We have all heard about optical illusion. It refers to the perception of visual images that differ from reality. Illusions can be false perceptions of shapes, (intensity of) colors, and dimensional misinterpretation. They are things humans perceive but rarely become aware of, such as subliminal images hidden in moving visuals as (illegally) used by the advertisement business. Subliminal images are recorded by human eyes and brains, but the person observing the image(s) does not become consciously aware of what (s)he sees.
Animation by Edward H. Adelson
In the image above, square A and B are the same shade of gray, but the eye / brain register A to be darker than B. Then there is the old trick to make font corners appear to be sharp; the corner of a letter looks like this when enlarged, while without the tiny stub the top left corner of the letter E (as is shown in the image below this paragraph) would be registered as rounded from a distance. There are many more known illusions that are the product of the eye and brain rather than the real color or shape of an object.
How is this important to airbrush artists? Human perception is capable of registering what exists, even though observations are at times filtered. What is not there, can not be registered, not even unconsciously. So whatever an artists applies to the surface he is working on will be registered and whatever he omits is not. Both consciously and subconsciously by the observers of the art work, the audience. It is why people see in a mere glance that detail was applied or not, even though most of them will remain unable to put to words what they actually see. This is how people distinguish quality work from sloppy art, especially in photo-realistic figurative art.
What this means to airbrush artists who spray figurative art work, is that they have to consciously apply - every single detail to give the audience the impression that they are looking at a piece of quality art work. The audience (and many artists as well, for that matter) may not consciously notice all detail on an aware level, but their eyes and brains register the difference between works of art in which that detail is applied and the ones in which such detail is lacking. Typical details that artists 'forget' to give a sufficient amount of attention are the corners of the eyes in a portrait and the corners of the mouth. It is why I use a computer to store the reference image from which I airbrush. It allows me to zoom in to observe more detail while zooming in also removes distracting impressions surrounding the detail I am spraying.
To be able to apply such detail it is imperative that the airbrush gun is capable of spraying fine detail. This is why many experienced artists who spray photo-realistic art tune their airbrushes, even the ones that are already excellent straight out of the box. Many of them also use special paint / additive mixtures to be able to create the detail they want to put in their art work. It is also why they apply accents with the hairy brush of (color) pencils and why they use stencils or masks in the entire art work or in some specific areas. With the exception of outlandishly talented artists like Alberto Ponno who only uses his tuned Paasche VJR only to create art work that is of a unique, high quality.