Pointing is more complicated than mere visibility; although many 
dé authors treat them together under the heading "acuity", 
we better split them into two groupss the visibility 
tasks and the visual-manual tasks. The objects usually 
   
| which often differ in shape, and basically differ in 
| 
d / | | consist of two types: à target and & measuring mark, 
alr ||| || 
NN | function. The MM has to be placed in such a position 
- on the target that a kind of symmetry is obtained. 
= but of minimizing differences and asymmetry by 
manipulating in the object. 
The target is a point, often blurred, as in an aerial photograph; a 
thick line which has to be bisected; an unsharp edge, the location of 
which has to be determined subjectively; and alignment of the Vernier- 
-type (slide rule) and coincidence (distance-setting on camera). 
The visual performance is often an order better than in the previous 
group; the standard deviation of repeated pointing normally is 2 to 10 
seconds of arc (10 to 50 prad), with the exception that the subjective 
edge location has a somewhat larger accidental error and a ten times larger 
systematic error (Hempenius 1968), because symmetry fails. Besides Image 
Quality and the Contrast Mechanism, the involuntary eye movements play 
& certain role, as does Judgement, another form of Thinking. 
MU Chr 
> < 
align ment pointing edge location coincidence 
  
[Recent research on the Contrast mechanisms and on the quantitative 
rules underlying the pointing operations made it necessary to 
distinguish between the acuity and the pointing group. Pointing is 
based on guessing equality of details; an important facet is the 
general form of Weber's Law: length, size, width, area, and the like 
can be compared and judged to be equal within 1 to 2%, if of 
moderate size and sharp (Davson 1962," p. 234). 
A new rule, based on the experiments by O'Connor (1962), discovered 
by Trinder (1965) and worked out by the author (Hempenius, 1968) says 
that the contrast mechanism of the slope discrimination on almost 
symmetrical intensity profiles senses differences of 10% in the 
relative slopes. These are two quantitative statements on the accuracy 
with which the visual sense can execute basic tasks. The advantage of 
a better understanding leads to the rules of pointing accuracy versus 
image quality, which are of basic value to photogrammetry. |