Following on from our last blog post ‘Barking mad’, we are going to look at visual acuity and the scale of tree bark textures therefore providing optimum viewing distances.
Using visual acuity to provide optimal positioning
With a bit of understanding about human vision and acuity, we can provide definitive distances for the optimum viewing of tree bark.
Acuity is the detail which a person can perceive form a distance. A higher acuity would mean being able to perceive detail from a larger distance, compared to someone with a lower acuity. 6/6 vision (or 20/20) is equivalent to an acuity of 1.0.
Using the equation:
acuity = 1/gap size in arc minutes
along with the equation:
visual angle in degrees/360 = arc length/2πr
we can calculate the maximum distance at which certain levels of textural detail can be seen by the human eye. See figure 1 to see how this has been applied in a landscape context.
In the calculations we have assumed not all people viewing the surface will have perfect vision, and as such have been inclusive of people with moderately impaired vision, which on the lower end of the scale would have a acuity of 0.125 (International Council of Ophthalmology, 2002, p.g.10). As you might expect, people with more severe visual impairment (a lower acuity value), would have a lower maximum distance for viewing.
The ‘minimum detail size’ is the smallest distance, be it a block of colour in the bark, distance from one fissure to another, or size of a strip of peeling bark you want to be viewed to get the full visual impact of the bark.
Unfortunately, the sizes/scale of bark patterns are not very well recorded, so make sure you take the opportunity to take any measurements if you see any unique bark textures when you’re out and about.
Acer capillipes bark has very narrow stripes, requiring close viewing to see the texture, whilst Betula nigra has larger peelings of bark, which are visible from further away
Please note whilst detail size is somewhat dependant on species, the age of the specimen is also an important factor, as bark changes over time. As such the example species in figure 1 are not representative of all age ranges. Fortunately for many species of trees (especially those with rough/fissured bark), the visual interest of bark often improves (becoming more apparent over larger distances) over time.
As you can see from figure 1, predictably the smaller the detail in the bark texture, the closer you need to be to see it clearly. These distances can be used to position trees relative to the static (seating), or slow moving (pedestrian paths) spaces. This demonstrates why trees with bark interest are so valuable in play areas – the extreme proximity of children with trees provides optimal viewing opportunities of the bark and provides the sensory variety considered especially beneficial within play areas.
International Council of Ophthalmology. (2002) ‘Visual Standards, Aspects and Ranges of Vision Loss with Emphesis on Population Surveys’ [online]. Available at: http://www.icoph.org/downloads/visualstandardsreport.pdf (Accessed 1st July 2020)