What does it stand for?:
HDR stand for High Dynamic Range, this is a large amount of different shades. One of the mayor disadvantages of digital photography is the lack of dynamic range, which means that the number of grey values or the differences in contrast between pure black and pure white are limited, a lack of tonal range. One of the reasons for this is the file format of jpeg, this is a 8 bit fileformat which can only hold a maximum of 255 : 1 tonal ranges. With a 16 bit fileformat the number of (grey) values increases to 65.535 : 1, which is far more then the human eye can distinguish, the eyes are limited to registering only 10.000 : 1 tonal ranges depending upon the light conditions, in reality the sunlight holds as much as 100.000 : 1 tonal ranges or more. A 16 bit fileformat would suffice to store more values then the human eye can register, but because of the fact that there have to be 3 colours registered the file format would rather have to be in the range of 32 bit to be able to have no loss of information. However the next limitation is determined by the tonal range of the ccd or cmos sensor which is a maximum of 400 : 1 which is also the maximum range of a 12 bit raw file, so with a raw file you get the most out of your ccd or cmos sensor. But this is still far from what the human eye is able to register. If you want to get more out of your photographs you wil have to make use of other techniques.
What can be done?:
With high contrast scene's like with photographing towards the sun there's a range of options:
The grey gradient filter, the polarizing filter, the flash, reflection screens and the digital darkroom.
The grey gradient filter:
A grey gradient filter has a gradual or sharp defined transition from normal clear glass to a grey value of 1 or 2 and sometimes even 3 stops. You place this filter in front of the lens with the grey part situated at where the high contrast area starts, with a landscape photograph this is usualy the horizon, this is also the subjetc (landscape photography) to which the grey gradient filter is applied the most. The filter lessens the difference between the dark en light areas thus providing for more tonal ranges and making sure that more detail is contained in the light and dark areas, the endresult is very pleasing to the eye. The effect of a grey filter can to some extent also be mimiced with the digital darkroom. A drawback of a grey gradient filter is that it can filter out the amount of light thus leading to longer shutter speeds unless you can compensate for it by choosing a higher iso setting. There are also grey filters without a gradual transition, they have a function in creating scenes with slower shutterspeeds and to enlarge tonal range. They can for instance be used in photographing water movement (rivers, waterfalls), the slower shutterspeeds you get in this way make the movement of the water flow together, creating a mist or haze instead of freezing the movement. This prevents water scenes from becomming clogged with messy movement. In this way you get to see the water movement in a way that you will never percieve it with the eye.
The polarizing filter:
The use of this filter lies not so much in the fact that it increases the dynamic range, but it prevents glare thus creating more colour throughout the scene, but in a certain way this also diminishes the contrast because the glare often appears blown out. But as a matter of fact it also works to highten the contrast especially in the sky.
Although the flash is mostly used to profide enough light in a dark situation so as to still be able to photograph, it can also be used to lighten up dark spots in a backlight situation. In this case the flash is used as fill flash creating a lot more detail in the dark parts of the scene. Usualy there is also an option to vary the amount of light the flash emits by either lessening or hightening it. When photographing close to a subject/foreground you can lessen the amount of light to prevent dropshadows, the same thing can be achieved by using a diffuser, whenever the subject/foreground is further away you can key up the amount of light.
These types of small screens can especially be usefull in close-
Enhance photographs with the aid of software:
Whenever you keep in mind that whilst photographing you try to keep the highlights correctly exposed so they won't become blown out, one can often regain the details in the underexposed parts of the photograph with the aid of software. With washed out highlights this will never be possible. On some camera's there is a nifty function which colours the highlights black whenever they are washed out, now by underexposing the scene until nothing black shows up in the viewer anymore you can determine the right exposure needed for editing the photograph. In case of (to much) underexposure one can retouch the photograph pretty well with the use of the level tool. The level tool shows a graph with the distribution of the light. In this case the graph would be situated to much to the left indicating the presence of to many dark areas. The thing to do now is to drag the right slider towards the left where the graph starts thus deviding the light more evenly troughout the photograph. You can also play a little with the brightness, contrast and gamma values by alternatingly increasing each of them. In this way you can get back a lot of detail, although there is ofcourse a limit to this as well and there's the fact to consider that dark areas are more prone to noise.
Then finally we come to the section where there is realy mention of HDR photograqphy:
Real HDR photography means stacking photographs, each with a different exposure, on top of eachother. This is very easily achieved with a camera that supports bracketing. Bracketing means making a series of photographs in which you underexpose 1 or 2 photographs, expose one as should be and overexposing another one or two. This can be done with 3 or 5 consecutive photographs. 5 photographs yielding the best result in which you have 1 photograph underexposed by 2 stops, 1 underexposed by 1 stop, 1 correctly exposed, 1 overexposed by 1 stop en 1 overexposed by 2 stops. The underexposed photographs hold the right details in the highlights and the overexposed ones contain the right details in the dark areas. It can also be done in smaller steps of 1/2 or 1/3 stops. A camera with bracketing will perform this automatically for you. Working in this way it is absolutely essential that you prevent movement and that he frames fit excactly on top of eachother so a tripod is indispensable here. In a photo editing program you stack these photographs on top of eachother, for this purpose the software should hold a special HDR function where you have to state how much under/over exposure was applied (1/3, 1/2 of 1 stop). The software now determines the endresult which can be adjusted to some extent. The result is a photograph with a lot of detail in the dark and light areas and with a well-
Nowadays there are camera's with a HDR function build into it, it automaticaly brightens the dark parts and darkens the brights parts even without having to take a series of photographs. This saves you the trouble of having to enhance the photographs afterwards, but you should keep into consideration whether the results wil be equaly as good as when you would have enhanced the photograph with a photo editing software, probably not, because with editing software you have more control over the endresult and you have far more computing power.
In the context of HDR I want to mention one more thing about a program called AutoHDR, it is a small executable with large results. One can use it to convert a series of photographs into a HDR, with even the possibility to line-