HDR and Focus Stack – Technologies to Enhance Photographic Possibilities.

 What is your intention when you take a photograph? Do you want to create a picture that gives an  impression of what you are seeing in a given situation? Do you want to create a work of art? Do you want to have something to remember? Something for documentation of an incident (or even an accident)?

Well, I think those possibilities (and maybe more) will be more or less in your mind, with different weightage due to your character, to the situation you are, to the object to be depicted, to your aims …

However, on many occasions, your picture does not manage to give you a real effigy of what your impressions in the situation have been. You may realize that there are details missing that you have been recognizing. For instance, if your impression was that of a beautiful landscape with a foreground of a dark lake or a dark pine forest, a shady rock or the like, in combination with a white, snow capped mountain in bright sunshine, you often will see that the photograph does not deliver you the details of all parts of the picture. Depending on the extent of exposure to light, you may get details on the snow combined with completely black areas on the lake or the rock or the forest, if the picture is underexposed, or you may get details of forest and lake and shady rock but an unstructured white area instead of details of the snow fields, if the picture is overexposed. However, you clearly remember having seen details on all those areas.

This phenomenon is the consequence of the fact, that the chip of your digital camera cannot cope with the same light range as compared to your eye. If you focus your glance on the dark trees, your eyes´ pupils will open a bit, so the retina of your eye will receive much more light, and you can manage to discern the details of the dark areas. If you, afterwards, focus on the snow, your pupils will maximally close, the retina receives much less light, and you can discern details even on the very light areas of your view. There are even more adaptive mechanisms apart from the pupils, on the dark side there is a second population of light receptor cells in the retina, called rod cells responsible for seeing at twilight or night, and on the light side, the sensitivity of the cone cells, responsible for hight light intensities, may furthermore decrease by far, even allowing some protection of the eye when dazzled by a flash.

What are the consequences of that for the impressions you get looking at a landscape as described above? You never look at all the details of your view at the same time. Your brain, however, makes a fusion of all the details of a given view to a virtual picture in your reminiscence, although put together by different parts with different focus of your view each of them with different eye sensitivity. Thus, your brain puts together a remembered picture composed of many different parts each  with largely different light intensities, thus coping with a tremendous difference of light exposure (BTW: this is also what a painter would do - he mainly paints what he is seeing!).

Although there are differences between digital camera chips, all of them by far cannot cope with that tremendeous span of light intensity our eyes can cope.

In last century´s Nineties, methods to overcome that problem were developed. They are mainly based on the combination (superposition) of several exposures of exactly the same motive by means of computer software. In short, three or more pictures with different light exposures are superimposed by the software program thus calculating a new image now containing detailled information on light areas by using the underexposed pictures, and dark areas by the overexposed ones. To achieve that purpose, a new picture data format had to be created, the “hdr - format”, which is capable to contain much more information about light intensity. Whereas the common “jpg – format” stores light intensity information (for each color) as an integer number between 1 and 256 (equivalent to 3 byte each containing 8 bit), the same information is stored as a floating point number in the hdr format, resulting in millions of possibilities. So, in this data format, the full range of light intensities of three or more divergently exposed pictures can be stored in one data file.

Now at this point, we however enter the next problem: Neither usual PC monitors, nor printouts or anything else can display such a wide range of light intensity. If you want to create a picture that makes use of all the information now included for the very dark and the very light areas, you have to enclose a second step of picture data processing by application of a method called “tone mapping”. Now, what´s this? “Tone mapping” reads all the information of the hdr format and creates a picture with low dynamic range (now displayable everywhere) that contains information on details of very dark as well as very light areas, altogether in one picture data file, which can be stored subsequently in tiff, jpg, bmp or any other picture data format, easily readable by your picture viewer program which now, with the narrow dynamic liht range, can display all details of light and dark areas altogether on your monitor or printout.

Now, let me explain all this with a few examples:

Here you see a photograph of Schneeberg, a huge mountain about 20 km from my home, as seen from the summit of Kienberg, in a “correct” light exposure with the automatic mode of the camera. If you move your cursor over the photo you can compare the result with another, overexposed picture. The overexposed version clearly displays the dark needle of the pine trees in the foreground, whereas the snow on the mountain is largely blurred, no details are seen there.

Once more the same picture, this time you see the underexposed version when moving the cursor over the photo. The trees now are black