|제목||Gamma, Chart Contrast and MTF Calculations|
e explanation of gamma,A fuller and more up-to-dat
including how to obtain it for slanted-edge MTF calculations
Gamma (the average slope of log pixel levels as a function of log exposure for light through dark gray tones) is used, per the ISO 12233 standard, to linearize the input data, i.e., to remove the gamma encoding applied by the camera or RAW converter. Gamma defaults to 0.5 = 1/2, which is typical of digital cameras, but may be affected by camera or RAW converter settings.
If the edge contrast is known and is ≤ 10:1 (medium or low contrast), you can enter the edge contrast in the Chart contrast (for gamma calc.) box, then check the Use for MTF checkbox. Gamma will be then be calculated for each individual edge in the chart image and displayed in the Edge/MTF plot. Full details, including the equation, are on Gamma and MTF Measurement.
Errors in Gamma
Small errors in gamma have a minor effect on MTF measurements (a 10% error in gamma results in a 2.5% error in MTF50 for a normal contrast target). Gamma should be set to 0.45 when dcraw is used to convert RAW images into sRGB or a gamma=2.2 (Adobe RGB) color space. If gamma is set to less than 0.3 or greater than 0.8, the background will be changed to pink to indicate an unusual (possibly erroneous) selection.
Gamma is the exponent of the equation that relates image pixel level to luminance. For a monitor or print,
Output luminance = (pixel level)gamma_display
pixel level = (RAW pixel level)gamma_camera ~= exposuregamma_camera
The total system gamma is gamma_display * gamma_camera. Standard values of display gamma are 1.8 for
1. Capture One LE set to Film standard
(the default). Gamma = 0.679.
See Kodak’s definition in Sensitometric and Image-Structure Data.
To obtain the correct MTF, Imatest must linearize the pixel levels— the camera’s gamma encoding must be removed. That is the
Characteristic curves for the Canon EOS-10D with three RAW converter settings are shown on the right. Gamma deviates
2. Capture One LE, Linear response. Gamma =
0.508. Recommended for SFR runs.
Confusion factor: Digital cameras rarely apply an exact gamma curve: A “tone reproduction curve” (an “S” curve) is
The transfer function may also be adaptive: camera gamma may be higher for low contrast scenes than for contrasty
To learn more about gamma, read Tonal quality and dynamic range in digital cameras and Monitor calibration.
3. Canon FVU set to Standard contrast.
Gamma = 0.642
|이전글||Gamma, Tonal Response Curve, and related concepts|
|다음글||Calculating Field of View (FoV) using distance settings|