Bokeh vs. Sunstars

Are optical designers designing (and re-designing) lenses today to create better bokeh at the expense of optical characteristics such as diffraction spikes (popularly known as star effects/sunstars/starbursts)? Is the trend toward creating “better” bokeh sending the lens-made diffraction spike the way of pay phones, SLR cameras, manual transmissions, and the internal combustion engine?

The photography world, especially on the Internet, is full of debates. Canon vs. Nikon. Full-frame vs. APS-C. Manual vs. Auto. DSLR vs. Mirrorless. Prime vs. Zoom. UV Filter vs. No Filter.

Is bokeh vs. diffraction spikes the latest?

Photographs © Todd Vorenkamp

What is bokeh?

“Bokeh” is a Japanese word that refers to the how out-of-focus highlights are rendered in an image. The aesthetics of bokeh are subjective. In the simplest terms, how a lens renders bokeh is a function of the optical design (lenses), shape of the aperture diaphragm, focus, depth of field, and finally, the background itself. I dig deep into the topic of bokeh in this article: Understanding Bokeh.

What are diffraction spikes?

“Diffraction spike” is the technical term for beams of light that extend outward from point light sources in photographs. As mentioned above, they are sometimes called starbursts, sunstars, star effects, sun flares, etc.

The camera renders them in photographs when light diffracts around the edges of the blades of an aperture. I dive deeper into this topic in the article 6 Tips to Create Compelling Star Effects, Sun Stars, Starbursts, Sun Flares, or Diffraction Spikes in Your Photographs.

Interestingly, your own vision can experience diffraction spikes when bright light shines through your eyelashes.

The Common Factor—Aperture Diaphragms

The overall shape and characteristics of a lens’s aperture can and will affect how the optic renders bokeh and diffraction spikes in photographs. Today’s iris-type aperture diaphragms are the lens’s most prominent factor in both optical effects.

I am not an optical engineer (or an engineer of any kind), so we are going to discuss this awesomeness in layman’s terms and avoid complex formulas and mathematical fun. If you are an optical engineer, please feel free to join the discussion in the Comments section after the article—I welcome your thoughts and input!

Aperture Diaphragms and Bokeh

The most common belief is that the more perfectly circular an aperture diaphragm is, the more beautiful the bokeh it will render. We know from my bokeh article and have already mentioned that there are other factors influencing how bokeh is rendered, but a primary player is the shape of the aperture diaphragm.

In the old days of photography, drop-in Waterhouse Plates with circular openings served to control or limit the amount of light that reached the film. Eventually, more modern lenses were equipped with adjustable iris-type aperture diaphragms.

The openings of mechanical iris-type aperture diaphragms have non-circular shapes—they form polygons. The number of sides of the polygon are determined by the number of blades of the diaphragm. Depending on the design of the diaphragm, that polygon can, and sometimes will, approximate a circle as it is opened and closed. How closely does the diaphragm opening approximate a circle instead of a polygon? That depends on the diaphragm’s design and, sometimes, the f/stop selected. Often the widest possible aperture is the most circular. Sometimes the narrowest aperture is also circular.

If you study the out-of-focus highlights of some lenses, you can see the actual polygon shape from the aperture diaphragm revealing itself in the bokeh. Although bokeh quality is subjective, the most common opinion seems to be that polygonal bokeh shapes can be harsh and distracting. I am in the minority here because I sometimes enjoy the polygonal shapes in some bokeh.

Aperture Diaphragms and Diffraction Spikes

Diffraction spikes are created when light passes the edges of the diaphragm. (Light is diffracted when it passes by an object, so a perfectly circular aperture will create diffraction spikes, but the spike on one side of the circle will overlap the spike from the opposite side of the diaphragm.) Usually, more distinct spikes are created by flatter edges of the aperture diaphragm.

Mobile-device cameras generally have circular apertures and do not create diffraction spikes. You can use portrait mode to simulate bokeh, but smartphones and tablets live in a world without sunstars!

The other profound effect that the aperture diaphragm has on diffraction spikes is that it determines the number of spikes visible. Diaphragm polygons with an even number of sides produce one spike per side of the polygon—a six-bladed aperture will produce six rays of light. Apertures with odd numbers of blades produce double the number of spikes than edges—a 7-bladed aperture will produce 14-spike sunstars.

Why does an odd number of blades create two spikes per blade while an even number of blades produce one spike each? With an even number of blades, the spikes on opposite sides overlap—creating a single spike.

Rounded Aperture Blades—Friend or Foe?

Bokeh has been one of the most discussed topics in photography (at least on the Internet) over the past few years. Likely because of this, lens manufacturers have been placing an emphasis on designing (or redesigning) lenses with an eye toward creating pleasing out-of-focus highlights—including by making the opening of the aperture diaphragm as round as possible.

There are two ways to encourage an iris-type aperture diaphragm to approximate a circle: 1) you can increase the number of blades or, 2) you can curve or round the blades. One downside of increasing the number of blades is added mechanical complexity. Therefore, many lens companies are rounding aperture blades on new lenses or when re-designing older lenses.

Rounding the blades helps create a more circular aperture opening and reduce the hard edges of the aperture polygon and, theoretically, improve the bokeh. Artie M., a frequent Explora reader, commented in the diffraction spike article saying, “A lot of lens manufacturers spent the 2010s rounding every aperture blade they could get their hands on.” A Pentaxian, he has seen the company re-design existing lenses primarily just to re-release them with rounded aperture blades (and new-and-improved coatings in many cases).

