Tuesday, November 16, 2004

Nice description of digital camera dynamic range

TidBITS: How to Buy a Digital Camera
For pictorial photography, dynamic range usually matters more than resolution. It doesn't matter if the sensor is able to resolve fine detail on an object if you cannot see the object at all because it is buried in shadow. You can do a quick-and-dirty test of dynamic range even in a camera shop by systematically underexposing photos of a photographic grey scale. I found the dynamic range of the SD-10 to be remarkable. If highlights are correctly exposed, shadows can be 10 to 11 stops darker yet still retain some coarse detail. The pair of screenshots (linked below) from Sigma's PhotoPro software show how easily and effectively this detail can be extracted. This photo was exposed perfectly for the highlights. The dark version shows a normal dynamic range, about what a colour slide would have shown. The light version shows additional detail in the shadows that was recorded by the sensor and brought out by the Tonal Adjustment sliders.

...(Note that with a digital camera, increasing the "ISO" speed does not make the sensor more sensitive, it amplifies the signal and, at the same time, it amplifies the noise. "ISO" 100 is normal for most cameras and speeds up to 1600 are commonly available, but any speed over 400 is not likely to look very good.)

Some cameras offer a choice of metering modes - spot, segment, averaging - so you can choose the one most likely to be accurate for the picture you are about to take. This is the sort of silly featuritis that makes so many electronic devices difficult to use. There is no point to trying to figure out how to set the meter to read a scene the most accurately, it's as fast and more certain to take a quick test picture and adjust the control that nudges the automatic exposure up or down. Automatic exposure-bracketing is almost as useless: there is rarely reason to bracket exposures when you can identify the correct exposure when you make it. The SD-10 dedicates push-buttons and primary display space to both of these "features."

Something else I can't see worrying about is how the camera reproduces colour. As I explained in "Colour & Computers" in TidBITS-749, this is tantamount to complimenting or castigating an amoeba on its figure. There is even less reason to worry about the colour reproduction of lenses. If a lens tints the image that it projects onto the sensor, the tint will be systematic and slight, and it will be corrected automatically by whatever software converts the raw image into a usable one...

One in a series of hard core digital photography articles printed in Tidbits. I've learned something with each one, and I'm not a complete novice.

There's lots of fine commentary in here. The author is writing about a very high end camera, but the commentary is relevant to every digital camera. I ended up thinking we really need to move away from the anachronisms of film photography and adopt new imaging parameters. The ISO control may be the worst of the bunch.

We're fine with 6 MPixel sensors for all the pictures I'm likely to do. Let's see the focus change (sorry) to dynamic range, image stabilization and light sensitivity. Forget the darned megapixels.

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