How it was done
Considerable effort is behind each of the photo pairs. In order to encourage similar projects, we summarize the steps which lead to the following photo comparisons.
- The photos should be repeated from exactly the same position. This may be difficult after many years. Even when taking the follow-up image from a building, such as an Alpine Club hut (which is a good marker and easy to remember), it may be difficult to recall exactly where the first one was shot. Or things may have been built which are now in the way. If the old location is no longer exactly known it may be possible to reconstruct it if a print of the first photo is taken into the field. Careful "triangulation" of landmarks will confirm that the old location is reached. Some of the image pairs presented here somewhat suffer from slightly different camera locations.
- Ideally the photos are taken at the same time of day in order to reduce changes in illumination. A cloudless sky would be best, but may be too much to ask for...
Digital image processing
- To achieve the results presented here, the second image was precisely scaled to the first one, as camera lenses used were not exactly identical - not surprisingly, after 25 years! This was done by measuring distances between clear reference points within each image and comparing the two values.
- Then the second image was rotated to achieve the exact same orientation as the first (the camera is never exactly horizontal). This was done by overlaying one image above the other, but setting the upper level to a suitable degree of transparency.
- Then the images were cropped to the exact same size, only using the part visible in both images.
- The older image was also de-vignetted, i.e. the corners were made brighter by use of a radial filter to compensate for the effects of an older lens, which suffered more from vignetting than the newer one.
- Then colours and contrast were adjusted to achieve - as much as possible - similar values. This proved very difficult for the sky which was usually treated separately from the landscape. Changes in film technology and development were very evident during this part of the work. The photos shown here may still look rather different in colours - but you should have seen them before they underwent this treatment!
- As observers usually perceive different things when comparing images side by side or when switching from one image to another in overlays, both methods of presentation are used here simultaneously.
- The upper images are identical, but scaled to 50% of the lower ones in order to minimize download time.