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Contrast Ratios and other display ratings


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Contrast Ratios and other display ratings
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macleod
Now in lower-case™!
 
Join Date: Feb 2006
 
2006-04-19, 16:42

I am looking for a new LCD display but have had trouble comparing models because of all the different rating so I figured I would ask you all what they mean.
First of all, what is contrast rating? Dell has ratings of 600:1 up to 1000:1 while the Apple Cinema Displays only have 400:1. What does this mean and how important or accurate are these ratings.
There is also a brightness rating? How important is this and what is a good number for this rating?
If you have any additional knowledge about these ratings or other ratings help would be appreciated. Thanks
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Dorian Gray
Veteran Member
 
Join Date: May 2005
Location: Paris, France
 
2006-04-19, 21:15

Contrast is expressed as a ratio of light emitted by a pure white pixel compared to a pure black pixel, so 600:1 means a pixel at its brightest setting emits 600x more light than when it's set to black (with the backlight still on of course). Contrast ratio is one of the most important factors affecting our perception of image quality, because it determines how inky the blacks are. A dark black creates a crisp, saturated image, which is especially nice when viewing movies or photographs.

That said, in practice, the contrast ratio is not very important, because all LCDs are about the same. A contrast ratio of 400:1 is almost identical to 600:1. 50% less light in the black pixels is a very slight change, because of the way our eyes work (we can cope with anything from sunlight to starlight: a contrast ratio of 2,000,000:1, by "compressing" the brightness range, so 50% doesn't make a lot of difference). In photographic terms, 50% represents a difference of 1/2 stop. You may be able to tell the difference if the two displays were side by side, in a dark room, but it's more likely that other variables would swamp that difference.

1000:1 would be noticeably better than 400:1, in a room with very low ambient light levels, if other factors didn't change. That's roughly a 1 stop difference, but because that difference is at the dark end, in a normally lit room you wouldn't be able to tell the difference due to screen reflection. Other factors (such as the viewing angle) would still make much more difference, so the lower contrast screen could still look better, and even appear to have higher contrast.

Plasma screens have contrast ranges far beyond any LCD display, with ratios like 10,000:1 quite common. A contrast ratio like that would make an immediately noticeable improvement over the contrast of an LCD (in a dark room), but I don't think plasma computer monitors exist (and they would have many other problems anyway).

Brightness (or luminance) is expressed in candelas per square metre (cd/m2). Even the less bright LCDs (some laptop displays excluded) on the market are much brighter than the CRT displays of old. A figure of about 250 cd/m2 seems very bright to me (slightly too bright in fact), but I have noticed that many people prefer brighter screens than I. Again, the difference between 250 and 400 cd/m2 is not awfully significant in practice, but it is noticeable (because this figure determines the brightness of the white pixels, and the difference is therefore clearly visible even in brightly lit rooms).

By far the most important factor in determining LCD quality is viewing angle. A large viewing angle means the display has a uniform brightness when looked at from straight ahead, which is a vital quality for photo editing, for example. Just as importantly, a large viewing angle ensures that the contrast and brightness specs hold true across the surface of the display, rather than just in the centre (when viewed straight ahead). If the viewing angle is low, the brightness and contrast taper off to low levels at the corners and edges of the screen, when viewed from the centre, especially on large displays or from a close viewing distance. Unfortunately, of all the paper specs, viewing angle is the most susceptible to optimistic inaccuracies, because the manufacturer can arbitrarily set a brightness standard, which they can then legitimately claim is being met at exaggerated angles. But if that brightness standard is low, the viewing angle is pretty meaningless. Incidentally, this is why Apple recently boosted the paper specs of the 30-inch Cinema Display without changing the hardware: Dell released a 30-inch screen with the same LCD panel and claimed higher performance than Apple.

The quality of the backlight itself is also very important, but often ignored in discussions of LCD screens. It must be pure white and of an even brightness across the display right into the corners. Backlights on cheap displays are often hideously uneven and nowhere near white.

Response time is relevant only for gaming.

The moral of the story is that you can't predict anything about the quality of a screen by viewing specs on the internet. You absolutely must go to a store and compare the screens in real life. So I guess my long-winded answer didn't help much!
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macleod
Now in lower-case™!
 
Join Date: Feb 2006
 
2006-04-19, 21:42

Wow. Thanks a ton. That was actually very helpful even if I do have to go look at the displays in the store. The only problem with that is that I can't find any stores that carry dell displays and I am deciding between the dell 20" or 24" widescreen and the 20' or 23" cinema display. Any suggestions on how to compare them or personal experience would be great!
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Windowsrookie
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Join Date: Oct 2005
Location: Minnesota
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2006-04-19, 22:13

Dell only sells there products online, this is part of why they are so cheap.
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ghoti
owner for sale by house
 
Join Date: Apr 2005
Location: Charlotte, NC
 
2006-04-20, 09:20

I hate to correct Dorian Gray, but ...

