The Ultimate Guide to the PENTAX vs iPhone 120mm F2.8- Optical Design Value Analysis No.130

A review article comparing and analyzing the 120 mm F2.8 telephoto lens in the 2023 Apple iPhone 15 Pro Max with the amazing 1977 PENTAX 120 mm F2.8 lens for single-lens reflex cameras.

Today (as of 2023), people all over the world have smartphones, but in recent years, the function as a camera has also improved rapidly, and there are more parts than cameras in terms of convenience.

On the other hand, it is a smartphone that everyone always wears, but there are very few people who know the mechanism and structure as a camera.

In the first place, even if you are told that it has a built-in camera, it is a size that makes you wonder, "Do you really have many lenses?"

Therefore, in this article, we estimate the design value of the optical system of the telephoto lens for Apple iPhone from the patent information and the production example of the real picture, and compare and analyze the lens performance with the lens for single-lens reflex by simulation.

Please enjoy the special information that can only be read on this blog in the world.

Each lens already has its own in-depth analysis article, so please take a look at it together.


The most representative smartphone, the apple iPhone, was also equipped with a telephoto lens equivalent to a focal length of 120 mm in the new model "iPhone15 Pro Max" in 2023.

At first glance, the iPhone telephoto lens is a very mysterious optical system folded by a tetraprism.

First of all, please take a look at the iPhone 15's tetraprism optical system created in a previous article.

The light that passes through the three lenses on the subject side (left) is reflected four times by the tetraprism and enters the image sensor.

The optical system of the telephoto lens, which is usually long, is folded short (thin).

Because the thinness of a smartphone is the most important, it can be stored smartly by a tetra prism.

In general, this folded optical system is called a Periscope lens.

Periscope means a so-called periscope mounted on a submarine.

This tetraprism optical system is developed and converted into a straight optical path diagram as follows.

If you expand it like this, it is a little closer to the familiar shape for those who have read our blog so far.

So far, I've been looking back at past articles.

The question here is, "Can we say that the lens is short as an optical system?" and "Can we say that the performance is good?" Although we can understand that the lens becomes short (thin) due to the effect of the tetraprism, such questions are being raised.

Therefore, in this article, we will compare and analyze the characteristics of the iPhone lens in the optical sense by comparing the telephoto lens of the iPhone with the lens of the single-lens reflex camera.

Document Survey

For a literature search, see the individual articles 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 left side of the above diagram shows the optical path of the PENTAX M 120 mm F2.8 (blue letters), and the right side shows the optical path of the apple iPhone 15 Pro Max 120 mm F 2.8 (red letters).

The PENTAX M 120 mm F2.8 was made in 1977, which is about 45 years older than the iPhone 15 released in 2023.

The reason why I chose PENTAX lenses this time is because I could only find single-focus lenses with a focal distance of 120 mm in PENTXA among major manufacturers.

Now, returning to the explanation of the figure, the above figure is drawn so as to be the actual size ratio.

The PENTAX on the left is a 35 mm film version for a modern full-size camera, and the diagonal length of the image sensor is about 42 mm. According to the working example of the patent, the diagonal size of the image sensor of the iPhone is 5.6 mm.

The optical system of iPhone is of course the size to store in a smartphone, so it is very small.

The focal length of an optical system decreases in proportion to the size of the image sensor. (Proportional Relationship)

The focal length of the PENTAX lens is 120 mm, but the actual focal length of the iPhone lens is 17 mm.

The 120 mm focal length written on the iPhone is a full-size conversion value, not an actual value.

It means that you can take a picture of the same size as the focal length of 120 mm in a full-size camera.

Since the image sensor and the optical system are proportional to each other, let's make the lens of the iPhone as large as the full size and compare them.

The left side of the above diagram shows the optical path of the PENTAX M 120 mm F2.8 (blue letters), and the right side shows the optical path of the apple iPhone 15 Pro Max 120 mm F2.8 (red letters).

This is the size ratio when you enlarge the lens of iPhone for full size.

PENTAX lenses are composed of 5 lenses in 5 groups. They are good old lenses and do not use special glass materials or aspherical lenses.

The iPhone lens is composed of three lenses in three groups. The first lens is a special material comparable to fluorite, which is considered to be a Super ED lens by NIKON and others, and the other two lenses are aspherical on both sides.

