Bokeh Photography 101: The Basics of Creating Good Bokeh in Your Photos

While sharpness is an essential part of most photographs, the creative use background blur or bokeh can be a very useful compositional element.

Until a few years ago, “bokeh” was a term very few photographers knew about or understood. These days, it is a very popular photographic component, which is why lens makers—from Nikon and Canon to Pentax, Sony, Olympus, Sigma, Tokina, and Tamron—are touting “good bokeh” as a desirable feature, and sometimes will even charge a premium for lenses that are capable of creating beautiful background blur.

The way a lens aperture is designed directly affects the look of the bokeh in your photos. In this article, written exclusively for the Adorama Learning Center, optical expert Bob Atkins offers a clear explanation about blur and how to achieve your desired bokeh effect in your own photos.

Not all blur is created equal, and the more you understand how different kinds of background and foreground blur work, the more control you can have over your photos.

In the article below, we’ll discuss the two main properties of background blur:

Blur Quality or Shape

The quality of the blur has come to be known as bokeh. The term is from a Japanese word that is spelled bokeh (pronounced bo-keh) in romaji (English characters) and means “fuzzy,” usually in the sense of being “touched in the head” or “senile.” It can also be used to describe someone who says stupid things or makes silly mistakes.

However, in the world of photography, the term refers to the fuzzy or confused nature of out-of-focus areas in photographs. Are they smooth and uniform or are they of some other shape and texture? Generally, smooth, uniform, and aesthetically pleasing blur is considered “good” bokeh while blur with odd shapes or structure is considered “bad” bokeh.

Factors that affect the quality of your bokeh:

Lens Optics

Typically, points and lines would blur smoothly as they fall out of focus, similar to smooth gaussian blurring. This is illustrated by the two images below. The left image shows a pattern in focus and the right image shows what ideal gaussian blurring looks like.

However, that’s not the way optics normally work. Blurring does not occur in a smooth gaussian manner as image elements fall out of focus. Here’s what really happens:

These two images show what you get when the focus isn’t on the pattern but is adjusted to be either further away (left) or closer (right).

As you can see, the lines don’t blur smoothly. They can interact to give patterns, as evidenced by the star shapes and square structures that appear where the lines cross. Chromatic effects are also visible and the color of blur patterns are different inside and outside focus. Aesthetically, these patterns are less pleasing compared to a smooth gaussian blur and don’t classify as examples of good bokeh.

The exact nature of the out-of-focus pattern depends on a number of things, but mainly on the design of the lens and the manner in which aberrations are controlled in the out-of-focus areas of the image. While all lens designs automatically attempt to minimize aberrations in the areas in focus, different bokeh elements will have different levels and types of aberration in the out-of-focus areas. This is why there are different types of bokeh.

Aperture Shape

The shape of the aperture or lens opening is also a factor in determining the appearance of the bokeh. The less circular the aperture is, the more the out-of-focus areas of the image are likely to be of poor bokeh quality. The aperture shape is reflected in the shape of out-of-focus areas with small areas of light, as you can see in the series of images of a star bokeh below:

On the left is a defocused image of a star in the center of the image, taken with a 50mm f/1.8 lens that’s wide open at f/1.8. In this case, the out-of-focus image is circular. However, notice that the point source blurring is not as smooth as you’d like for your ideal bokeh. It’s pretty uniform in illumination and actually appears to have a brighter ring around the outside.

In the middle is the same defocused image, also at f/1.8, but with the star in the corner of the frame. This time, the out-of-focus star image is lenticular in shape. This shape is due to how the aperture looks to oblique light rays, which causes vignetting in the corners of an image when wide open.

This particular lens has five aperture blades, so the lens opening is pentagonal when stopped down. Hence, the shape of bokeh lights often follow the shape of the aperture or DIY bokeh cards. This can clearly be seen in the image shot at f/8 (right, above) and of the bokeh hearts below.

While shaped apertures create interesting bokeh shapes, this likely leads to less pleasing bokeh than a bigger, more circular aperture would.

Better lenses often have more aperture blades, and those blades are often curved to give the lens opening a more circular shape. Very cheap cameras often have four or even three-blade apertures, resulting in square or triangular out-of-focus highlights that don’t make for good bokeh.

