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Old August 7th 06, 04:20 PM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
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Default Measurung dynamic range...

wrote:

wrote:


Really measuring dynamic range is quite a difficult job.



In reading my post I see I really left out a lot, and didn't explain
myself very well. The problem is accurate radiometry/photometry.
Accurately measuring light is not a trivial problem- the instruments
are expensive and need frequent calibration. The big problem is
getting black references. White references are easy to do, and don't
need super accuracy. The problem is, what is the brightness, or even
reflectivity of a black reference.

To simplify the math, lets look at Black and White, say an 8 bit grey
scale. There can be up to 256 values in the image. But the best
available black inks or paints have a reflectivity of about 2-3%. If
we use a single even illumination, then a black and white printed chart
has a dynamic range of 50:1 or less. Thus, we can't use a printed
chart to test a camera with a dynamic range of 250:1 or 500:1.

I am working on a low cost "light trap" using a pringles can and
self-adhesive black felt. If this works out I intend to write an
article on it for a camera magazine, along with some stuff on measuring
dynamic range and flare performance. I intend to use this first on my
scanner, but will use it for some tests on my cameras too.

Procedures for Evaluating Digital Camera
Sensor Noise, Dynamic Range, and Full Well Capacities
http://www.clarkvision.com/imagedetail/evaluation-1d2

Gets around these problems, but you need access to the raw
data. Other tests typically use transmission targets.

A good way to make a light trap is a cone painted with
glossy black paint. The reflections from the painted surface
go deeper into the cone. At the bottom of the cone, have
a deep hole ~1mm diameter. A long pipe also works, as in
Figure 1, 2 at:

Dynamic Range and Transfer Functions of Digital Images
and Comparison to Film
http://www.clarkvision.com/imagedetail/dynamicrange2

Roger