In Part Three, we started looking at how manufacturers classify their monitors and why. In this article, we will complete what we started.
When a pixel displays light, it doesn’t do it instantaneously. There’s a rise and a fall. The time it takes for this to happen is the response time, often quoted in milli-seconds (ms). A 60 Hz display capable of showing 60 fps will have a time per frame of 16.7ms. The total response time should be lower than this value for zero ghosting. As we have seen, a Grade 1 and Grade 2 monitor should have a ‘measured’ value of less than 10ms.
Here’s the problem:
- Manufacturers can measure the rise and fall in various ways. Sometimes, they only measure rise or fall.
- Response time isn’t consistent over different luminous values. E.g., ‘white to black’ is different from ‘black to white’ is different from ‘white to grey’ is different from ‘grey1 to grey2’, and so on. There isn’t a single standard response time for any one panel. So, which one is the manufacturer quoting, and why?
- Color complicates the measurement of response time by a factor of three.
As you can see, there is no point in wasting your time on this value. Forget about it.
How many stops of light can a monitor display? That is its dynamic range or contrast ratio.
As we have seen in Dynamic Range of the Human Eye, the human eye can only see about 1000:1 at any given time, even though the static dynamic range of the eye is about 1,000,000,000:1 (30 stops).
But there are manufacturers claiming 5,000,000:1 in contrast ratio. Another might meekly claim 1000:1. Is it really 1000:1, or is he applying the same multiplier? It is unethical to market incorrect data, so manufacturers measure data differently, and choose to display the one that they feel will show their product in best light. One trillion:1 contrast ratio… as long as you watch it with eyes closed, and so on.
1000:1 is the bare minimum for a professional display monitor, while Plasma monitors can go up to 5000:1, and OLEDs having the potential for even higher. Let’s stay in the ‘thousands-range’ for now. As we have seen earlier, a Grade 1 monitor must have a contrast ratio of 2000:1.
Should you worry about this? Not really. Let’s say you filtered down your choices to two monitors from different manufacturers. One claims 1000:1 while the other claims 2000:1. They’re not in the millions, so could they be true, you ask yourself? The only way to find out is measure it scientifically, or look at them side by side in a controlled environment with the same signal. Tell me: when was the last time any dealer let you do either of these things?
If you look at one monitor in one setting, and then on another day saw the second monitor, can you say for sure which is best? Don’t kid yourself. Most people can’t say even with the same monitor on different days!
As we have seen in Part Two, the brightness of monitors depends on the viewing environment. If you’re viewing your video in a dark environment, you’ll only need 70-100 cd/m2 (nits). To know more about brightness, read How to Measure Light and The Units of Photometry.
In general, I wouldn’t advise using a monitor which has a brightness rating of less than 100 nits. In a well-lit studio, you might need 200 nits or more.
When a manufacturer claims 24 bits of color, that’s spread over three color channels. This means, each channel (R, G or B) is 8-bits in size. Today, most consumer grade displays support 8-bit color depth. You don’t need more than 8-bits for any kind of work, including demanding cinema work. Why? Read: Color Bit Depth of the Human Eye.
So why do people pay more for 30 bits (10-bit per channel)? This is because color bit depth isn’t the only property of a monitor, just as the size of your nose isn’t the only defining feature of your body. The end result of what you see is a combination of many technologies working in tandem. Sometimes, manufacturers club higher-end technologies together, so the average user thinks this correlation means causality.
E.g., a crappy product can be placed on the shelves of a high-end boutique store, to be sold for more. People who don’t know the inherent merits or demerits of this product will assume, incorrectly, that it is worth its place and price just because it sits alongside other expensive items. This same principle is used in advertising. If your favorite star is seen drinking that beer or driving that car, the marketers want you to associate (correlate) that product with the star. You end up thinking if you used the same product, people will associate you with the star.
Therefore, one can have the best monitor in the world in 8-bit color depth, but that is unlikely until a manufacturer feels they value right over money. Not going to happen, for which reason I say: Don’t worry about the bit depth as long as it’s 8-bits per color channel.
Color Space, Gamma and Gamut
Most consumer-grade panels don’t display full 100% sRGB, therefore Rec. 709. For HDTV work, even if your video is only destined for the internet or DVD/Blu-ray, I recommend a monitor that can display 100% sRGB/Rec. 709. Such a monitor is a professional display monitor.
If your work involves PAL, 100% Rec. 709 will be pretty close. However, if your work involves NTSC, you need a wider gamut monitor, one which can display 100% NTSC. Such a monitor isn’t cheap, so you’d better be sure your product is intended for national broadcast or something similar. Just as a thumb rule, if a monitor displays 98% or more of Adobe RGB, it is almost as good as 100% NTSC.
For Cinema, you need a monitor or projector that conforms to the color space for delivery. The most widely known is the DCI specification, for which the color space is CIE XYZ (another way of saying: everything). To narrow it down, projector color spaces correspond to the DCI P3 color space, and you should be looking for that label.
As for Gamma, check out What is Display Gamma?
Ideally, you’d want 180o, but how many human beings do you know who’d like to watch a screen 5o off the edge? As a quick test try reading this article from the edge onwards. At what point are you comfortable, regardless of the fact what the specified viewing angle is?
You have to decide whether your video monitor is for everyone or a client or for yourself. If it’s just for one or two people, you don’t need to worry much about the viewing angle.
The refresh rate is directly related to how many frames per second your monitor can display. If you want a 60fps video to play as it is, you’ll need a monitor capable of 60 Hz. The refresh rate is the number of times the pixels can be refreshed to ‘zero state’ per second. Nowadays, most monitors are capable of at least 60 Hz. Some go up to 75 Hz, but if you’re on a DVI connection, or if your GPU limits you, you might only get 60 Hz.
For 3D Stereoscopy, you’ll appreciate 120 Hz monitors (60 Hz for each eye), otherwise there aren’t any consumer display systems for anything greater than 60fps. Even 60fps progressive is rare.
Pixel pitch directly correlates with ppi (pixels per inch), which correlates (vaguely) with the resolution of the human eye. Since we’ve covered this already, let’s move on.
Just as a side note, if you’re considering a display greater than 1080p, take a look at What is 4K TV?
The lower the power, the better. If you’re running a single-monitor shop, you might feel this is not your problem. If you’re running a post house with many devices, the power saving on each device will add up significantly. So always take a look at the power rating, and keep your conscience clean.
In Part Five, the final chapter of this series, we’ll go deeper and understand what ‘delta’ means, and why you see it often in charts and the like.