That elusive "3D pop" defined

"Chris" with the Pentax-FA 43mm Limited

Photographers have argued for decades about a certain appealing quality of an image called "3D pop". Some claim that it doesn't exist. Others concede that it does exist, but deny that lens design has anything to do with the effect. Those that promote 3D pop sometimes resort to hyperbolic claims about the characteristics of certain vintage lenses that (they claim) cannot be found in contemporary glass. The subject is so heated that a few years back it spawned a parody article which, in my opinion, did more harm than good.

Here's my take: 3D pop exists and can be well described. Certain lens designs encourage the production of this effect. While it's not necessarily true to say that older lenses are required for 3D pop, specific vintage lenses are desirable, for reasons that will be explored below.

3D pop and its requirements

We can define 3D pop as that characteristic of an image where the subject has a certain dimensionality, a sculptured profile, that allows it to be clearly distinguished from its environment. Given that a photograph is only a two-dimensional representation of a three-dimensional scene, we rely on visual cues to indicate depth. We can make a more general claim that interpreting images is an experiential process, one that must be learned and is culturally determined. Nonetheless, our understanding that visual interpretation is contextual doesn't mean that 3D pop is illusory or can't be defined.

Let's examine the components that go into producing this effect by first explaining factors exterior to the lens itself: light, subject placement, and perspective. Then we can examine lens design.

Almost all good photography requires suitable light. This is why photographers and film-makers alike have long favoured the "golden hour", that special period in the early morning or evening. During this interval sunlight travels through more of the Earth's atmosphere to reach us than if it was, say, directly overhead. This provides the light with a warm tint, which can be favourable to skin tones. The light also becomes more diffuse, reducing harsh shadows. There are other cases where light might be especially appealing and most of these have to do with directionality. Consider the classic three-point lighting used in portraiture.

Second, it is helpful to have a certain longitudinal subject to environment distance, which is to say space around the subject that acts to physically separate it from the background. Consider the portrait above. The background is blurred, though one can still see just enough to get context. Quite often images with large areas of bokeh are put forward as demonstrating 3D pop, but this is not a sufficient condition. The following photograph demonstrates that it's not necessary to blur the background into obscurity. Even the small distance between the stone and the rock surface behind is enough for a clear and distinct delineation between the two. It also helps that the textures and colours of the objects differ substantially. 

"Shadows cast" with the Pentax-FA 43mm Limited

Third, we must choose our perspective, the position of the camera relative to the subject, so as optimise the angle of light and distances involved. In the photo above the light is at 45 degrees to the camera axis, which aids in distinguishing the stone from the surface.

Even when all external factors align, some photos have the elusive "pop" and others do not. The remainder of the effect can be attributed to the lens itself. Some lenses I've used never pop. Other lenses make it rather more likely.

Diving into the depths

3D pop relies on depth of field (DOF) which is worth explaining in some detail.

DOF refers to that distance before and behind a focused subject in which the photograph appears sharp. But note that the plane of focus actually has no physical depth. Everything not in the focal plane is out of focus... but to a greater or lesser degree.

Hence DOF is perceptual. It depends on the "circle of confusion", defined as the smallest region that can be clearly discerned by the average person under normal conditions. Since this is subjective, different standards exist for specifying the size of the circle of confusion. Furthermore, not all conditions are "normal". We can expect DOF perception to vary with factors like lighting, as discussed above. And since we are not all "average", some people will see 3D pop easier than others.

DOF is further determined by two objective measurements: the magnification factor of the image on the sensor and the lens aperture. For a given circle of confusion, magnification itself depends on lens focal length, subject distance, and the size of the sensor that is registering the image.

Shooting with a longer focal length can help a subject pop, since it compresses the subject-background distance in the resulting image. But there are plenty of counter-examples, such as the 28mm lens described below.

Modulation Transfer Function

An MTF chart displays lens information we can use to evaluate optical performance. Typically such a chart will have three curves:

• 10 lp/mm for large structures, called macro-contrast, global contrast, or simply contrast
• 20 lp/mm for medium structures, called micro-contrast, local contrast, or resolution
• 40 lp/mm for small structures, called acutance

The unit is line pairs per millimeter. We get these values by measuring contrast transitions between edges of parallel lines. Sensors have resolution (AKA spatial frequency) of 100 lp/mm, more than sufficient for anything our eye can discern. The lens we use is the limiting factor.

Not every manufacturer will use the same line pair curves. The standard 40 lp/mm was chosen because it is sufficient for an 8x10 inch print enlarged eight times to be perceived as sharp. This bring sup yet another important consideration: the type of display, size of display, and viewing distance all have a significant impact on 3D pop. An image viewed on a phone screen might not pop, but when printed it might.

