Image resolution and human perception

Sometimes we view a poster or picture from afar and are amazed at the level of detail, or the crispness of the features, yet viewed from up close this just isn’t the case. Is this a trick of the eye? It has to do with the resolving power of the eye.

Images, whether they are analog photographs, digital prints, or paintings, can contain many different things. There are geometric patterns, shapes, colours – everything needed in order to perceive the contents of the image (or in the case of some abstract art, not perceive it). Now as we have mentioned before, the sharpest resolution in the human eye occurs in the fovea, which represents about 1% of the eyes visual field – not exactly a lot. The rest of the visual field until the peripheral vision has progressively less ability to discern sharpness. Of course the human visual system does form a picture, because the brain is able to use visual memory to form a mental model of the world as you move around.

Fig.1: A photograph of a photograph stitched together (photographed at The Rooms, St.John’s, NFLD). .

Image resolution plays a role in our perception of images. The human eye is only able to resolve a certain amount of resolution based on viewing distance. There is actually an equation used to calculate this: 2/(0.000291×distance(inches)). A normal human eye (i.e. 20-20 vision) can distinguish patterns of alternating black and white lines with a feature size as small as one minute of an arc, i.e. 1/60 degree or π/(60*180) = 0.000291 radians.

So if a poster were viewed from a distance of 6 feet, the resolution capable of being resolved by the eye is 95 PPI. That’s why the poster in Fig.1, comprised of various separate photographs stitched together (digitally) to form a large image, appears crisp from that distance. It could be printed at 100 DPI, and still look good from that distance. Up close though it is a different story, as many of the edge features are quiet soft, and lack the sharpness expected from the “distant” viewing. The reality it that the poster could be printed at 300 DPI, but viewed from the same distance of 6 feet, it is unlikely the human eye could discern any more detail. It would only be useful if the viewer comes closer, however coming closer then means you may not be able to view the entire scene. Billboards offer another a good example. Billboards are viewed from anywhere from 500-2500 feet away. At 573ft, the human eye can discern 1.0 PPI, at 2500ft it would be 0.23 PPI (it would take 16 in2 to represent 1 pixel). So the images used for billboards don’t need to have a very high resolution.

Fig.2: Blurry details up close

Human perception is then linked to the resolving power of the eye. Resolving power is the ability of the eye to distinguish between very small objects that are very close together. To illustrate this further, consider the images shown in Fig.3. They have been extracted from a digital scan of a vintage brochure taken at various enlargement scales. When viewing the brochure it is impossible to see the dots associated with the printing process, because they are too small to discern (and that’s the point). The original, viewed on the screen is shown in Fig.3D. Even in Fig.3C it is challenging to see the dot pattern that makes up the print. In both Fig.3A and 3B, the dot pattern can be identified. It is no different with any picture. But looking at the picture close up, the perception of the picture is one of blocky, dot matrix, not the continuous image which exists when viewed from afar.

Fig.3: Resolving detail

Note that this is an exaggerated example, as the human eye does not have the discerning power to view the dots of the printing process without assistance. If the image were blown up to poster size however, a viewer would be able to discern the printing pattern. Many vintage photographs, such as the vacation pictures sold in 10-12 photo sets work on the same principle. When provided as a 9cm×6cm black-and-white photograph, they seem to show good detail when viewed from 16-24 inches away. However when viewed through a magnifying glass, or enlarged post-digitization, they lack the same sharpness as viewed from afar.

Note that 20-20 vision is based on the 20ft distance from the patient to the acuity chart when taking an eye exam. Outside of North America, the distance is normally 6 metres, and so 20-20 = 6-6.

A ballad of the senses

When you’re an infant those memories made aren’t really that accessible when you get older. That’s because humans generally suffer from something scientists term infant amnesia. Something to do with rapid neuron growth disrupting the brain circuitry that stores old memories, making them inaccessible (they are not lost, but tucked away). Of course you don’t want to remember everything that happens in life… that would clog our brains with a bunch of nothingness. But we all have selective memories from infancy which we can visualize when they are triggered. For me there are but a couple, and they are usually triggered by an associative sense.

The first is the earthy smell of a cellar, which triggers fleeting memories of childhood times at my grandmothers house in Switzerland. The second is also of the same time and place – the deep smell of wild raspberries. These memories are triggered by olfactory senses, making the visual, however latent, emerge even if for a brief moment. It is no different to the other associations we make between vision, smell, and taste. Dragonfruit is a beautiful looking tropical fruit, but it can have a bitter/tart taste. Some of these associations have helped us survive over the millennia.

Raspberries on a bush.
Mmmm… raspberries… but you can’t smell them, or taste the ethyl formate (the chemical partially responsible for their flavour)

It makes you wonder then if these sense-experiences don’t allow us to better retain memories. If you travel to somewhere like Iceland, and take a picture of a geyser, you may also smell faint wisps of sulphur. There is now an association between a photograph of geyser, and physically experiencing it. The same could be said of the salty Atlantic air of Iles de la Madeleine, or the resinous smell of walking through a pine forest. Memory associations. Or maybe an Instagram of a delicious ice cream from Bang Bang ice-cream. Again an association. But how many of the photos we view lack context because we don’t have an association between the visual, and information gathered from our other senses. You can view a picture of the ice cream on Instagram, but you won’t know what it tastes or smells like, and therefore the picture only provides half the experience.