What the Heck is Bokeh?
There is much written on the net about this elusive subject, and I won't attempt to provide the etymology of the word (is it Japanese or not?) to compete with it. Suffice to say that different lens designs have an effect on the appearance of the out-of-focus areas in photographs.
Some photographic styles commonly use selective focus to bring the viewer's attention to the subject. That means the subject is focused but the non-subject areas are not. If those unfocused areas are busy and distracting, the objective of bringing attention to the subject is lost. That's why portrait photographers in particular care about bokeh.
There are a couple of myths about bokeh:
- Bokeh is controlled by the roundness of the lens diaphragm.
- Faster lenses have better bokeh.
Both of these are, at the very least, not the whole story, as we will see.
An out-of-focus highlight will be represented as a disk. If that disk is uniformly bright, the bokeh is, in my opinion, neutral. This is common with highly corrected prime lenses. The literature suggest that over-correction of spherical aberration causes the disk of the out-of-focus highlight to have a bright edge. I call this bright-edge bokeh, and it's a close cousin of another distracting effect, known as double-line bokeh, where out-of-focus linear details are rendered as parallel lines. These are examples of poor bokeh.
Lens designs that provide good performance but leave a residue of uniformly under-corrected spherical aberration produce a disk with a faded edge, that I call soft-edge bokeh.
Some lens designs are associated with good bokeh, while others are associated with poor bokeh. The three lens types that are usually found in normal and short telephoto lenses are the traditional Tessar, a double-gauss design like the Planar, and the traditional Sonnar. Sonnars are thought to have the best bokeh, while Tessars are described over and over again as having "clumpy" bokeh that is acceptable to some and not to others. Double-gauss lenses are hard to pin down. Some double-gauss designs have reputations for terrible bokeh, like the Shneider Xenotar, and others are famous for good bokeh, like the Canon Serenar 50/1.8 that was used on their rangefinder camera back in the 50's and 60's.
The Tessar design is a refinement of the old triplet design. It minimizes chromatic aberration and provides good sharpness, but does not perform really well at wide apertures. It used few elements (only two or three groups), so it worked well in uncoated lenses. The Planar/double-gauss design is quite old--over 100 years--but it required 6 elements in at least four groups. Performance without anti-reflection coatings was poor, with lots of flare. So, this design did not gain in popularity until lens coatings were used. The Sonnar design was intended to provide excellent wide-aperture performance without so many air surfaces. The first really fast normal lenses were Sonnar designs, and even as late as the 50's most fast normals were variations of it. But the Sonnar design had a fault when used as a normal: The glass got too close to the film and did not allow room for a reflex mirror. Thus, the Sonnar design could not be used for normal lenses in single-lens reflex cameras what had a mirror box between the lens and the film.The Sonnar design has therefore shifted to longer focal lengths where sufficient clearance could be attained.
In the last couple of decades, lens design has enjoyed a couple of breakthroughs. One is computer optimization, using the modulation transfer function instead of ray-tracing. MTF provides a measure not only of acutance and resolution, but also how they interact with contrast to accurately represent fine detail in the photograph. The other breakthrough is low-dispersion and high-refractive glass. These two breakthroughs, coupled with advanced multi-coatings, have made complex designs possible, especially in with zoom lenses.
To demonstrate and compare bokeh, I have assembled a range of lenses all in the 120-135mm focal length. These are commonly used as short portrait lenses in medium format and long portrait lenses in 35mm. (They are very long for portraits using an APS-sized sensor, but the longer working distance proves to be important in keeping depth-of-field narrow). I chose this focal length for several reasons: 1.) it sits between the popular 80-100mm range in small cameras and the 150-180 range in medium-format cameras, and thus I could throw in some examples from those categories without the change in focal length ruining any hope of comparison. 2.) This focal range is considered a working portrait range in both small and medium-format cameras, and so lenses in this range are more likely to be designed with good bokeh in mind. 3.) I happened to own a wide variety of lenses in this range, including at least one example from the three major lens designs described above.
The lenses in the test are as follows:
Lens Lineup (from left): Zeiss Jena 135 Sonnar, B&L Tessar on Pentacon Six bellows, Zeiss Jena 120 Biometar, Nikkor Series E 75-150, Vega 120, Canon 70-200/4L, Canon 135SF, and Jupiter 9 85/2
Zeiss Jena 180/2.8 Sonnar added to the test, here compared with the 135/3.5 Sonnar. It's heavy in addition to being big.
