Calibrations are based on images submitted by users. Distortion changes when lens elements move and this can happen when you zoom or focus. ConsequentIy images should be taken at a distance of at least 25 feet (8 meters) so the correction will be accurate for architectural subjects. Only one image is required for prime lenses but for zoom lenses 10–12 images, each at a different focal length, is appropriate. Extra images may be needed near the wide end as that's where distortion changes the most. When processing images, and a non-calibrated focal length is used, PTLens will do a linear interpolation between the nearest calibrated values.
A suitable target contains a single straight-line continuous feature that runs from end-to-end at the top of the frame in landscape orientation. Modern buildings work well. Brick walls are a no-no. The red dots illustrate points on the image where distortion has been measured. Excessive tilt should be avoided but a small amount of tilt is okay to bring the feature to the top of the frame. No tripod is necessary. Note the selfie.
Today many cameras automatically correct native lenses for distortion in JPEG images. For example, some Nikon cameras automatically correct for distortion in Nikon lenses. Third party lenses such as Sigma or Tamron and Raw images are not corrected for distortion. Note that Raw images, when opened by a developer such as Photoshop, may also be corrected for distortion. Other developers, such as RawTherapee, do not correct for distortion.
To cover the bases I ask that users submit both JPEG and Raw images for native lenses. For third party lenses only JPEGs are needed. JPEG images should be downsized to 2000 pixels wide, medium quality. If you do this, and there are no Raw images, you can send them via email as an attachment. Otherwise you can post them to the cloud, such as dropbox, and furnish a link. Be sure to check that EXIF information is intact. A 2000 pixel width is sufficiently accurate for normal use. Consider an error of 2 pixels in a print 1 meter wide. The error would be 2/2000 x 1 meter, or 1 mm.
Over the years I have calibrated a multitude of lenses and it has been my privilege to scrutinize the facades of many buildings. From time to time I can see the photographer in a reflection or shadow and this is one of my favorites.