Note though that a perfectly round aperture doesn’t automatically guarantee good bokeh. While all lenses used wide open show circular out-of-focus points in the center of the image, not all lenses can create good bokeh since out-of-focus aberrations also have a significant effect on the result.

Mirror or Refractive

Another classic example of the aperture shape yielding unappealing bokeh is seen in images taken with mirror lenses. A mirror lens’s circular aperture appears to be donut-shaped mainly due to the circular blockage right at the center.

This “donut” shape can often clearly be seen in defocused areas of images shot with mirror lenses, as shown in the pair of images below.

The image on the left was shot using a 500mm f/8 mirror lens while the image on the right was shot using a conventional refractive 500mm lens at f/8. I think most people would agree that the background blur in the image shot with the mirror lens is less attractive than the one shot with the conventional refractive lens.

Distance from Focus Plane

Photographers will often argue about the quality of the bokeh produced by a particular lens. This is probably because the out-of-focus image quality also depends a lot on exactly how far out of focus the subject is and whether the blur results from objects being closer or further away from the plane in focus. The same lens may even produce different bokeh when used in different situations where foreground and background objects are placed at different distances.

So when comparing two lenses, it’s still quite possible for one to produce very pleasing bokeh and still lose against the other under different shooting conditions. Some confusion also comes from the fact that differences in bokeh can be pretty subtle at times. Below is a comparison of two 50mm lenses at f/2 and f/2.8.

These images are crops from full-sized images to show the same area of out-of-focus foliage. I think most people would say that lens “B” has the better bokeh. It’s slightly smoother, especially at f/2, but such differences probably wouldn’t be obvious to most people when looking at a print unless they were looking closely.

Bokeh Quantity or Size

While the quality of blur is a “fuzzy “concept in itself and is something that’s quite difficult to predict or control, the quantity of blur can be calculated quite easily as the photographer has control over this through his choice of focal length and aperture.

Most photographers are familiar with the concept of depth of field. It’s the range of distances over which objects are rendered acceptably sharp. It might be natural to assume that the smaller the depth of field, the more blurred objects outside the depth of field would be, but that would be an incorrect assumption.

It is true that the amount of blurring of objects that are close to but just outside the region in focus is greater when the depth of field is smaller, but it’s not true for distant objects. What determines how blurred distant background objects are is the physical size of the lens aperture. This is simply the focal length divided by the f-stop. For example, a 50mm lens used at f/4 has a physical aperture of 50mm f/4 = 12.5mm.

For a lens that’s focused on a nearby subject while the depth of field is fairly small:

1. The degree of blur of objects close to the main subject is determined by the f-stop of the lens in use. The faster the lens, the smaller the depth of field and the greater the local blur.
2. The degree of blur of objects far behind the subject is given by the physical size of the aperture of the lens in use.
3. The degree of blur at intermediate distances behind the subject has to be calculated. There are no simple rules, except that it’s more for fast lenses with large physical apertures.

Here’s an example: Let’s say you’re taking a portrait and you wonder whether a 50mm f/1.4, 85mm f/1.8, 135mm f/2, or 135mm f/2.8 will better isolate the subject by blurring the background. To keep things fair, longer focal length lenses are used from further away to keep the subject size (magnification) the same in each case.

The size of the blur is the size that the defocused image of a point would be on the camera sensor. A DSLR with a 1.6x multiplier APS-C sensor is assumed (e.g. Canon EOS 80D) for depth of field calculations.

As you would expect, at the same magnification, the faster the lens, the smaller the depth of field. This also means that the background close to subject will be blurred more by the faster lens.

In this case, the 50mm lens at f/1.4 gives slightly greater blurring for objects up to about 1 meter behind the subject in focus. However, as you go further back, the lens with the largest physical aperture starts to show the most blur, and by the time you’re at infinity, the 135mm lens at f/2 lens will give almost twice as much blurring (actually 1.9x as much).

The following images show the effect quite clearly. All three were shot to produce the same magnification of the camera box and so have essentially the same depth of field, but the image shot with the larger physical aperture (longer focal length) lens shows the greatest degree of background blur.

Calculating the magnitude blur is a little tricky but I’ve written a simple program which will do the calculation for you. It calculates both foreground and background blur (as well as depth of field and magnification) and it can be downloaded here.

Bob Atkins

To see more of Bob Atkin's work visit his website at www.bobatkins.com