Rounded aperture blades have been in lenses for years, but, interestingly, the current rounded blade movement (or at least the marketing promoting it) seems confined to the mainstream lens manufacturing companies (Canon, Nikon, Pentax, FUJIFILM, Sony, etc) while “third party” lens companies seem content with straight blades or not promoting their curved blades.

Testing Diffraction Spikes vs. Bokeh

Artie continues, “Starbursts, run and hide! The bokeh mob is coming after you!”

The mission: To find if there is evidence that rounded blades are improving bokeh while silently killing diffraction spikes, which seem to thrive in diaphragms with straight aperture blades.

To answer this query, I decided to get a selection of my own lenses, plus some new ones, outdoors and in the studio to see what the images revealed.

The “Controlled” Experiment, Part I—Pentax 21mm f/3.2 smc vs. HD + Pentax 21mm f/2.4 HD

Wouldn’t it be great to conduct this test with optically identical lenses that have different aperture diaphragms?

Ricoh recently released a version of its popular Pentax 21mm f/3.2 pancake lens. The Pentax smc-DA FA 21mm f/3.2 AL Limited‘s replacement is the HD Pentax-DA 21mm f/3.2 AL Limited. The new “HD” lens features the same optical design—now with HD and SP coatings—and, you guessed it, rounded aperture blades.

Here are the results:

Conclusion

While the rounded diaphragm of the new 21mm HD lens did not kill the diffraction spikes, it degraded them―they are not as sharp as they were on the 21mm smc version of the lens. Bokeh is a bit smoother with hints of diffraction spikes removed in the HD version, but 21mm lenses aren’t usually known for being bokeh machines. Flare is better controlled with the new coatings of the HD version, as well.

The “Controlled” Experiment, Part II—Pentax 31mm f/1.8 smcP vs. HD

Almost identical to the re-design of the 21mm, Ricoh’s Pentax smcP FA 31mm f/1.8 Limited’s replacement is the HD Pentax-FA 31mm f/1.8 Limited—new coatings and a rounded aperture on the “HD” version.

Pentax published the following at the time of the new HD 31mm’s release:

It seems that the official Pentax press release promises degraded diffraction spikes.

Here are the images.

Conclusion

Interestingly, the rounded blades of the new HD 31mm seem to have enhanced the diffraction spikes while almost imperceptibly rounding the bokeh and removing hints of the spikes surrounding the bokeh. Flare is better controlled with the new coatings of the HD version, as well.

The “Legacy” Comparisons

Taking a look at some of my lenses and photographs, we see that the Nikon AF NIKKOR 50mm f/1.8D (and its older, optically identical, manual focus Nikon NIKKOR 50mm f/1.8) has always been one of my favorite lenses for sunstars (now superseded by the 9 blades of the Nikon NIKKOR 50mm f/1.2). Here is a comparison of its straight 7-blade aperture (our spec sheet says they are rounded) to the rounded 9-blade diaphragm of the Nikon AF-S DX Zoom-NIKKOR 17-55mm f/2.8G IF-ED lens.

The Nikon AF Zoom-NIKKOR 80-200mm f/2.8D ED and my original version of the AF-S NIKKOR 70-200mm f/2.8G ED VR (Version II linked here) were both equipped with 9-bladed diaphragms. While the Internet disagrees about rounded vs. straight blades for both lenses, the degraded sunstar performance would indicate that the 70-200mm lens has rounded blades and—but still a nice diffraction spike—while its older sibling, the 80-200mm, has straight blades and prominent spikes.

Are Rounded Blades Wrecking Diffraction Spikes?

While there is anecdotal evidence around the Internet that rounded aperture diaphragms are the enemy of good diffraction spikes—and a cross-section of my own lenses/photographs agree—the test of the two Pentax 31mm versions shows that this is not the case for all optics.

Can diffraction spikes and good bokeh cohabitate in the same lens?

The Ricoh website for the HD PENTAX-D FA 21mm f/2.4ED Limited DC WR lens claims that the 8-bladed aperture diaphragm has been designed for good bokeh and good diffraction spikes.

Bokeh vs. Diffraction Spikes

Spending days and nights thinking about this photographic debate, I have come up with a short list of miscellaneous thoughts about bokeh vs. diffraction spikes.

• The aesthetics of bokeh are completely subjective. Is rounded bokeh better than a subtle hint of a polygon? That is up to the viewer. And, as mentioned above, the rendition of out-of-focus highlights is a function of more than just the shape of the aperture diaphragm.

• Some photographers love diffraction spikes. Others feel disinterested in them. One could argue that they enhance some images but create distractions in others. Like bokeh, it is subjective.

• Wide-angle lenses have never really been known for highlighting bokeh, so I am perplexed as to why wide-angle lenses would be candidates for rounded aperture blade treatment unless part of the goal is to reduce diffraction spikes.

• Bokeh is more prominent at wider apertures and diffraction spikes are more prominent at narrower apertures. This naturally plays into benefiting both phenomena since the diaphragm shape becomes more circular as it gets wider, making the case for rounded blades more complex.

• In the case of some lenses, rounding the aperture blades does not ruin the lens’s ability to create diffraction spikes. In the case of others, it certainly appears to have prioritized bokeh over spikes.

Some lenses certainly prove that diffraction spikes and good bokeh can coexist in the same lens, but, for fans of diffraction spikes, it looks like the trend of rounding aperture blades is making good diffraction spikes harder to find in modern lenses.

Are you a sunstar fan? Are you forever in search of perfect bokeh? Are you an optical engineer who can shed further light on this subject? We would love to hear from you. Please share your experiences with us in the Comments section, below!