Originally posted by sany And a Bokeh is controlled by the size of the aperture. The bigger the size the more bokeh and the smaller the size the bokeh effect is minimal. Also it has to do with the distance of the background to the subject.
Senior experts, please correct me if I am wrong. I just mentioned above what I've been learning from you.
Those are both true, quantitative aspects of out-of-focus features but are not what's commonly thought of as Bokeh which is the
quality of out-of-focus features.
Good Bokeh is usually considered an out-of-focus spot with soft edges and bad Bokeh when such spots have hard of bright edges.
Bokeh sampler page
Sometimes hard edged Bokeh adds to the photo and sometimes not.
Bokeh is mostly a characteristic of a lens. Neutral Bokeh (out-of-focus disks smooth to the edges) is what's usually sought by lens designers.
If Bokeh is not neutral (soft or hard edges) it will often flip from one type to the other for underfocus vs overfocus spots (a result of a lens defect called spherical aberration.)
Finally, Bokeh will vary with f-stop; the size of the out-of-focus spot is directly proportional to the actual size and shape of the aperture and as a result will often be a funny shape for wide open apertures.
Here's a great example of bad Bokeh detracting from a photo by Just1moreDave*
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/141924-other-th...l#post1488235:
Hard-edged Bokeh leading to double images this severe is common with mirror lenses.
You were absolutely correct in your comment that the out-of-focus features shown in the above examples would have been smaller had they been caused by subjects closer to the plane of focus but their nature would have been pretty much the same.
* this is a marvelous photo example - perhaps the most extreme example I've ever seen of bad bokeh overwhelming a photo to the extent that one can barely make out the photo's intended subject.