Friday, 25 June 2010

Dynamic range in digital photography

In photography, dynamic range is the difference in brightness between the darkest and the brightest part of a scene. There are two problems with dynamic range: capturing it in the camera's sensor, and reproducing it on screen for people to see.

It's useful to distinguish between three levels of dynamic range.

  • Low dynamic range: the scene can easily be captured in one exposure, saved as an image with 8 bits per colour, and reproduced on a good monitor without any loss of dynamic range. A photo of such a scene doesn't need any special treatment. However, tone-mapping can still be useful to "improve the lighting".
  • Medium dynamic range: the scene can be captured in one exposure and saved in a camera raw file with 12 or 14 bits per colour; but reducing it to 8 bits per colour would clip the dynamic range. In this case you can save the raw file as several different files with different exposure corrections, and then combine them using tone-mapping or exposure fusion. These techniques compress the original dynamic range in different ways to give a result that looks pleasing.
  • High dynamic range (HDR): the scene can't be captured in one exposure without clipping the dynamic range. In this case, you can take multiple exposures with different shutter speeds, and then combine the exposures using tone-mapping, in order to compress the original dynamic range in a pleasing way.

A problem with the use of multiple exposures for HDR photography is that there are often moving objects in the scene (people, cars, leaves, waves) that cause blurring or ghosting when the exposures are combined. Photomatix tone-mapping offers some degree of automatic correction for this problem, but Photomatix exposure fusion does not; so I wouldn't use exposure fusion for true HDR photos, because there are so many things in a scene that may move. Unless you take photos inside a building with no moving objects in sight.

Yes, you can use laborious manual methods to deal with ghosting in your HDR photos. If you want to spend that much time on each photo.

Most normal scenes have either low or medium dynamic range. Anyone who often takes photos of high-dynamic-range scenes is probably going out of his way to look for them. They might be scenes with bright sunshine and deep shadow (perhaps looking into the sun), or night scenes with bright artificial lighting.

Thursday, 17 June 2010

Small white hunter

A memory fragment from my mother, early 1940s:

Hate to remember it now but on holidays in Wiltshire (aged around 9 to 11) I used to go out with my aunt Rhoda's husband Ted and shoot rabbits. Meat rations were stingy and my grandmother (she and Rhoda were staying in Wiltshire for the war) welcomed succulent rabbit to stew and feed us all.

She adds later:

... it's a good job my rabbit-shooting days were so long ago because, never mind it being illegal now, it was illegal then. We were trespassing on someone's land for starters and at my age I should never have been allowed to handle a gun, let alone use it. I don't know whether one needed a licence for a gun then but I'd bet my bottom dollar that Ted didn't have one.

I understand from Wikipedia that a gun licence has been required in the UK since 1870; originally anyone could have a licence who paid for one, but from 1920 it became necessary to get approval from the police.