The Ultimate Guide to the HD PENTAX-DA 21mm F3.2 AL Limited - Optical Design Value Analysis No.048

This is a performance analysis and review article of the HD PENTAX-DA 21mm F3.2 AL Limited.

You hardly understand the specific differences in how the lenses work and how their performance differs from each other, do you?

Even if you look it up in magazines or on the Internet, all you will find are similar "word-of-mouth recommendations" and articles like that.

In this blog, while researching the history of lenses and their historical background, we estimate lens design performance based on patent information and actual shooting examples, and analyze lens performance in detail from a technical viewpoint through simulations.

Professional lens designer Jin Takayama will carefully unravel optical characteristics such as optical path diagrams and aberrations, which are generally not visible, and explain the taste and descriptive performance of lenses in a deep and gentle manner.

Now, please enjoy the special information that you can read only on this blog in the world.


The PENTAX Limited Series is a lens that can be said to be a modern masterpiece that pursues the "joy of ownership" by wearing a high-quality and well-made exterior while keeping a balance between the depiction performance and the size of the product.

There are two types in the Limited series: FA-Limited for full size and DA-Limited for PAS-C size.

At the time of writing (2021), FA-Limited for full-size is selling 3 lenses of large diameter specification with focus on blur.

smc PENTAX-FA 77mmF1.8 Limited
We have analyzed FA 31 mm in the past. Please see the link above.

As for the other DA-Limited for PAS-C size, 6 lenses are sold.

  • HD PENTAX-DA 15mmF4ED AL Limited
  • HD PENTAX-DA 21mmF3.2AL Limited
  • HD PENTAX-DA 35mmF2.8 Macro Limited
  • HD PENTAX-DA 40mmF2.8 Limited
  • HD PENTAX-DA 70mmF2.4 Limited
  • HD PENTAX-DA 20-40mmF2.8-4ED Limited DC WR

The DA-Limited is designed with a small diameter to ensure resolution performance while focusing on the "mobility" required for APS size systems.

I would like to take up 21 mm F3.2 from DA-Limited in this analysis.

Private Memoirs

This article is the first APS size lens analysis of PENTAX lenses.

In the Limited series, there are many lenses with specifications that other companies do not have, so I was wondering which one to choose, but I chose this DA21mm, which is treated like a standard lens in the DA-Limited series.

Another reason was that I wanted to compare the relationship between FA-Limited and FA31mm.

Since the DA is a lens for the APS size sensor, the focal length converted to full size is equivalent to 32 mm, and the focal length setting is close to the FA-Limited FA31mm.

However, since the FA31mm is a large diameter specification of the F1.8, I think that the design philosophy and taste are greatly different. I decided to analyze it from the research heart that it will be a very interesting comparison to see how the difference in philosophy appears in the optical design.

Oh, there was one more thing. If you say, "This lens looks cool," you don't need a reason anymore.

Document Survey

PENTAX is a manufacturer that issues patents in an easy-to-understand manner. The relevant document is Japanese Patent Application Laid-Open No. 2007 / 225960. Example 1 looks very similar to the cross-sectional view of the product. Assuming that Example 1 is commercialized, the design data will be reproduced below.


The following design values have been selected and reproduced from the appropriate patent literature and do not correspond to the actual product. Naturally, the data is not guaranteed, and I am not responsible for any accidents or damages that may occur by using this data.

Analysis of Design Values

Optical Path Diagram

The above diagram shows the optical path of the PENTAX DA 21 mm F / 3.2 Limited.

Composed of eight lenses in five groups, the aspherical lens is placed closest to the image sensor.

Surprisingly, it has a similar layout to the FA31mm (7 groups of 9 lenses). The FA31mm has a large aperture of the F1.8, so it has a large number of lenses, but the difference is not as much as one lens. Before the analysis, I expected that there would be a larger difference in the number of lenses.

