AVS Special Member
Join Date: May 2008
Location: San Francisco - East Bay area
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If you use full fields with local dimming turned on, you will get whacky gamma results because the TV will dim for the darker grayscale steps and brighten for the brighter grayscale steps in spite of using full fields. Using full fields does not stop the TV from "gain riding" the luminance changes in full-field patterns. Local dimming will dim small areas, but it is not limited to darkening or brightening small areas. It will darken or lighten the entire image area if you use full frames... depending on how close to white or black the full field pattern is. In fact, one of the issues with calibrating plasma displays is that they have brightness limiting that kicks-in when images get increasingly bright. You can't stop it from happening. In LCD displays, if you leave dimming on, you essentially turn them into a plasma TV with essentially the same issue to deal with. If you turn dimming off, you then have 1 specific black level and 1 specific white level and you can then build your gamma curve between those 2 points so gamma is accurate.
Now this is the part that trips-up a lot of people. Let's say you turn dimming off to calibrate an LCD TV. And let's also say you are able to get a nice 2.25 gamma all the way through the grayscale. OK... now lets say you are viewing Picture-In-Picture and you have a hockey game in the large main screen (bright white images most of the time) and let's say you have a darker movie running in the PIP window. So with dimming turned on, the image will want to be at maximum brightness for the hockey game because of all the "white" ice. But because you calibrated with dimming off, the hockey game images will be calibrated to the 2.25 gamma standard (we'll it's my standard, not official, but it should be the standard for gamma setting) . The whole image might be brighter than an average image (if your TV's dimming works that way), but it will still measure pretty close to 2.25... it won't be exactly 2.25 Gamma, but it's likely to be pretty close. Now the dark movie in the small window... that also will be close to 2.25 gamma because when you dim, you reduce luminance, but you maintain the same luminance relationship between grayscale steps and you'll still have pretty close to 2.25 gamma within the darker PIP images. So if you calibrate with dimming off, you get reasonably similar gamma with fully bright images or fully darkened images.
But if you leave local dimming on during calibration, you get a wrong (errors) gamma all the time no matter whether the TV images are dark or light or medium, average luminance. The bright hockey game's gamma will be wrong, and the dark PIP image will also have a bad gamma.
So my advice is to ALWAYS turn off auto dimming, local or full screen, for calibration to get the best looking images. Once calibration is done, most people will probably find gamma looks fine with dimming turned on and that the dimming really helps to improve black level noticeably.
"Movies is magic..." Van Dyke Parks
THX Certified Professional Video Calibration
Widescreen Review -- Home Theater & Sound