Making fine prints in your digital darkroom
Background to monitor calibration and gamma
by Norman Koren
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In this page we cover some background (reference) areas related to gamma. We discuss why a new chart was needed (many old ones had an error), then we present two additional charts: one for three luminance levels and one for three colors (R, G, B). We illustrate what luminance steps look like for different gamma settings. We moved the Praxisoft WiziWYG instructions to this page: since I put up QuickGamma, WiziWYG is no longer my choise for monitor calibration. Finally I discuss pages by Timo Autiokari, which contain some dubious advice on gamma.
Most
gamma charts consist of an arrangement of black and white patterns and
solid tones. You estimate gamma by viewing the chart from sufficient
distance
(typically one to two meters from the monitor) so you don't see the
pattern
details, then either locating the position where the average luminance
of the pattern matches the solid tone or operating a slider to obtain a
match. Many of these charts use checkerboard patterns, typified by the
pattern on the left: a portion of the gamma chart supplied with Epson
scanners,
enlarged 3x to make the pattern
plainly
visible. This type of checkerboard
pattern
gives an erroneous estimate of gamma!
I owe a debt of gratitude to Pete
Andrews for pointing this out.
The
problem is the risetime of the video card and CRT monitor. In
most
cases the brightness cannot change from pure black to pure white in the
short distance of one pixel (or even two or three). Since CRTs are
scanned
horizontally, abrupt vertical features are affected.
The situation is illustrated in the box on the right, which consists of four quadrants, each of which contains equal amounts of Black and White. 1: vertical alternating B and W. 2: horizontal alternating B and W. 3: Vertical B and W alternating every second line (same as 1 magnified 2x). 4: Horizontal B and W alternating every second line (same as 2 magnified 2x).
If risetime were not a problem, all four quadrants would appear equally bright when viewed from a distance. But on most CRT monitors, quadrant 1 (alternating vertical lines) appears quite different from 2 (alternating horizontal lines)-- usually darker. Quadrants 2 and 4, both containing horizontal lines, should be similar. Conclusion: patterns with vertical features are sensitive to risetime and cannot be used reliably to estimate gamma. Liquid crystal displays (LCDs) don't have this problem, but most have another, worse, problem-- sensitivity to viewing angle. For more details see Pete Andrews' Monitor calibration and Gamma assessment page.
I was dismayed to discover that my monitor's gamma, which measured around 2.1 with the old chart, was closer to 2.6. I corrected it using my video monitor's software (illustrated above ). I'll be recalibrating my printer and editing many images in November. Because of the gamma error I set Contrast to +12, well above the default setting (0). I'll undoubtedly reduce it. Annoying and a little embarassing, but the end result will be improved print quality.
Three level gamma estimation chartThe three level chart on the right enables you to estimate display gamma for three normalized luminances: 0.25 (left), 0.5 (middle) and 0.667 (2/3) (right-- 0.75 was too light to be legible). Gamma is estimated for each zone by locating the position where the average intensity across the zone is constant. The corresponding gamma is shown on the right. The section for luminance = 2/3, on the right, is difficult to read.I use this chart mainly as a diagnostic tool-- to see if gamma is consistant at different tonal levels. If your monitor is working well, gamma should vary by no more than ±0.1 between the zones. For normal gamma estimation I use the combined gamma/black level chart, on the previous page. The solid areas are calculated from the equation, pixel level = 255*luminance(1/gamma). |
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color gamma chart
A gamma chart with separate R, G and B patterns is shown on the right. It could be useful if you need to adjust gamma separately for each color. The blue pattern is dark-- difficult to read. I don't find this chart very useful for diagnostics; if gamma is different for different colors, you'll see color variations in the gray chart. But it could be useful in adjusting gamma in special cases, like my ATI Radeon video card, where the software doesn't allow the gamma adjustment to be coupled for the three colors. |
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Note the large relative luminance steps at low levels in the lower row, particularly between the first and second box. This is what you would get for evenly stepped pixel levels with monitor gamma = 1, resulting in banding in dark areas (below). In the top row, the relative luminance steps at low levels are relatively small; the second box should be perceptibly lighter than the first, but still very dark. On the whole, gamma = 2.2 is more perceptually uniform than gamma = 1. Gamma = 1.8 (the value used in Macintosh) has near optimal perceptual uniformity; it has a slight edge over gamma = 2.2.
The middle (11th) box in the upper row (pixel level 127) has a normalized luminance of 0.218. In photography, the 18% gray card is considered "middle gray." Reflective exposure meters are designed to give correct readings when the gray card is used; it is regarded as being close to the average density of the "average scene." The flip side of the gray card is 90% reflectance white. The normalized reflectance for middle gray is therefore 18/90 = 0.2: very close to the 0.218 luminance of the middle box-- pixel level 127-- for gamma = 2.2.
The following table zooms in on the above table to display the worst
case banding. The top row, for highlights, has evenly spaced pixels
from
236 to 255 in steps of 1. The bottom row, for shadows, has evenly
spaced
luminance steps when viewed with gamma = 2.2, the normal setting for
Windows
and the Internet. It illustrates the banding that takes place with
gamma
= 1.
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I find the highlight banding to be barely preceptible, if at all. But the shadow banding-- what you would get with gamma = 1-- is obvious and unacceptable. There would be no visible shadow banding with gamma = 1.8 or 2.2.
. Timo
Autiokari and AIM (Accurate Image Manipulation)
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visits to this page since October 3, 2002 |
Images and text copyright © 2000-2005 by Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording technology for high capacity data storage systems until 2001. He has been involved with photography since 1964. |  |
