Chasing bokeh: Fuji 50/2

TEAS wrote:

Does sterile mean it is sharp lacks distortion? I generally find that these vague, undefined words are meaningless in terms of scientific reality.

Photography is not a science - it is an art. Of course one might argue that there are multiple aspects to photography but at the end of the day even in plain simple documentation - the more artistic the presentation the better the product.

And of course as an art form there are subjective descriptions which are used to describe the quality of an image. If you are not familiar with the language used to commonly describe the emotional context of art - then this might help.

https://www.words-to-use.com/words/art/

Sharpness of a lens is an engineering concept. Sharpness normally results from removal of aberrations of a finite aperture lens. However, it might just be some of those aberrations that generate interference in out of focus areas that result in what some call pleasing bokeh. In fact one of the most squeaky soft, fuzzy concept in photography is bokeh which is an important factor in the some consider when assessing the quality of a lens.

While bokeh is a concept that arises from interference relationships in out of focus areas and can be non-linear it be explained by non-linear Fourier optics.

http://www.phys.unm.edu/msbahae/Optics%20Lab/Fourier%20Optics.pdf

https://en.wikipedia.org/wiki/Fourier_optics

A lens can be be described as a Fourier transform operation on the incident light on the lens. This is the basis of optical signal processing which basis of such things as fiber optics communications and processing of ultra wideband signaling technologies.

That is the lens takes the Fourier transform onto the image plane. When a point source of light is incident on a lens with a finite aperture- the Airy disk that shows in the image plane the Fourier transform of the impulse conceived with the impulse. This results in the Airy Disk whose width is in a one to one relationship with the focal length of the lens and the size and shape of the aperture opening. Different shapes of aperture opening have different Airy Disk. This is known in photography as diffraction. What happens in front and behind the focal plane is different waves coming from different parts of the lens will have differing delays - depending on the abbreviations of the lens which can result in differencing rendering of the bokeh on the sensor plane (because of destructive interference) patterns.

Lens design is quite complex and today simplified ray tracking algorithms is what dominates lens design. Ray tracing fine. Ray tracking models the light through a lens systems as rays of light that are focused on a sensor plane. The object in the focal plane are in focus and the goal is to design the lens so that the resolution on the focal plane is maximized. This however, ray tracking is a very simplistic model for light and not an accurate model for what goes on in the out of focus areas. Although it requires the computerized design tools require massive compute resources to solve the equations of the interactions of the focal plane. The description is incomplete - in fact ray tracing does not predict the Airy Disk as part of the theory or interference patters in the out of focus areas. For that the more complete theory of Fourier optics is needed.

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/raydiag.html

When it gets right down to it - unless you are designing a specialized copy lens - there are many aspects in the rendering of an image which become important. Out of focus rendering is not important for a copy lens - flat field is the prime consideration. You can turn an enlarger lens around, mount it on a bellows and it will become a fantastic copy lens. However, to express ones self there are many attributes of a lens that come into play that go into a pleasing rendering. What bothers one person or what one person might consider "harsh" or "sterile" another might not be bothered by it. Part of the quality in a lens is giving the photographer the ability to control the out of focus areas so the eye (and attention ) is not distracted away from the main theme of the composition. Bokeh is a fundamental element of composition.

https://www.thephoblographer.com/2013/06/01/the-basics-of-photography-b-for-bokeh/

https://www.imaging-resource.com/news/2013/09/11/understanding-bokeh-the-art-and-science-behind-the-beauty-of-blur-part-1

http://kurtmunger.com/bokeh_samplerid22.html

That may or may not be correlated with the resolution MTF and there can often be a trade off between out of focus control and in focus MTF.