Warning: Reading this article may diminish your enjoyment of some photographs you view in the future because you will start to observe a flaw that would previously have gone unnoticed.
A camera lens has many different attributes. There is focal length, zoom, minimum/maximum aperture, sharpness, and chromatic aberration… and that’s just off the top of my head. Lens manufacturers only tell the basics like focal lengths and apertures. Third party reviews will often get in to sharpness and “fringing”. But most of these reviews are concentrate on subjects that are in focus. They usually ignore those parts of the photograph that are outside the range of “acceptable sharpness” since they’re assumed to be unimportant. After, all, if they important, they would be in focus. Besides, it’s all blurry anyway, right? Not always.
A blurry background is not necessarily un-sharp.
At first read, this seems like a contradiction in terms. How can something that is “blurry” be at the same time “sharp”? Why depends on the reason something is blurry. When something is out of focus, the light rays coming through the lens are projected on the film/sensor in a circular area instead of at a single point. If the rays are dispersed evenly, you get a circle like that on the left. If the rays are concentrated towards the center, you get a circle like that on the right. In the center is an example of a perfect in-focus point.
Bokeh: The Forgotten Lens Attribute
The rendering of out-of-focus points by a camera lens is called “bokeh” and it is commonly ignored by lens users and lens designers for the simple reason that “good bokeh” (the images on the right) is created by imperfect lenses, or lenses that exhibit “spherical aberration”. A technically perfect lens (corrected spherical aberration) will render points evenly, thus causing the images on the left. Most lenses will exhibit this “neutral” bokeh.
This quality is of special importance to portrait photographers who almost always want soft backgrounds. In their case, any sharpness will detract from their subjects and so many of these photographers demand lenses with uncorrected spherical aberration for their work.
Now… If the images on the right are examples of “good bokeh” and the images on the left are examples of “neutral bokeh”, then what would be “bad bokeh”? In this case (over-corrected spherical aberration), instead of getting a solid circle, you would get a ring. Unfortunately, the nature of physics says that if an out-of-focus point in the background exhibits good bokeh, then an out-of-focus point in the foreground will exhibit bad bokeh, and vice versa. That is why professional portrait lenses like the Nikkor 105/135 DC (“defocus control”) allow adjustment of the amount of spherical aberration correction in both the forward and backward directions from center. I own a Nikkor 105DC and it is an absolutely beautiful lens; slow to focus but sharp and beautiful!
Now that you know about Bokeh, though, you’re probably going to start noticing it in photographs you see, including your own. You’re going to be distracted by things that are supposed to be out of focus, and there isn’t anything you can do about it. Still, the distraction would have been there whether you knew what was causing it or not, and now that you know what it is, perhaps you can do something about it. If you don’t have or can’t afford a “dc” lens like the above-mentioned Nikkors, things like this can be somewhat corrected with post-processing in Gimp or Photoshop by applying a gaussian blur to these areas, such as what I did here.