March 8th, 2015
Last month, we talked about resolution, and its lessening relevancy as an indication of image quality. In that discussion, I mentioned the term dynamic range, a concept I would like to further explain. Unfortunately, as with many scene elements, cameras are currently unable to match the human eye's perception.
Dynamic range is the capability for a camera to resolute differences between the brightest (whites and highlights) and darkest (blacks and shadows) areas in a photograph. This ability is shown both in-camera, and through post-processing software, as a histogram (shown below with the originating picture).
The histogram is organized with the blacks at the left, then the shadows, mid-range, highlights, and whites. This histogram is typical of a well-exposed, normal photograph, showing a wide peak in the middle, and no lines running up the sides. Dynamic range in cameras is limited by a number of "stops" of light, a unit of light intensity measurement. Higher end cameras have higher dynamic ranges, but none are able to simultaneously capture pure white and pure black. As such, cameras either lose detail in the shadows or in the highlights, resulting in areas that are either blacked out or blown out, an effect called clipping.
This photo has lost detail in the shadows, as evidenced by the peak running up the left side. This can be corrected in post, by increasing the brightness in the blacks and shadows, hopefully gaining detail in the bee's head, and the area behind the flowers in the bottom left corner. Doing so will introduce noise to these areas, so the optimal solution would instead be to avoid clipping in the field. Or, if the dynamic range of the scene outpaces the camera's capabilities, HDR techniques can be used (detailed later).
This next example, in part due to the harsh sunlight, has right side clipping, where detail has been lost in the highlights. Unfortunately, highlight clipping is difficult to correct. When detail has been lost to pure white, it is gone completely. It is essentially impossible to regain this detail, and the only bandaid solution is to try and minimize the visual impact of the areas by darkening them a bit.
Here, detail has been lost in both the highlights and lowlights, due to the black butterfly's wings and bright, penetrating sunlight above.
To counteract detail loss in high contrast compositions, high dynamic range (HDR) techniques may be employed. The idea of HDR is to intentionally both under-expose and over-expose the scene, in multiple photographs, and stitch them together. In the under-exposed photo, highlight detail will be preserved at the cost of shadow detail (and vice versa for the over-exposed shot). Some cameras have internal HDR capabilities, but most HDR is done in post. The final result, provided the scene is not rendered unrealistically diverse in range, can be quite extraordinary.
In this final example, a strong peak in the mid-range dominates the tones in this photo. The peak may be spread out by increasing contrast, but you have to be careful. Increasing contrast too far can clip highlights and/or shadows. Also, uncompressing a peak to generate a more gradual tonal range results in a degree of posterization, depending on the amount of increased contrast.
With dynamic range, as with many areas in photography, there are a few similar takeaways. It is better to get the exposure right in camera, or employ HDR techniques if absolutely necessary. Also, the artistic vision, actual scene, and produced image will all differ slightly, due the different ways cameras see the world differently than we do. In the aspect of dynamic range, however, cameras have gotten much better at reproducing the tonal range we see ourselves (even if we aren't quite there yet).
Thoughts about dynamic/tonal ranges? Take a moment to leave them in the comment section below.
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Hope to see you back next time!