On the subject side, a convex lens and a concave meniscus lens, which are often used in classical wide-angle lenses, are arranged side by side, but on the front and back of the aperture, a lens is arranged neatly, giving it a modern look with no gaps.

The reason why the number of lenses is more than expected and the configuration is complicated is probably due to the "difficulty of APS single lens reflex lenses."

For single-lens reflex cameras with a mirror, many companies make the mount the same for full-size and APS, but then APS lenses have a relative disadvantage.

Do you know what the "disadvantage" is?

For the same mount, the flange back (the distance between the mount and the image sensor) must also be the same.

As a result, even though the APS lens has a small image pickup device, it has the disadvantage that "the flange back is the same = relatively long".

Therefore, the APS lens cannot make the entire lens system as small as the ratio of the image pickup device, but rather the design conditions are relatively disadvantageous as compared with the wide-angle lens.

The lens can be used if the mount is the same, but there are also disadvantages as mentioned above.

Be analyzed in this section The DA21mm is equivalent to 31 mm in terms of conversion, so it is becoming a bit difficult with a wide angle, but by making the Fno a modest F3.2 and introducing an aspherical lens, it seems that the policy was to emphasize the compactness while not sacrificing the performance.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

The spherical aberration has a moderate Fno and aspherical lens, so it is not very small, but it is moderate.

The axial chromatic aberration is at a satisfactory level for practical use.

Although the specifications are slightly different, I think it is easy to understand the "moderate condition" if you compare it with the representative classic lens analyzed in the past such as NIKKOR 28 mm F2.8D.

Related article: NIKKOR 28 mm F2.8D

Field Curvature

In field curvature, there is a slight tilt to the minus side around the corner of the screen, but the difference between the sagittal and tangential directions (astigmatic difference) seems to be small.


The distortion is acceptable as a wide-angle single-focus lens, and it is a little larger than a modern high-performance lens. However, I like this slight distortion because it is more like a wide-angle lens.

Lateral Chromatic Aberration (Magnification Chromatic Aberration)

The lateral chromatic aberrations is a little large, but this aberration is noticeable when you narrow it down. On the other hand, this lens has an open Fno of F3.2, which is rather dark, so there are few opportunities to narrow it down for practical use, so it is hard to notice.

In this case, the price, size, and specifications are very different. This is a classic PENTAX selection.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Let's look at it as lateral aberration.

Since the Fno is modest, the aberration is quiet overall for the small number of lenses.

The comatic aberration around the middle image height of 7.8 mm in the tangential direction is a little annoying, but it seems that this comatic aberration is intentionally left to diffuse the strong color bleeding due to the large lateral chromatic aberrations.

Spot Diagram

Spot Scale 0.3 (Standard)

The results of the optical simulation will be shown from here, but let's look at the spot diagram first.

Lateral chromatic aberrations, which I was concerned about, seems to be diffusing by leaving comatic aberration in the g-line (blue) when looking at the lateral aberration, and as a result, it seems to weaken the condensing action when looking at the spot image.

There is a strong V-shaped feeling in the corner of the screen, but it's not a big deal unless you intentionally put a bright spot such as a star in the corner.

Spot Scale 0.1 (Detail)


Maximum Aperture F3.2

Finally, let's look at the results of the MTF simulation.

As can be seen from the spherical aberration, from the center of the screen to the middle of the screen, the characteristics are high. Although there is some degradation in the peripheral area, the flatness of the entire screen seems to be good enough for practical use.

Small Aperture F5.6

When you narrow it down, the mountain becomes higher, but the peripheral part seems to move to the minus side. You can get the effect of improving the resolution by narrowing it down, but you need to be careful.


The Fno is also a moderate and reasonable lens, but it is a product that makes you want to reflect on the recent big-gun principle with the design of Limited's exquisite balance.

With this light and accurate lens, you don't have to worry about focus accuracy or depth control. You can enjoy true snap shooting without any worries.

Sample Picture

Example photos are in preparation.

If you are looking for analysis information on other lenses, please refer to the table of contents page here.