With the term real HDR i mean the standard form of the high dynamic range (HDR) technique to control all tonal ranges in an image -- i will stay away from the well-known "artistic" effect of HDR tone mapping exaggeration the web is full of. I will expose a way to use GIMP to merge images captured with standard shots, to produce a final image with a controlled tonality on the three general tonal ranges: shadows, midtones, highlights.
Exposing the technique for the simple case basically, i am going with an example of combining data from three shots. Theoretically any number of shots can be used, but that would be a tedious task that might not be needed. My intention is to illustrate a fast and effective method to surpass the inherent technical disability of your typical photocamera to capture a wide exposure range on one shot, so as to produce images that you want to be tonally perfect.
This HDR technique is meant to be enganged under certain situations, if your experience tells you that tonal control is critical, like in landscape photography or commercial photography, in order to produce an important image that might be evaluated and appreciated from every aspect. The basic problems of histograms as clipping and gaps mentioned in Lesson 5 can be eliminated. With this technique it is possible for you to produce professional grade images, using simple and ordinary technical means -a camera on a tripod- in any light conditions.
This is not the right place to present the theory of HDR and bracketing. You can find on the web numerous resources to delve. But even if you do not know much, you will get the point of HDR.
So, what is the point?
When you shoot a frame that contains areas under high contrast, it is naturally hard for your camera to capture the whole dynamic range of the frame on the same shot -- it is the so-called LDR, low dynamic range, or STR, standard dynamic range. This happens because the outcome of the exposure metering process is not equally adoptable across all tonal levels. The camera's sensor works best on the midtones, but not best on the extremes: the shadows may get underexposed (too dark) and/or the highlights may get overexposed (washed out). So you may have a good detail in midtones but you may lose detail in the shadows and/or in the highlights. This is a technical disability of the sensor, period.
Still more, there is another aspect as well. In reality, it does not only have to do with the objective side of your equipment, it has to do also with the subjective side as to the way your eye works. The human eye adopts automatically to luminosity levels, so we do not perceive luminosity levels at will. In a sense with HDR we go a step further on our optical function, from what our eyes see to what we could see or what we want to see inside a single frame.
So someone somewhere somehow had this clever idea as a technical possibility: to obtain one shot for every tonal range adjusting the exposure so that the most detail is captured for every tonal range, and then to find a way to selectively use these areas to produce an image, controlling the luminosity of every one of those tonal ranges by selecting their source shot. So in the simplest scenario of three shots, we take:
- one shot with normal exposure,
- one shot 1-3 exposure steps underexposed,
- one shot 1-3 exposure steps overexposed
and then:
- from the shot with the normal exposure, we pick the midtones,
- from the underexposured shot we pick the highlights, and
- from the overexposured shot we pick the shadows.
Before shooting: things you should know about HDR
- All three shots should be better made on a sturdy tripod on one session, to ensure a perfect match of the three images. If not, later on your computer you will probably have to match the images -if possible- by moving and rotating layers, something that might be a very tedious task.
- Due to the same necessity of perfect matching, HDR can be used only in static subjects. Any movement in the frame creates the problem of shadowing or ghosting. For example in landscape photography, trees moving because of the wind make multiple shot HDR shooting rather impossible.
- As for the exposures, modern cameras afford a so-called auto exposure bracketing function, to help take a given number of shots automatically and with a given exposure step. Sometimes professionals oriented towards perfect result do not rely on this exposure step approach, instead they define the exposures with spot metering focused on certain areas inside the frame.
- Some contemporary devices afford a more sophisticated function for taking a "HDR photo on one shot", actually combining three shots automatically to yield a file. For best quality, it is suggested to deactivate this function, because it is better to control the HDR process by yourself afterwards on your computer.
- The varying exposures must be taken with the same aperture size (f/stop) -- that is to say, varying the aperture speed. This is necessary just to have the same "actual picture" based on the same lens's depth of field.
After shooting: ready for action?
Having again the intention to simply illustrate the method, my example is going to be the simplest possible, with three simple JPG files taken with my simple camera, out on a simple setup in the morning sun. So having taken my three shots, let me open them in GIMP to do the job. It should not be more than a matter of minutes.
No comments:
Post a Comment