Quote:
Originally Posted by Dorian Gray
That said, in practice, the contrast ratio is not very important, because all LCDs are about the same. A contrast ratio of 400:1 is almost identical to 600:1. 50% less light in the black pixels is a very slight change, because of the way our eyes work (we can cope with anything from sunlight to starlight: a contrast ratio of 2,000,000:1, by "compressing" the brightness range, so 50% doesn't make a lot of difference).
That's correct overall, but it implies that there could be a contrast of 2,000,000:1 that we could see at the same time, which we can't. If you compare bright sunlight to starlight, you get a ratio of about 10,000,000:1 (a few million don't matter here, this is about orders of magnitude), but our eyes don't have nearly the dynamic range to see all of that contrast at the same time. They can adapt, though, so you can even see things quite well in the middle of the night when there's a full moon (even without light from a city spilling over).

Quote:
Originally Posted by Dorian Gray
In photographic terms, 50% represents a difference of 1/2 stop.
This is nitpicking, but since this is a root scale, a 1/2 stop more light is about 67% more ...

Quote:
Originally Posted by windowsrookie
Dell only sells there products online, this is part of why they are so cheap.
That's not completely true, they also have those "Mini Stores" in malls. While you can't easily compare a Dell to another brand this way, you could look at the monitors there.
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Dorian Gray
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Join Date: May 2005
Location: Paris, France
 
2006-04-20, 10:07

Quote:
Originally Posted by ghoti
I hate to correct Dorian Gray…
It would be remiss of you not to ghoti!

Quote:
Originally Posted by ghoti
That's correct overall, but it implies that there could be a contrast of 2,000,000:1 that we could see at the same time, which we can't.
Of course, and I didn't mean to imply that. If one walks from bright sunlight into a house one has trouble seeing for a few moments, and starlight is much darker than the inside of a house.

Quote:
Originally Posted by ghoti
If you compare bright sunlight to starlight, you get a ratio of about 10,000,000:1
Sounds right: the 2,000,000:1 ratio (21 stops from "Sunny 16") was the figure I used to calculate exposure for starlit landscapes, but of course I wanted the photos to appear like night, not bright sunlight. I had to add a lot of exposure to compensate for reciprocity failure too, so exposure wasn't always an exact science.

Quote:
Originally Posted by ghoti
…but our eyes don't have nearly the dynamic range to see all of that contrast at the same time.
What is the instantaneous dynamic range of the average human eye? I've seen estimates from 100:1 (less than 7 stops) to over 10,000:1 (over 13 stops). A natural sunlit scene with deep shadows can have a dynamic range of about 15 stops (beyond even low contrast black and white film, with a range of about 12 stops optimally).

Quote:
Originally Posted by ghoti
This is nitpicking, but since this is a root scale, a 1/2 stop more light is about 67% more…
I wouldn't call that nitpicking.

Regarding the Dells and the Cinema Displays, they both have a good reputation. The Apple displays have a nicer design, but they're significantly more expensive. The Dells are cheap and cheerful, but have good LCD panels and good backlights in the more expensive models. There are also connectivity differences which I think generally favour the Dells (check this). You may be overly worrying about this: any decent LCD is way better than a CRT for normal computer use in my opinion. Sharper, brighter, no flicker, less energy use (and hence less heat), and only marginally less colour saturation these days. CRTs have retreated to print shops and the like, and that's as it should be, in my opinion.
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ghoti
owner for sale by house
 
Join Date: Apr 2005
Location: Charlotte, NC
 
2006-04-20, 10:19

Quote:
Originally Posted by Dorian Gray
It would be remiss of you not to ghoti!
Exactly!

Quote:
Originally Posted by Dorian Gray
What is the instantaneous dynamic range of the average human eye? I've seen estimates from 100:1 (less than 7 stops) to over 10,000:1 (over 13 stops). A natural sunlit scene with deep shadows can have a dynamic range of about 15 stops (beyond even low contrast black and white film, with a range of about 12 stops optimally).
The number I have seen is 10-11 stops, or about 1,000:1 to 2,000:1 - which seems believable to me. 13 stops or more seems a bit too much, especially if you compare it to the approximately 5-7 stops of contrast slide films have - and those generally look pretty good. That would mean at least six to eight stops more than slide film, which I can't believe.
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Dorian Gray
Veteran Member
 
Join Date: May 2005
Location: Paris, France
 
2006-04-20, 10:30

I have a Leica Pradovit P 600 with the legendary Super Colorplan lens (90 mm f/2.5), from a few years ago when I worked for a living and had some disposable income, and I can say that even in a dark room with dark walls, the densest part of an E6 slide is far from black. The highlights are close to the limit of my vision in such conditions, but I could see detail in at least a couple of stops below the shadows. Perhaps flare in the taking lens (mostly Nikon manual focus stuff) reduced contrast a bit though.

Completely off-topic: viewing photos on this set up is an experience to be treasured! Beautiful, just beautiful.
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