Similarly, 20/40 is half as good with the subject being able to read at twenty feet what someone with normal vision could read at forty. 20/10 vision is therefore twice as good as 20/20. 20/10 means that same subject can read, from a distance of twenty feet, the line that a subject with 'normal' vision could only read from ten feet. 20/20 means that a subject can read the line that defines average vision from the prescribed twenty feet away. The smallest line that can be read accurately defines the acuity of vision, which is expressed in a quasi-fractional manner.

Using this tool an optometrist or physician would ask you, from a standardized distance of twenty feet (six meters in countries that use the metric system), to read the 'letters' on the chart. The most familiar is called the Snellen chart. Vision is measured using a few different tools. Unlike hearing, our visual acuity is unambiguous and relatively simple to measure. It can have no real part in improving the viewing experience. Beyond our ability to see it, increased image resolution is simply an academic exercise. The Dictionary of Visual Science defines visual acuity as 'acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain.' Simply put, our eyes have a resolution limit. To fully understand the implications of high resolution and high definition we must first explore the limitations of human vision.

I can buy 1080p gear, but will I see the difference? The answer to this question is a bit more ambiguous. Expectations of a real performance improvement based on such an investment will likely go unfulfilled, however! In the consumer electronics world we have to ask a similar question. The most obvious qualification: Is this performance improvement manifest under real world viewing conditions? After all, one can purchase 200mph speed-rated tires for a Toyota Prius®. But, (there’s always a 'but') there are qualifications. 1080p can be significantly better that 1080i, 720p, 480p or 480i. '1080p provides the sharpest, most lifelike picture possible.' '1080p combines high resolution with a high frame rate, so you see more detail from second to second.' This marketing copy is largely accurate.

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