Adjusting brightness and contrast...the proper way. (Updated)
Brightness and contrast are among the most
misunderstood controls on most monitors and projectors. Here's a way to ensure
that they are set up properly...
First, ignore the names. No, really, ignore what
they are called - it REALLY doesn't
help.
Instead of brightness think "black level" b = blacklevel.
Instead of contrast think "video gain" c = crushing the video (i.e. squeezing it down or up)
and you will be much closer to understanding what they do.
The brightness control adjusts the video level that the monitor treats as true back. The goal here is to match things that the sending equipment thinks is black to actual black on the monitor. There are various reasons why these might not be the same to start with.
The contrast control is like a gain control on the video. It adjusts the overall range of the incoming signal in terms of the histogram of brightness. The more 'contrast' the more gain is applied to the incoming signal and vice versa. Think of it as 'turning up the volume' on the video and you are not far off.
The key then, is to get the black level right so the left hand end of the histogram is in the right place, and then adjust the 'volume' so the right hand end is in the right place too.
The good news is that you don't really have to understand any of the above to get it right.
So...how do you do that?
The way to achieve this is to use a test chart.
I am amazed at how many people try to adjust projector setting by eye without using a decent test image to make it easier
Here's what you need
A grayscale test chart.
Here it is as a Powerpoint file, all set to load up on your PC and display on your projector.
grayscale test chart.ppt
and here it is as a PNG image
(Just save this image)
Stick this up and follow the instructions on-screen. It's that easy
You want the brightness to give you a good dark black, but still be able to just distinguish the next along black bar, and the contrast to give a good strong white, but still be able to distinguish the next along white bar. These two controls interact so be prepared to fiddle a bit.
Do this for ALL display equipment - monitors, projectors, plasmas etc. and you will be a lot closer to getting good dynamic range in your video signals.
NOTE: When you do this, you may end up saying " now my white isn't white enough to be seen". That is a problem with the brightness of your projector, not with the settings. You can compensate for lack of brightness by boosting contrast, but be aware that this will blow out all the light parts of the image, which may look fine for song words, but will look terrible for video.
P.S. I made this test chart myself in Photoshop, rather than ripping off an existing one.
feel free to use it, but don't distribute it and pretend it's yours, or sell it to anyone.
P.P.S. If you are using this test chart through a scan converter, remember that the black and white levels are going to be dependent on the scan converter settings too.
That touches on the subject of 'video gain' i.e. how the signal get's handled as it travels through the system - that will be the subject of another post.
Instead of brightness think "black level" b = blacklevel.
Instead of contrast think "video gain" c = crushing the video (i.e. squeezing it down or up)
and you will be much closer to understanding what they do.
The brightness control adjusts the video level that the monitor treats as true back. The goal here is to match things that the sending equipment thinks is black to actual black on the monitor. There are various reasons why these might not be the same to start with.
The contrast control is like a gain control on the video. It adjusts the overall range of the incoming signal in terms of the histogram of brightness. The more 'contrast' the more gain is applied to the incoming signal and vice versa. Think of it as 'turning up the volume' on the video and you are not far off.
The key then, is to get the black level right so the left hand end of the histogram is in the right place, and then adjust the 'volume' so the right hand end is in the right place too.
The good news is that you don't really have to understand any of the above to get it right.
So...how do you do that?
The way to achieve this is to use a test chart.
I am amazed at how many people try to adjust projector setting by eye without using a decent test image to make it easier
Here's what you need
A grayscale test chart.
Here it is as a Powerpoint file, all set to load up on your PC and display on your projector.
grayscale test chart.ppt
and here it is as a PNG image
(Just save this image)
Stick this up and follow the instructions on-screen. It's that easy
You want the brightness to give you a good dark black, but still be able to just distinguish the next along black bar, and the contrast to give a good strong white, but still be able to distinguish the next along white bar. These two controls interact so be prepared to fiddle a bit.
Do this for ALL display equipment - monitors, projectors, plasmas etc. and you will be a lot closer to getting good dynamic range in your video signals.
NOTE: When you do this, you may end up saying " now my white isn't white enough to be seen". That is a problem with the brightness of your projector, not with the settings. You can compensate for lack of brightness by boosting contrast, but be aware that this will blow out all the light parts of the image, which may look fine for song words, but will look terrible for video.
P.S. I made this test chart myself in Photoshop, rather than ripping off an existing one.
feel free to use it, but don't distribute it and pretend it's yours, or sell it to anyone.
P.P.S. If you are using this test chart through a scan converter, remember that the black and white levels are going to be dependent on the scan converter settings too.
That touches on the subject of 'video gain' i.e. how the signal get's handled as it travels through the system - that will be the subject of another post.
Posted: Sat - May 21, 2005 at 10:45 PM |
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This site contains my personal views, ideas, tips, tricks and rants about doing live video, IMAG or graphics at live events on a very low budget. They are published as I get time, in a style closely approximating a weblog. Dave C.
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My name is Dave Chalmers and I live in Edinburgh in Scotland, UK. My background is in Electronics, but for 10 years I have been helping to put on live video at events in central Scotland in my spare time. Through the course of these events, nearly all on very constrained budgets, I have learned a few things about what works and what doesn't. While I would not claim to be able to give you all the answers, I can at least help you avoid some of the mistakes.
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Total entries in this category: 16
Published On: Jul 06, 2006 11:37 PM
Total entries in this category: 16
Published On: Jul 06, 2006 11:37 PM