Prepared 2017-03-07, revised 2017-03-11 by Bill Claff
The purpose of the collaboration is measure where neutral gray appears in the linear data relative to the nominal 18% that we might expect.
DxOMark calls this "Measured ISO" and determines it in a very rigorous scientific way.
The approach taken here is less scientific but is highly repeatable and provides real-world information.
Choose a lightweight
prime lens. Not extremely wide as we want to avoid a large light falloff,
For example, I used a 30mm f/1.4 to collect my data.
If your camera has a fixed lens your choices are more limited; if it is a zoom lens don't use the widest focal length.
Use RAW or RAW+JPG.
Only the RAW files are used by the analysis.
For cameras like Fujifilm set Dynamic Range to 100% not Auto, 200% or 400%.
For Nikon cameras it's probably best to set Active D-Lighting (ADL) to Off.
You will be using
Aperture mode at f/4, Manual focus focused at infinity at ISO 200.
The test is sensitive to metering mode but unfortunately the best mode seems to vary.
Center-weighted metering is probably ideal but Matrix metering seems to work OK with most camera, for example Nikons.
When in doubt just use your typical one.
We're going to take
at least 18 images of various gray patches on the computer screen.
Having the camera a
consistent distance from the screen is very helpful.
I simply put the filter ring of my lens directly against my computer screen.
You may want to use a tripod, but hand-held is OK.
Use the Gray Patch Tool.
This puts a window
on the screen with an RGB value of (0, 0, 0).
The values are shown in hexadecimal. So at this point you will see 000000 @ 0F.
The ‘a’ or ‘-‘ keys will reduce the gray value by 0F on each key press.
The ‘z’ or ‘+’ keys will increase the gray value by 0F on each key press.
Enlarge or maximize
pressing the ‘z’ or ‘+‘ key between each image.
At the default step size there will be 18 images from 000000 to FFFFFF.
Exposure time will start at about 5s and drop to about 1/30s; depending on your screen brightness.
If you feel ambitious you can use the 'd' key to decrease the step size and increase the number of images.