Seeing Image Sharpness

2006.11.25

One of the most common complaints about digital cameras is that they produce soft-looking images. While softness, or lack of sharpness, is somewhat subjective, most people can agree which of two images appears sharper on the medium on which they are presented. What most people do not realize is that the medium significantly affects perceived sharpness.

The number one factor that affects perceived sharpness is obviously visible size.The larger an image appears to our eyes, the less sharp it will appear. This has to do with both size and viewing distance. The close you stand to an image, the larger it appears. This is why looking at unscaled full-resolution images on a computer is frequently disappointing.The more megapixels a camera has, the larger its unscaled images appear on a set display. This explains why many people upgrading to a higher resolution digital camera report that images from the new camera are not as sharp.

Once the effect of size is understood, the natural tendency is to scale all images to the same size before comparing them. It turns out this is only a partial solution. The problem is that scaling always affects image sharpness but not all scaling affects image sharpness equally. This is generally not a problem if you are judging images on your final medium, since this is exactly how they will appear to their intended audience. Say you showcase your images on 9"x12" prints, then judging the sharpness on such prints is fair, judging sharpness on a 9"x12" display is not.

It turns out I do not have to write much more about this subject because Ken Rockwell already did a good job with his How to Fix Unsharp Images article. There are a few things worth adding though.

Modern displays either use VGA (D-SUB) or DVI connectors. According to the article linked above, pixel clocks do not need to be synchronized when using DVI cables. Actually, this is half-true because there are 2 types of DVI connections: DVI-A and DVI-D. The former actually passes an analog signal through a DVI cable. In that case, pixel clocks must be synchronized. That connection was introduced to ease the transition from D-SUB to DVI but frequently results in confusion as you can buy an analog-only display with a DVI input. Even if your display supports both, it may be the case that your graphic card only outputs analog. With DVI-D, a digital signal is used and there is no need to worry about pixel clocks.

While much more LCDs are being sold than CRTs, using the latter should be expanded upon since they are still very common. Even though CRTs are analog devices, they have a fixed grid of phosphors which produce the image we see. The distance between these phosphors is important and is called dot-pitch. Manufacturers which use Trinitron tubes measure dot-pitch as the horizontal distance between adjacent pixels. Other manufacturers measure the dot-pitch as the diagonal distance between pixels. To know if you have a Trinitron tube, look for 2 thin black lines that cross your display image near the top and bottom third of the monitor. This is easiest to see against a white background.

A CRT display reaches its optimal sharpness when its resolution is set so that pixels exactly match the positions of its phosphors. That is why knowing the dot-pitch is important. To determine the optimal resolution of your display, measure the diagonal, for regular tubes, or the width, for Trinitron tubes. Then, divide you measurement by the dot pitch. The result is roughly the optimal horizontal resolution. For example, lets say a Trinitron CRT measures 400mm in width and its has a 0.2mm dot-pitch. Dividing 400 by 0.2, we get 2000. This indicates that the optimal resolution is between 2048x1536 and 1920x1440. The result is not perfect because there is no 2000-wide resolution and part of the screen area may not be usable. If you use a CRT's adjustment to scale the display accross its entire display area, the likelihood of distortion increases. In terms of sharpness and geometry, LCDs are perfect. Where they are generally weaker are color, contrast and black-level, though this is slowly changing.

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