Data in Skin Diseases: Distortions

• Even if efforts are made to standardize the image collection (same position), there are distortions which can arise. These can sometimes be corrected but it is important to be aware of them.
• But you can have distortions added to the images and these distortions can be optical or can be perspective distortions. And basically the classical optical distortion is also called the lens distortion.

• Optical distortion
  • And as you can see on the bottom right, the woman looks totally distorted with a 50 millimeter lens from a photo camera.
  • And that distraction really bothers you when you see it instead with a 350 millimeter lens, she’s still distorted, but it’s a distortion that you accept, your mind accepts the most.
  • But actually the way she really looks, it’s more like here on the bottom left with a 70 millimeter, between 50 and 70 millimeter lens.

• You can get, of course, optical distortions also with mobile phones and you usually get them when you get very close to the subject. That’s why you always need to get away from the subject, at least two meters to get the proportions, the correct proportions. Otherwise the same fellow looks with a huge nose in comparison to the way he really looks.

• Perspective Distortion
  • Perspective distortion has to do with the position of the camera in relation to the subject. And you can see how… And perspective distortion is probably a little bit easy to correct with softwares afterwards. The best photo is the one that is good from the beginning, that you don’t need to go in post-processing, but still there are ways nowadays to correct perspective distortion.
  • The presenter shows an image: a very distorted image where the hand is smaller than the eye.

Paola Pasquali, MD. Digital Photography for Teledermatology Systems and Devices. 8th World Congress of Teledermatology, Skin Imaging and AI in Skin diseases – November 2020