod and camera mount. Next, take the camera out of the mount and carefully rotate it 90 degrees to take the portrait image of the same area without bumping the camera or lens. You can rest the camera body on the mount in order to keep the camera still whilst you’re taking the portrait photo. The view should still look sharp when looking through the viewfinder.
Move 1/5th the object distance as shown in the above figure to camera station 2, before taking any imagery refocus the camera using AF and then switch back to MF once you’re ready. The view should look sharp when looking through the viewfinder of the camera. Take a landscape and portrait photo covering the same area done in station 1. Repeat this process for all 6 stations forming a rough triangle configuration as shown in the above figure.
If required, the calibration imagery can be sent to ADAM Technology for processing for a fee. ADAM Technology will then perform the camera and lens calibration. Providing the imagery has been taken correctly according to the above instructions, a .cal file will be produced that can then be read into either 3DM CalibCam or 3DM Analyst.