Carl Zeiss Jena
|M42 mount and adapted to Canon EF. This lens is a four-element Sonnar design with multicoating, and was intended for use in Practika and Pentacon 35mm cameras made in the former East Germany. Produced in the 1980's, but designed in 1965.|
Bausch and Lomb
|A barrel lens mounted on a Pentacon Six bellows, which are adapted to Canon EF. This lens is by far the oldest in the test. It's a pre-war uncoated lenses intended for use on enlargers. Enlarger lenses would not be expected to be optimized for bokeh, and this lens does nothing to refute that assertion. I use it for copy work in medium format. Produced in the 1930's.|
Carl Zeiss Jena
|This five-element double-gauss lens is a variation on the Planar design. The diaphragm was malfunctioning on this lens and so it was tested only wide open. The mount is Pentacon Six medium format, adapted to Canon EF. Produced in the 1980's from a 1950's design. Many buy this lens to escape the often poor bokeh of the Vega (see below), but is it really better?|
Nikon Series E
|The Nikon EM was intended to be a low-cost entry into the SLR market many years ago. The Series E lenses for it were also targeted to a low price point, which is why they are not Nikkors. I included it because when it comes to bokeh, assumptions without testing are dangerous, and it might turn out to be a gem. It has a Nikon AI mount, but I have an adaptor. The lens dates probably from the early 80's, and I expect the design was a stock design in the Nikon lens range. It's a good lens to test whether lens complexity means bad bokeh, especially if the lens is cheap.|
|Arsenal is a Ukrainian manufacturer, and also produces the Kiev line of cameras. The lens is a double-gauss design with six elements in four groups. It is known for being sharp, and in some circumstances for producing unsatisfying bokeh. It's in an Pentacon Six mount. The Vega is likely a 70's or earlier design, and the lens dates from the 80s.|
|This is a highly regarded L-series premium zoom lens from Canon, designed in the late 1990's. It is by far the most expensive lens in the lineup, but also the most complex with 14 elements.|
|The soft-focus effect on this lens seems to be switchable spherical aberration. I find the effect unsatisfying, but the lens is still quite usable with the soft-focus switched off. It's faster than the Zeiss Jena 135 at the top of the list, but is it better? This lens was designed in the 80's or 90's.|
|This lens is a copy of a Sonnar, and is made in Russia for use on Zenit cameras with an M42 mount. It has a preset aperture with many blades and produces a round aperture at all settings. A proper test of this lens would compare it with the Canon 85/1.8, which is known for good bokeh, and various 80mm medium-format double-gauss lenses. I'll save that for a future test.|
Carl Zeiss Jena
|This medium-format portrait telephoto is perhaps the most classic of all Sonnars, being a direct descendent of the original Olympia Sonnar developed for us in the 1936 Olympics. F/2.8 is quite fast at this focal length, and with coverage for medium format, the lens is quite large. I included this lens to compare it with the Canon 70-200/4L zoom, and to compare it with the faster (f/2.8) lenses in the test. It is famous in medium-format circles for having outstanding bokeh, and even non-photographers can pick images made with this lens out of a crowd.|
Test conditions include a near item that would serve as a focus target. A far item would provide a sampling of out-of-focus details to provide the comparison. There were three test scenarios. The first is a wine bottle used as a focus target, with a crystal wine glass and crystal candlestick in the background. The facets of the crystal provides a range of out-of-focus highlights. This test was conducted at f/4 for all the lenses (except the Tessar, at f/4.5). This test did well for looking at out-of-focus specular highlights, but did not reveal general rendering of out-of-focus backgrounds clearly.
The second scenario used a small Hopi Kachina doll as the focus target. A larger doll in the background tested out-of-focus patches of color and a range of details. A glass vase next to the doll in the background provided some out-of-focus specular highlights. I used a striped Mexican blanket as a background to provide out-of-focus linear elements. This test was conducted at f/4 and at f/11. This test gave different results, demonstrating that lenses perform differently in different scenarios. To render the background Kachina doll more out of focus, a third scenario was set up.
The third scenario is a slight variation of the second, to get the items within the field of view and minimum focus distance of the 180mm Sonnar, and to put the background doll more out of focus. This test was conducted at f/4 only with the Sonnar and the Canon 70-200/4L zoom, and then it was repeated with a selection of the lenses at their maximum apertures.
Scenario 3 Subject arrangement. Camera was about four feet to the left of the frame.
Lighting for all tests was the same: A bank of tunsten floodlights to provide plenty of specular highlights.
The camera was a Canon 10D 6-megapixel digital camera with a 15x23mm sensor. 135mm is nearly five times the normal focal length of 28mm for this format. White balance was set to tungsten (2800 degrees Kelvin). Images were shot in raw mode, and batch converted with no processing to high-quality JPEG files of 500x333 pixels. There are some JPEG artifacts in the images, but the JPEGs do not lead me to different conclusions than the original images. The camera was mounted on a very sturdy Bogen tripod (the largest in Manfrotto's line and intended for large-format view cameras). Exposure times were generally 1/8 second at f/4, 1 second at f/11, and 1/15 at f/2.8, with an ISO setting of 200.
Test Scenario 1, Wine Glass at f/4.
|Edge: Slight bright-line
Aperture shape: Slight
Overall effect: Pleasing
Bausch and Lomb
|Edge: Pronounced bright-line|
Aperture shape: None