Check out Koren and Mansurov in the references for a more detailed explanation of MTF and related subjects.

"sheep" with the Pentax-FA 77mm Limited

Sharpness and all that

So what is sharpness? Though often used colloquially without clear meaning, sharpness is defined as a combination of resolution and acutance. A necessary (but not sufficient) condition for 3D pop is that the focal plane be rendered with both high contrast and high sharpness. This is known as "bite".

A lens with bite has other beneficial properties. It's easier to achieve manual focus since the subject snaps crisply into focus. Autofocus mechanisms similarly benefit, especially those based on contrast detection. In his article "Micro Contrast and the ZEISS ‘Pop’" Lloyd Chambers defined 3D pop almost exclusively in terms of such attributes.

The aperture used is not as important as many think. We don't need to use a lens wide open because 3D pop does not require blurring the background into oblivion (as we've already seen). A tack sharp lens can still produce 3D pop when stopped down. This example from the Chambers article was taken at F5.6. What is perhaps more important is that the aperture is chosen to provide a DOF that ends at that border between subject and background. The subject will bite, but the background won't.

An intriguing observation can be made here. You can view photographs taken with two different lenses (at same aperture and focal length) and they will appear to have different depths of field. Greater bite can increase the perceived DOF. 

Sharpness and contrast are negatively impacted by any number of optical defects, including spherical and chromatic aberrations. Hence a lens optimised for 3D pop needs excellent correction in all areas. A lens hood is mandated to avoid flare. Internal reflections can be a further problem, especially when adapted vintage lenses onto mirrorless cameras.

Given the above design challenges, it's not surprising that a lens suitable for producing 3D pop might well be expensive. Which is also why many fledgling photographers might not be able to demonstrate this effect for themselves, with their kit lenses. This leads to scepticism that 3D pop even exists. The need to spend money and be specific about lens choice leads deniers of pop to accuse believers of simply asserting their prestige. We can now understand why this might be so.

"Dave on his birthday" with the Pentax-FA 77mm Limited

Pentax Limited designs

The next optical factor is rather more obscure. It's something that I learned from Jun Hirakawa of the Asahi Optical Industries Research and Development Centre. Hirakawa designed several amazing lenses for Pentax, including the SMC Pentax-FA 1:1.8 43mm Limited and 77mm Limited models that I've used to illustrate this article. Here's a translation of page 80 of a technical report he wrote in 2000:

The Limited lenses have achieved a level of aberration correction unattainable by earlier concepts. That is to say, without giving priority to resolving power, MTF values and other numerical evaluations, they attain a level of correction in actual photo capture that remains in your mind. This is because currently the subject plane is the target of this numerical evaluation, and thought not to be a suitable evaluation of the depiction of solid objects. Certainly, we think evaluation of object depiction by the numerical value method should be established urgently, but for the time being, that comes after the design.

Simply stated, MTF gives us information about the focal plane only, but since objects are three-dimensional there is more to rendering than this. Standard lens design of the era (1990s) had a goal of a flat subject plane, so that MTF curves looked better to users... and reviewers. I would wager that this is even more true of contemporary designs. 

As a technique to correct aberration, the meridional subject plane is fully corrected and the sagittal subject plane is left slightly under corrected which makes the mean subject plane virtually flat. However, with this correction method sagittal and meridional subject planes open up at middle angles leaving an astigmatic difference. Thus the subject plane is flat making it excellent in numerical evaluation, and you can take a picture with uniform field, but in reality it will lack spice.

In the pursuit of better MTF scores, designers allowed astigmatism to remain in the optics. But what if instead we made the opposite compromise: sacrifice the flat plane of focus in order to fully correct astigmatism? This is exactly what Hirakawa did.

With the limited lenses, even though small amounts of field curvature were left, meridional and sagittal picture fields were fully corrected. In this case, evaluation of the plane isn't so great, but when capturing solid objects astigmatic difference is completely gone, allowing the point of focus to be sprucely depicted.

The 43mm and 77mm Limited lenses are distinctive in being specifically designed to allow field curvature. So long as the subject is in the centre of the frame, it will bite. The curvature will only help to enhance the distinction between subject and environment by pulling (or pushing) the latter out of focus. This allows a "sharp but gentle" depiction of the overall scene.

Furthermore, CA was corrected in some new manner, left unexplained. Just as well, because these lenses do unfortunately demonstrate colour fringing.

The Limited lenses weren't merely corrected for axial chromatic aberration and aberration of magnification, correction tor various wavelength characteristics were carried out. Focus point for each color and out of focus boundaries were aligned, allowing a gentle transition from the solid subject to the out of focus portion. The above aberration corrections which are a bit different from previous ones, were carried out with the Limited lenses.

