The total resolving power of the visual system

You are here: Nature Science Photography – Visual acuity – The resolving power of the visual system

Now that we know the limiting variables of our resolving power, we are naturally interested in the result of their interaction. Regrettably, we cannot calculate it by multiplying the individual values, as their effects vary depending on the individual. Fortunately, we can measure visual acuity directly, unlike many other parameters of our visual system, which, due to their nature as sensory variables, can only be measured in comparative values. The classic eye test, as performed by ophthalmologists or opticians, and the already mentioned determination of the contrast sensitivity function serve this purpose.

You must distinguish printed or projected standard vision signs, which are precisely defined in terms of size, brightness, shape, and contrast, during the eye test. Visual cues can be specific letters, numbers, or, most commonly, the so-called Landolt ring. This is a circle with an opening whose size is exactly 1/5 of the diameter of the circle and which points in eight successive images in directions offset by 45° from each other. By recognizing the direction in which the opening points, the subject proves that his visual resolution is at least equal to the width of the gap. Test results typically express visual acuity as a fraction, where the numerator represents the actual distance from which the subject recognizes the sign, and the denominator represents the standard distance from which a person with visual acuity 1.0 or 100% could recognize the sign. Alternatively, one can express visual acuity as a decimal number. If a test person recognizes a visual sign for which the standard distance is 6 meters from exactly this distance, his visual acuity is 6/6 or 1.0. If, on the other hand, he recognizes a sign for which the standard distance is 15 meters from a distance of only 6 meters, his visual acuity is 6/15 or 0.4. Your optician or ophthalmologist should perform an eye test to equip you for the implementation of visual acuity in image reproduction, as we will discuss in the following sections. You will then know about your own physiological prerequisites independently of the average value and can calculate your personal maximum image resolution.

Many tests have found that a resolving power of 1 minute of arc (1/60°) is a reliable average value for normal-sighted middle-aged individuals. Therefore, the eye test assumes this resolution is 100% (or 6/6, or 1.0). Younger people can have up to 50% better visual acuity, while in older people it can be up to 50% below average due to degeneration of the cornea, lens or retina. Practically, the measure of 1 minute of arc means that someone with this visual acuity can distinguish two points at a distance D of 200 mm as separate if they are 0.0582 mm apart. The mentioned distance holds significance as it represents the average closest focusing distance for an adult with normal vision:

Calculation 2

With two further calculations we can relate this value to the grid pattern known from the section „Contrast“ with different spatial frequencies per millimeter or per degree of visual angle (Lp/mm; Lp/°). We encounter them even more frequently when considering the resolving power of the photographic components:

Calculation 3 – Calculation of the number of line pairs per millimeter:

Formula 3 – Calculation of the number of line pairs per degree of viewing angle:

Calculation 4

Conversely, the number of line pairs/° can of course also be used to infer the visual acuity:

Formula 4

These figures indicate that an individual with an average resolving power of 20/20 can only resolve a good 20 Lp/° when reducing contrast to 1/10 of the maximum value, and only 4.5 Lp/° when reducing contrast to 1/100. With the approximate best visual acuity of 20/10, this results in 40 Lp/° and 9 Lp/°, respectively.

To calculate your personal maximum resolving power, first determine your own closest focusing distance (D) by measuring the minimum distance at which you can still see a page of a book in sharp focus, for example. Have your trusted ophthalmologist perform an eye test, then enter the results into formula 3 to determine the number of line pairs per degree:

Calculation 5

Now put the two previously determined values into the following formula and calculate the number of line pairs/mm for your closest focusing distance D:

Calculation 6

Next What the visual system does with object edges

Main Visual acuity

Previous The contrast

If you found this post useful and want to support the continuation of my writing without intrusive advertising, please consider supporting. Your assistance goes towards helping make the content on this website even better. If you’d like to make a one-time ‘tip’ and buy me a coffee, I have a Ko-Fi page. Your support means a lot. Thank you!