"What is contrast ratio and what is the best and worst?"

If you take the brightest white on a screen, and the darkest black and compare the luminosity, you get the contrast ratio.  For example, a 1000:1 contrast ratio means that the brightest white is 1000x brighter than the darkest black. 

Is higher always better?  That's the idea; however, there are instances where even a high contrast ratio can look pretty bad.  On a poorly built plasma set, for example, blacks are usually only a dark gray because  of the inherent plasma technology (plasma maintains a minimal charge to each plasma-gas cell, and typically emits a small amount of light from that cell when the TV is on).  On the other hand, a plasma's picture is very bright (again, thanks to the technology), so the contrast ratio remains high.  This doesn't mean that plasmas are "bad" - when you're watching a plasma, the brighter colors make the darks "seem" much darker.  Consider a movie theater - the screen is silver or off-white... but it looks black when the movie is playing, and since black isn't a projected color (it's the absence of any projected color), the "black" you see is still white or silver - it's just that your eye doesn't distinguish it because the other colors on the screen are so bright. 

On the other end of that, there may be a set that has a very very dark black, and a not-so-bright white - the contrast ratio could still be very high, but the brightest white on this set may not match the brightest white on another set... although the overall contrast ratio could be better.

In a high-end projector/set, the darkest black may be immeasurable, thereby creating an infinite contrast ratio (think of it as trying to divide by 0 for your ratio - 10,000:0 doesn't make sense). 

Consider this as well - the amount of ambient light and reflected light in a room will change the perceived contrast ratio on a set - in a sunny room with a matte big screen, the screen will reflect sunlight, therefore making the "darkest black" a bright gray, and therefore the contrast ratio drops significantly.   Obviously, manufacturers don't take this into account - the specs from the manufacturer are of the TV in an ideal environment (dark room), and they measure only the light being produced by the screen, not the "real world" reflected light (sunlight, overhead light, ambient light). 

In summary - Higher is better, most of the time, but if you're purchasing a TV, bring a favorite DVD or two with dark and light scenes, and see how it appears to YOU - regardless of numbers, specs, and what anyone else says about a picture, everyone has a different perception of what a "good picture" is.  All that matters is that you're happy with it.  Also, consider that most stores tuck TVs back in the corner, in a dim environment, and bump the contrast to maximum level to oversaturate the picture, and make it look "brighter".  Brighter is not always better!  Remember that the set you pick will be going in your home, and chances are it's going in a room that will get some sunlight and indoor light reflection!

Good luck!

Contrast ratio is the difference between the blackest black and the whitest white, or to put it another way, the number of shades of grey in between.

A contrast ratio of, say, 10000:1 (commonly seen on plasma screens) is better than 1000:1 or 500:1

Definition: Contrast ratio is a measurement of the difference in colors displayed on a screen at the same time. Most accurately, it is measured as the difference between the brightest and darkest colors on the screen. Contrast ratio is a term associated with LCD, Plasma and projection televisions as well as computer monitors. Televisions with a higher contrast ratio are considered to be better than ones with lower contrast ratios.

Think of a contrast ratio as the amount of shades between black and white. More shades between black and white means the television won't sacrifice quality in when displaying images with extreme differences between light and dark. Many televisions (excluding CRT tube) have trouble displaying an accurate representation of black, especially when there is bright images on the screen at the same time.

Examples: Examples of contrast ratios are 800:1, 1000:1, 5000:1, etc. In this example, the 5000:1 contrast ratio would be perceived as better than the 800:1 or 1000:1.

But...does it really matter anyway?
See this great article-
http://tech.yahoo.com/blogs/null/29651


What Does Contrast Ratio Really Mean?

Thu Jun 14, 2007 6:52PM EDT

Reader Jim writes: How much of a difference does the contrast ratio on LCD TVs make? Is 8000:1 really that much better than 1000:1 for contrast, and worth the extra price?

Contrast ratio is perhaps the most misleading and overhyped data when it comes to televisions. In theory, contrast ratio measures the difference between the brightness in the super-bright portions of a screen vs. the super-dark portions. A 1000:1 contrast ratio would mean that a perfectly white pixel is 1000 times brighter than a perfectly black pixel.

That's the theory, anyway. The problem is that these brightness levels are hard to calculate, and they're subject to all sorts of interpretation. Ultimately, contrast ratio has become little more than marketing-speak, and the numbers are now largely meaningless. Things have gotten out of control to the point where various vendors have claimed 1,000,000:1 contrast ratios. It's getting to the point where escalating contrast ratios are now an industry joke, though not a very funny one.

Gizmodo has an extensive piece on the topic
http://gizmodo.com/gadgets/feature/contrast-ratio-shoot+out-everyone-loses-259495.php
which rightly points out that high contrast ratios don't mean high brightness. Rather, companies have focused more recently on decreasing the brightness of the "1" side: making the blacks blacker instead of the whites whiter. As the post notes, "Cutting the darkest dark on a screen by .5 effectively doubles the contrast ratio." In comparing an 8000:1 TV to a 1000:1 TV, you're probably looking at a TV that the manufacturer claims has richer, truer blacks.

Of course, it may not. The numbers could be calculated differently, inflated, who knows. You'll need an independent source (probably from a high-end home theater publication) to get a real sense of a TV set's contrast, but even then the numbers probably wouldn't be very meaningful because of the difficulties in performing such a test.

Ignore manufacturer's contrast levels and focus instead on how a TV looks to your eyes, whether it has the connections you need, and if it's the right size for your room.