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Bokeh: The Not-So-Secret Secret Lens Feature That Improves Your Photos
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Bokeh: The Not-So-Secret Secret Lens Feature That Improves Your Photos

Sharp lenses are good, but here's why you should pay attention to background blur and bokeh

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


Until a few years ago, Bokeh was a term very few photographers knew about or understood. Now, however, 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 charge a premium for such leses. The way a lens's 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.

 

Not all blur is created equal, and the more you understand how different kinds of background and foreground blur (also known in Japanese as Bokeh) work, the more control you can have over your photos.



There are two main properties of background blur, the shape (or quality) and the size (or quantity).

Blur Quality: Bokeh

The quality of the blur has come to be known as bokeh. The term is from the Japanese word which in romaji (English characters) is spelled bokeh (pronounced bo-keh) and which means "fuzzy," but usually in the sense of "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 ofphotography, it refers to the fuzzy or confused nature of out of focus areas in photographs. Are they smooth and uniform, or are they some other shape and texture? Smooth, uniform, aesthetically pleasing blur is "good" bokeh while blur which shows evidence of ugly shape and structure is "bad" bokeh.

Ideally, points and lines would blur smoothly as they fell out of focus--in the manner, for example, of a smooth gaussion blurring. This is illustrated by the two images below. The left image shows a pattern which is in focus and the right image shows what an ideal gaussian blurring would look like.


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However, that's not the way optics work. Blurring does not occur in a smooth gaussian manner as objects fall out of focus. Hear's what really happens:





These to 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). The lines don't blursmoothly. They can interact to give patterns as is shown by the star shaped and squarestructures that appear where the lines cross. Aesthetically, these patterns are lesspleasing than a smooth gaussian blur would be and so would not be classified as examplesof good bokeh. In this case, chromatic effects are also visible and the color of the blur patterns is different inside and outside focus.

The exact nature of the out-of-focus pattern depends on a number ofthings, but mainly on the design of the lens and the manner in which aberrations arecontrolled in the out-of-focus image. While all lenses designs attemt to minimizeaberrations in the in-focus image, different designs will have different levels and typesof aberration in the out of focus image. This is why there's different bokeh.

The shape of the aperture is also a factor in determining bokeh. Thefurther from circular the aperture is, the more the out of focus image is likely to showpoor bokeh. The shape of the aperture is also reflected in the shape of out of focusimages of small areas of light. This is shown quite dramatiaclly below in a series ofimages of an out-of-focus star.




On the left is a defocused image of star in the center of the imagetaken with a 50mm f/1.8 lens, wide open at f/1.8. In this case the out of focus image iscircular. However notice also that it's not a smooth blurring of a point source as you'dlike for ideal bokeh It's pretty uniform in illumination and actually appears to have abrighter ring around the outside. In the middle is the defocused image, also at f/1.8, butwith the star in the corner of the frame. This time the out-of-focus image is lenticularin shape. This is due to how the aperture looks to oblique light rays and the shape iscaused by the same factors that result in vignetting in the corners of an image when alens is used wide open.

This particular lens has five aperture blades, so when it is stopped downthe aperture is pentagonal. In the image on the right this can be seen quite clearly. Again this is likely lead to less pleasing bokeh than a circularaperture would. Better lenses often have more aperture blades, and those blades are oftencurved to give an aperture that's more circular. Very cheap cameras mayuse four- or even three-blade apertures resulting in square or triangular out-of-focus highlight--not good bokeh.

Note though that a perfectly round aperture is no guarantee of goodbokeh. All lenses used wide open show a circular out of focus points in the center of theimage, though not all lenses have good bokeh. The out of focus aberrations also have asignificant effect.

The classic example of aperture shape yielding unappealing bokeh is themirror lens. The aperture of a mirror lens is donut shaped, a circular aperture with acircular blockage right in the center as shown below in this defocused image of a star.


 

This "donut" shape can often clearly be seen in defocusedareas of images shot with mirror lenses, as shown in the following pair of images. Theimage on the left was shot using a 500mm f/8 mirror lens, while the image on the right wasshot using a conventional refractive 500mm lens at f/8. I think most people would agreethat the background blur in the image shot with the mirror lens is less attractive thanthat shot with the conventional refractive lens.





Photographers will often argue about the quality of the bokeh for aparticular lens. This is probably because the out of focus image quality depends a lot onexactly how far out of focus the image is, and whether the blur results from objects beingcloser or further away than the plane in focus. The same lens may show different bokehwhen used in different situations with foreground and background objects at differentdistances.

So when comparing two lenses, it's quite possible that one will produce morepleasing bokeh under one set of conditions, but the other lens may be better under adifferent set of conditions.

Some confusuon also comes from the fact that differences inbokeh can be pretty subtle at times. Below is a comparision of two 50mm lenses at f/2 andf/2.8. These images are crops from full-size images and show the same area ofout-of-focus foliage. I think most people would say that lens "B" has the betterbokeh. It's slighly smoother, especially at f/2, but unless they were looking closely,such differences probabaly wouldn't be obvious to most people when looking at a print.





Blur Quantity

While the quality of blur--bokeh--is in itself a "fuzzy "concept and something that's quite difficult to predict or control, the quantity of blur can be calculated quite easily and it's something that you, the photographer, have control over through your choice of focal length and aperture.

Most photographers are familar with the concept of depth of field. It's the range of distances over which objects are rendered acceptably sharp. It might benatural 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'snot true for distant objects. What determines how blurred distant backround objects are is the physical size of the lens aperture. This is simply the focal length divided by thef-stop, so, 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 and when 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, 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.


For 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 thing fair, longer focal length lenses are used from further away to keep the subject size (magnification) the same in each case.

Thesize 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 30D) fordepth of field calculations.

  50mm @ f/1.4 85mm @ f/1.8 135mm @ f/2 135 @ f/2.8
Distance to subject 2.05m 3.5m 5.5m 5.5m
Field of view 24x36" 24x36" 24x36" 24x36"
Magnification 0.025x 0.025x 0.025x 0.025x
Physical Aperture 35.7mm 47.2mm 67.5mm 48.2mm
Depth of field 9.73cm 11.2cm 12.5cm 17.5cm
Blur 25cm behind subject 0.097mm 0.079mm 0.073mm 0.052mm
Blur 50cm behind subject 0.18mm 0.15mm 0.14mm 0.1mm
Blur 1m behind subject 0.29mm 0.26mm 0.26mm 0.18mm
Blur 3m behind subject 0.53mm 0.56mm 0.59mm 0.42mm
Blur 10m behind subject 0.74mm 0.87mm 1.1mm 0.78mm
Blur at Infinity 0.89mm 1.18mm 1.69mm 1.21mm


As you would expect, at the same magnification, the faster the lens, the smaller the depth of field. This also means that the backgound close to subject will be blurred moreby the faster lens. In this case the 50mm lens at f1.4 gives slightly greater blurring for objects up to about 1m 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 f2 lens will give almost twice as much blurring (actually 1.9x as much). The follwing 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 aperure (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 blue (as well as depth of field and magnification) and it can be downloaded here

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