Tutorial: Calibrating the Lens

4.1. Tutorial: Calibrating the Lens

Problem

Standard zoom and wide-angle lenses exhibit significant distortion of the image. Straight lines at the edges of the image can appear curved. Lens distortion effects can impact on the accuracy of the modelling process.

Solution

3DSOM Pro can automatically recognise many standard lenses using EXIF tags in JPEG images — information included in the images by the majority of digital cameras. For lenses in the database, 3DSOM Pro will automatically correct for lens distortion.

For undetermined lenses, 3DSOM Pro provides a Lens Calibration Wizard to allow the user to take additional photos for automatically calibrating the lens. Once the lens has been calibrated, new photos will be automatically undistorted when they are loaded from disc.

In this tutorial we will show how to calibrate a zoom camera lens.

Starting Point

Launch the Lens Calibration Wizard using the File > Calibrate Lens... menu item. When the Lens Calibration Wizard Window is opened choose the first option, "New calibration", to calibrate from photos. Then click the Next > button to proceed to the next page of the wizard.

Step 1

The first step is to print out the lens calibration grid (a regular grid of dots) onto a plain sheet of white paper. Hit the 1. Print calibration grid button to open the print dialog.

Ideally, you should try to print out a lens calibration grid roughly the same size as a typical object you will be modelling. That way you can take photos of the grid at a similar distance to images you will take of your objects. However this is not essential.

Hit the "Change..." button to modify the page setup and margins allowing you to print larger grids over multiple pages. Hit the OK button to print.

Step 2

Stick the lens calibration grid onto a flat surface such as a rigid piece of card or a wall. If you printed over multiple sheets be careful to line up the sheets accurately using the guides.

To calibrate a fixed focal length lens you need to take up to 3 front-on shots (at an angle less than 10 degrees).

To calibrate a zoom lens you need to take front-on shots of the grid at a range of zoom settings. Note that if your camera does not export JPEG images with EXIF tags you will need to fix the zoom setting for all photos and treat as a fixed focal length camera.

General points to note...

Make sure the grid pattern is flat.

Make sure the camera view is reasonably front-on so that there is not much perspective distortion.

Make sure the dots fill the whole image.

You don't need to see the whole grid but make sure each visible row or column includes at least 6 dots.

For zoomed-in shots you may need to move the camera away from the grid pattern.

Take one shot for each zoom setting.

Use a tripod if available.

Example photos for a digital compact zoom camera are shown in Figure 4.1.1.

Figure 4.1.1. Images taken at focal lengths of 5.8mm, 9.2mm, 18.4mm and 58.0mm

Step 3

Once you have taken the calibration photos, transfer them to your computer and load them by hitting the 3. Load photos button.

Step 4

Once 3DSOM Pro has processed the photos you will be presented with the following page.

You can see the input photos and undistorted images resulting from lens calibration. The dots should now appear in precise straight lines.

You can save the calibration data for future projects using the Save button. To reload a calibration you can use the "Load existing calibration settings" option when the wizard is launched.

Hit the Finish button to continue with your project. Imported photos (using the New project... button or the Images > Add Images... menu item) will now be lens corrected using the results of calibration.

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