The lens focal length and focusing distance of a photo are recorded in the EXIF data of your images. However, this information is not always accurate. For example, different but close positions of the focal length ring (say, 17 and 18 mm) could result in the same value (say 18 mm) being recorded in the EXIF data. In this case, the distortion correction may be less than optimal. In the same manner, the focusing distance might be recorded in the EXIF data with insufficient precision, and similarly lead to an imprecise correction. In both cases, to improve the effectiveness of the optical corrections, you can provide more accurate values in one (or both) of the sliders that appear in the Geometry palette:
Who can tell with such an accurate precision Focal length and Focusing distance for a picture ???
For focal length, the display on focusing ring is imprecise and for focusing distance, you can measure it in close photography.
Any way the displayed value is recorded in EXIF data and I can’t understand !
More, I don’t think that camera/lens tests are every 10 cm distance ou every half millimeter focal length !
With 14-24 zoom on FF camera the difference in distortion @14mm and @15mm is noticeable (about 0.35% in my case). The Focal length slider lets you fine-tune the correction according to your judgement or goal. It may be important in those few cases, where you want the correction to be perfect.
They interpolate. To be more precise, they probabably use the measurements to get several coefficients, which are used to calculate the correction, the new coordinates. The actual lens model used is probably their secret.
Focal Length and Focusing Distance are needed to compensate compromises that lenses exhibit. DxOmark.com has a DB with measurments of lenses, one of them having the following characteristics:
This lens’ distortion varies with zoom setting. Distortion, CA etc. can also vary with FL, and on some of my lenses I can see how distortions change when I drag the FL slider.
Lens characteristics are measured according to what has been published in DxO’s blog…which seems to have moved away from where it was found before. Some info can be found - with a sales pitch - in here: DxO MODULE - DxO
It means that, amongst useful information, they want unsuspecting readers to get into PL or PR. DxO seems to be desperate and/or obnoxious with placing sales traps on almost every page of their website. It’s their right to do so, but we could debate style, endlessly, but I won’t.
Nevertheless, the info given helps to understand DxO’s way to create their modules.
