Aspect Ratio

[RJ Emery]

When it comes to computer monitors, I am confused about aspect ratios. It seems CRTs have a different ratio than flat panel displays, and I do not know the visual difference. I wish I could put an equiavlent sized CRT and flat planel display side-by-side, each displaying the same image full screen. Perhaps then I would see something.

If someone could help me understand the difference in aspect ratios for the two types of monitors, I would be most appreciative.

Why, for example, IS there a difference?

Does it matter?

Why should it matter or not?

[Stephen Owades]

Originally Posted by RJ Emery

When it comes to computer monitors, I am confused about aspect ratios. It seems CRTs have a different ratio than flat panel displays, and I do not know the visual difference. I wish I could put an equiavlent sized CRT and flat planel display side-by-side, each displaying the same image full screen. Perhaps then I would see something.

If someone could help me understand the difference in aspect ratios for the two types of monitors, I would be most appreciative.

Why, for example, IS there a difference?

Does it matter?

Why should it matter or not?

The answer to your question depends on what you mean by "different [aspect] ratio." Almost all CRT and LCD computer displays use "square pixels"--i.e., the number of pixels per inch is the same horizontally and vertically--in their "normal" display modes. And you want square pixels, to avoid seeing everything squished or stretched disproportionately. So the physical aspect ratio of the screen should correspond to the ratio of horizontal to vertical pixels. A "standard" monitor, either CRT or LCD, tends to have an aspect ratio of 4:3 (1600x1200, 1024x768, 800x600, 640x480) or close to it (1280x1024).

Because people are starting to use their computer displays (especially laptops) for widescreen movie viewing, and manufacturers are making LCD panels for video use, we're starting to see more widescreen (16:9) displays on computers. Such displays should be fed with the appropriate ratio of pixels, like 1920x1080 or 1280x720. If such a display receives a signal with 4:3 proportions (e.g., 1024x768), it's either going to show black bars at the ends or stretch everything out; neither option is ideal, but stretching is worse.

If your computer's graphics card and drivers are capable of generating widescreen pixel ratios, you can use a widescreen LCD monitor properly. Whether you want to is up to you. Standard 4:3 displays are available in both CRT and LCD forms, and should produce equivalent images when fed with standard pixel ratios.

If you have something else in mind, please explain your question more clearly and I'll try to help.

[Richard Waller]

A parallel question. My smart new TV has a screen 23 x 13 which is not 4 x 3 or 16 x 9 so either looses the bottom of the the picture, or shows black top and bottom, or when on Auto has a picture smaller than is acceptable. Who designs these things?

[Michael Rowley]

Richard:

a screen 23 x 13 which is not 4 x 3 or 16 x 9

It's about as close as you can get: actual, 1.769; required, 1.777.

[Richard Waller]

OK. So why does the Auto feature screw up here?

[Michael Rowley]

Richard:

So why does the Auto feature screw up here?

I don't know what 'auto' is supposed to do, but I presume it's detects whether the width:height ratio of the signal fed to the monitor is (much) greater than 1.33. On a CRT, any slight difference between a perfect ratio of 16:9 and the actual ratio is dealt with by adjustment of the width or height.

[RJ Emery]

Originally Posted by Stephen Owades

If you have something else in mind, please explain your question more clearly and I'll try to help.

I will try to rephrase the question, but not being all that knowledgeable and certainly being no expert, I am having difficulty articulating what I am trying to express.

IIRC, the last time I checked, which was about two years ago, a CRT had a 4:3 aspect ratio, while a "corresponding" flat panel display had a 5:4 aspect ratio. In my mind for a given image, that meant the flat panel monitor would display a "distorted" image vis-a-vis the CRT.

I understand now that some flat panel monitors can be wide screen, therefore displaying a much different aspect ratios.

I DO NOT WISH TO DISCUSS SUCH WIDE SCREEN FLAT PANEL MONITORS IN THIS THREAD. Anyone who has a desire to do just that should feel free to go off and start another thread.

I wish to understand how 4:3 CRTs and 5:4 LCDs can possibly display the same view given the same resolution (e.g., 1024x768). That one issue stopped me from considering LCDs for DTP work, among other projects sensitive to WYSIWYG graphics.

[gary]

The pixels on the LCD are not perfectly square.

After running Auto-Adjust
The image on my 19" Samsung LCD measures 374mm x 297mm; this is a 1.259 ratio (closer to 1.25 than 1.33)
This means each of the 1280x1024 pixels is 0.292mm wide and 0.290mm tall (1.0069 aspect)
This means 86.9 pixels/inch horizontal versus 87.6 pixels/inch vertical

The viewable area on my 17" ViewSonic CRT is 318mm x 242mm; this is a 1.314 ratio
This means each of the 1024x768 pixels is 0.311mm wide and 0.315 mm tall (0.987 aspect)
This means 81.8 pixels/inch horizontal versus 80.6 pixels/inch vertical

So, do you prefer wide LCD or tall CRT pixels?
1/0.987 = 1.013 so the CRT pixels are less square than the LCD

Of course I never have to tweak my LCD for bowed sides or rhomboid distortion.

[RJ Emery]

Originally Posted by gary

The pixels on the LCD are not perfectly square. ... So, do you prefer wide LCD or tall CRT pixels?

Gary,

I appreciate your response.

The pixels on the CRT are round if not perfect circles. Your calculations, subject to a slight error, demonstrate that. If the LCD pixels are not square, then it suggests a distortion. Perhaps there would be a distortion given square pixels or not.

On your CRT, at any specific resolution, if you had a (JPEG) image of a perfect circle whose diameter was just less than the viewable height of your CRT, and without making any other kind of adjustments, would it not appear and be measurable as a perfect circle?

If you then took that same image at the same resolution and without any further adjustment, displayed it on your LCD (which has a different aspect ratio than the CRT), would that image then also be measurable as a perfect circle? If so, how and why?

[gary]

Using GIMP I created an 800x800 image and displayed it pixel-for-pixel. It displayed a box 187mm x 187mm (on the LCD) as near I can measure.

I connected the CRT to a system and booted Knoppix 3.9 then used GIMP to create a 720x720 image display pixel-for-pixel. It display a box 138mm x 140mm. Spending a few minutes adjusting horizontal width, position, vertical height, position, trapezoid, pincusion, parallelogram, rhomboid I get a box 147mm x 148mm.

That was a lot of work to get the CRT looking as clean as the LCD. The good news is that you should only have to do this once.

Note that some (Hitachi?) CRTs use rectangular pixels.

And that brings up another issue - there is a one-to-one mapping between virtual and physical pixels on the LCD whereas that is not necessarily true on the CRT. The grey background on the LCD is uniform whereas on the CRT I can see the composition of red, green and blue dots. On the CRT the red pixels are not in the same location as the blue pixels.