The priority given to rendering real-world dimensional objects and not the idealised focus plane results in "a gentle transition from the solid subject to the out of focus portion" of the image. If the lens also bites, we get an image like the following.

comfy chair [SMC Pentax 1:2/28]

The case of Hollywood

Pentax was not the only company to design lenses in this way. In fact 3D pop is most often attributed to Zeiss lenses, sometimes as though it is an exclusive property of that brand.

Consider the Carl Zeiss Distagon T* 28/2.0, produced for the Contax/Yashica mount. This lens has achieved a legendary reputation for its rendering, as much prized for cinematography as photography. Designed by Erhard Glatzel, the Distagon marks the only collaboration between Pentax and Zeiss.

In 1976 Pentax were moving their system from M42 screw mount to a brand new bayonet mount. For the launch of their first K-mount camera, the K1000, Pentax wished a distinctive and superior line of lenses. The resulting K-series are still highly regarded today.

The Pentax counterpart to the Distagon was the SMC Pentax 1:2/28, which stayed in production for five years. But photographers soon desired more compact lens designs. Pentax provided these with their M range, while Zeiss persisted with lenses that were often significantly larger.

Reviewer Jannik Peters provides a relevant description:

The Distagon shows a significant amount of field curvature. The plane of focus is comparable to a sphere around the camera. The extreme corners are focused significantly behind the center which will result in a greater subject isolation towards the edges (in comparison to a lens with a very flat field of view). The impression of depth of field is more shallow than the f/2 value may suggest. This property of the lens defines it’s character and is great for close-up photography with shallow depth of field since it conveys a three dimensional impression.

Indeed, this lens readily produces that coveted 3D pop, even though it renders a wide angle of view. This effect is perhaps easier to achieve at near distances, where the floating element provides for excellent close focus performance. In the interior shot above, the comfy chair pops out from the background in a most pleasing manner. 

Limerick roundabout [SMC Pentax 1:2/28]

But even more interesting is how the dimensionality remains present for distant objects... even when the lens is stopped down. One might not wish to call this "pop" but there is something special about the dimensionality of the Limerick roundabout photo above, though shot at F8. 

Conclusions

3D pop is a desirable characteristic of an image where the subject is rendered with a pleasing dimensionality that allows it to be clearly distinguished from its environment. This effect is created by a combination of ideal light, appropriate subject to background distance, and perspective. Furthermore, it requires a lens that is free from optical defects, with high measurements for sharpness and contrast, at least in the part of the frame where the subject is located. Furthermore, a lens that does not perfectly correct field curvature can enhance 3D pop.

Background blur and separation are not the same as 3D pop, though they might enhance the effect. A wide open aperture is not essential and might even be counter-productive. Instead, choose the aperture that most clearly isolates the subject.

And what about contemporary lenses? Have they somehow forsaken this look? I would largely reply "yes" but not because they have more lens elements, as sometimes proposed. New materials and computer-aided design techniques allow the manufacture of complex optics with a precision simply not possible decades ago. Hence the best modern lenses are highly corrected and also have a flat field of view. This produces a "better" lens on paper, and likely also in operation, but the results might not pop like certain vintage lenses. 

That's fine, because though desirable in some photos, we don't always want 3D pop. Sometimes the goal is a homogenous image from corner to corner. This is why photographers might own several lenses that overlap in focal length, each useful for a different purpose. My Pentax-A 28mm F2.8 is better for landscapes than the Distagon, and is much smaller as a bonus!

I wrote this article because I tired of reading simple assertions. As a practice, photography is complex, nuanced, and subjective. Every photo is a combination of myriad factors including decisions made by optical designers years ago. If nothing else, I trust this article has demonstrated that truth.

bee on yellow flower [Pentax-FA 77mm with macro adapter]

References

Chambers, Lloyd. 2017. "Micro contrast and the ZEISS ‘pop’," Lenspire [website], 29 August 2017. available

Hirakawa, Jun. 2000. "Lens technical report. SMC Pentax FA77mm, F1.8 Limited and FA43mm, F1.9 Limited," Photographic Industries 58.1, 78-81. abstract scans

Koren, Norman. 2013. "Introduction to resolution and MTF curves," Norman Koren Photography [website]. available

Mansurov, Nasim. 2020. "How to read MTF charts," Photography Life [website], 12 February 2020. available

Peters, Jannik. 2016. "Review: Contax Zeiss Distagon 2.0/28 T* AEG (C/Y)," Phillip Reeve [website], 30 April 2016. available

RELATED POSTS