There are situations in photography in which the dynamic range of the subject is beyond the capacity of any film or sensor; the range from dark to light is just too great. Faced with such a situation, you have a number of choices: you could give up and look for an easier subject; you could take an average exposure, knowing that neither the highlights nor the shadows will be right, but at least you’d have something to remember it by; or you could choose between the right exposure for either the shadows or the highlights, knowing that the other extreme will be lost.
A choice between black, muddy shadows or burnt-out highlights is not much of a choice
Actually, there are three options I haven’t mentioned:
graduated filters, simulated graduated filters and High Dynamic Range (HDR) photography.
The first is a traditional “hardware” solution, which places a glass or gel filter between the lens and the subject. Simulated graduated filters are a software solution that attempts to duplicate the effect of a traditional filter while HDR uses software to combine multiple exposures made at different camera settings into a single image able to display the entire exposure range present in the original image.
Each has advantages and all should be in the arsenal of any serious photographer, although the simulated graduated filter is probably best thought of as a rescue technique for images that didn’t have the advantage of the real thing, since a digital sensor or film that is "blown" or "washed out" (i.e., reading maximum values―white) captures no usable data in the washed-out areas, something that cannot be corrected with post processing.
Even so, increasing numbers of photographers have given up carrying traditional filters and rely on PhotoShop, PaintShopPro and the like for their graduated filter effects. The easiest way to try the effect is probably to download Picasa, Google’s free photo organiser, which includes an easy to use set of adjustable graduated filters in the Effects menu.
Picasa’s filters can be any colour you like, and physical filters come in a number of colours, too. The most popular (and useful) are Grey, Tobacco and Neutral Density (ND), all the following Intel on ND filters applies equally to filters of any colour.
Graduated neutral density filters, also known as graduated ND filters, split neutral density filters, graduated filter, or just grads, are optical filters that pass more light at one side than the other. Typically they are evenly graded from dark to clear. The transition can be sudden or gradual.
Place the dark side over a bright sky and the clear side over the land, and both the sky and the rest of the photo can be properly exposed. Centre filters are a variation where the slightly opaque section is in the centre and the edges are clear. They are used to compensate for light fall off that is natural with large optics and to control the over-bright central area at sunrise.
Grads come in a variety of shapes, sizes and densities and can be used in all types of photographic applications including still photography, motion photography and scientific applications. They can be separated into two categories: hard edge with an abrupt change in brightness and soft edge, with a wider smoother change from light to dark.
A hard edge is appropriate for scenes which have a clearly defined light and dark field, such as a field with a horizon to a bright sky. A soft edge is preferred when the light and dark portions are not distinctly separated such as mountain to sky. A soft edge filter is less noticeable than a hard edge. It also has the benefit of making the sky more intense, making the sky increasingly dark toward the top of the picture.
Grads are not usually supplied as screw-in filters, but as square filters made to fit into Cokin-type filter holders. this allows the transition point to be positioned at, above or below the centre of the image, and angled to match the needs of the subject.
High Dynamic Range
Major image software like PaintShopPro and PhotoShop and a number of stand-alone programs are able to go beyond the capacity of grad filters, provided they have enough material to work with. They require several files shot at exposures that cover the entire range of the subject.
This is best achieved using the camera’s automatic Exposure Bracketting feature, but can be managed if you don’t have this facility. Check your manual for the exact procedure for your camera, but basically, with the camera on a tripod or similar firm fixture make an exposure based on the darkest area of your subject. Without moving the camera, and without changing the focus, repeat the shoot, exposing for the brightest area. Make further exposures for the mid-range values.
If you don't have a separate exposure meter, focus on each area and record the settings the camera recommends before you set up the camera on your support. Then take consecutive shots at these exposures. Disable autofocus after the first picture.
In the computer, your software uses the EXIF information recorded by your camera from each of your bracketed exposures to determine their shutter speed, aperture and ISO settings. It then uses this information to assess how much light came from each image region.
This light may vary considerably, so the HDR file is built using 32-bits to describe each colour channel instead of the usual 16 or 8-bits. As a result HDR files have a relatively open-ended brightness scale.
Powerful as this is, it is clear that it is not a technique that is going to help with moving subjects, or even with subjects that contain some moving elements, like waving branches or flowing water. Sometimes there is no substitute for ‘old’ technology.
