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Purpose of a chart?


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#1 David Calson

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Posted 21 April 2008 - 01:59 PM

I'd like to know the purpose of a chart.

I'm guessing the shades of gray from white to black ensure you have the right exposure, nothing blown out or crushed.

There are 3 black bars, why?


Why all the colors and different shades?

I'm guessing the chart itself is gray for color correction references.


The striped looking cones are for backfocus?

How are charts and color bars related?



What would happen if you didn't have a chart?

*If my attachment didn't go through, go here to see what I'm talking about, http://www.dsclabs.c...s_in_action.htm

Any advice would be greatly appreciated.

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#2 Michael Nash

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Posted 21 April 2008 - 04:15 PM

The ChromaDuMonde chart includes several different references all in one chart.

http://www.dsclabs.c...romadumonde.htm

In simplest terms, it's useful to see how your imaging system (film or video camera) is responding to known real-world values. The basic things you want to look for are exposure (gray), contrast (dynamic range and gamma), color response (hue and saturation), and sharpness (resolution and detail).

There can be different charts for each thing you want to measure, and you often want to measure them across the entire frame, not just the middle of the image (since optical distortions are one thing you're trying to evaluate).

Grayscales let you see tonal distribution as well as dynamic range. They're not used for exposure per se.

Color charts let you see color response around the full circle of the spectrum, and at different saturations and luminances.

The striped "trumpets" are for resolution, detail and aliasing artifacts in different directions. A Siemens Star is usually used for backfocus.

Color bars are used as a signal reference, independent of the camera or film's performance.

You don't need to use a chart if you already know how your imaging system is behaving and what it will give you. But if you don't already know, or some variable in the system has changed, then a chart can provide a real-world known reference to compare against.

Charts are especially useful when you have multiple cameras that you're trying to match. The references on the chart are more precise and easier to see (especially with test equipment) than subjectively matching general images.
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