You can see that the lenses of the iPhone are filled with the essence of modern technology despite the small number of lenses.

However, if you compare the size as a pure optical system, the iPhone's optical system is surprisingly larger.

The actual iPhone lens is designed to be the smallest (thinnest) when folded by a tetraprism, so it's not a fair comparison to compare the unfolded state, but it's hard to design a telephoto lens with three lenses.

In addition, in the following articles, we will compare aberrations, etc., but various graphs of iPhone lenses have adjusted the display size ratio of the scale so that they can be compared side by side with full-size lenses.

If you redesign an iPhone lens for full size, you'll see this difference.

Longitudinal Aberration

Graphs of spherical aberration, image surface curvature, and distortion

Spherical Aberration , Axial Chromatic Aberration

Let's take a look at the resolution at the center of the screen and the spherical aberration, which is an index of bokeh. Looking at the d-line (yellow), which is the reference ray, the PENTAX lens is not at the minimum level, but the 5-lens configuration and the Fno are bright, but they are doing well.

It's not that the iPhone lens is extremely bad, but it will be bent in a complicated way.

In general, PENTAX is a basic correction method for optimizing the balance between the aesthetics of bokeh and field curvature.

Axial chromatic aberration, which represents the color bleeding in the center of the screen, is like a model of the classic correction method in which the PENTAX lens overlaps the wavelengths of the d-line (yellow) and the F-line (light blue) to increase the resolution, and the g-line (blue) is overlaid on the C-line (red), which is conspicuous in color, to relax it.

On the other hand, the lens of the iPhone is slightly under-corrected for the d-line (yellow) and the F-line (light blue), and the C-line (red) and the g-line (blue) are far apart.

Field Curvature

Field curvature, which is an indicator of the flatness of the entire screen, is comparable to PENTXA because iPhones use aspherical lenses.


The distortion aberration, which is an index of the distortion of the entire screen, is almost the same for both, because it is difficult to occur with a telephoto lens.

Lateral Chromatic Aberration (Magnification Chromatic Aberration)

The lateral chromatic aberrations of the color bleeding indicator for the entire screen is a little large only for the g-line (blue) of the PENTAX lens, but it is not at the level of the times.

On the other hand, as for iPhones, modern cameras often supplement lateral chromatic aberrations by image processing, but they seem to correct it to a sufficient level as a lens.

Transverse Aberrations

(Left)Tangential direction, (Right)Sagittal direction

Let's look at it as a lateral aberration, a measure of the convergence of the rays at a representative point in the screen.

In the left tangential direction, PENTAX has less coma aberration (asymmetry) at the d-line (yellow), but the flare (tilt) at the g-line (blue) is large due to lateral chromatic aberrations.

The iPhone also has a large coma aberration from the image height in the middle of the screen, and the halo (inclination) around the screen seems to be strong.

Right column Sagittal direction is about the same for both.

Spot Diagram

Spot Scale 0.3 (Standard)

Now let's look at the optical simulation results from the spot diagram that shows the actual behavior of the rays at representative points in the screen.

The difference in the correction policies of axial chromatic aberration and lateral chromatic aberrations appears clearly. The g-line (blue) of PENTAX is conspicuous, but the F-line (light blue) and C-line (red) of iPhones are larger.

Since the g-line (blue) is low in terms of the intensity (visibility) perceived by the human eye, it does not have much effect on resolution degradation, but the F-line (light blue), which has high visibility, has a significant effect on resolution.

In addition, the C-line (red) is a color that strongly stimulates human sensitivity, so it is easy to stand out and you need to be careful.

Spot Scale 0.1 (Detail)

This spot diagram has been scaled and enlarged.

This enlargement scale is for modern ultra-high performance full size, so it is too strict condition for the lens of the old era like this time and the lens for the smartphone.


Maximum Aperture F2.8

Finally, let's check the result of the simulation by MTF in which the resolution performance at the representative point in the screen is scored.

If you look at the blue line graph showing the performance in the center of the screen in the MTF characteristic diagram for the open aperture

Because the PENTAX lenses are put together correctly, they are inferior to modern lenses, but only five lenses depict a solid MTF characteristic.

The iPhone has a little hard characteristic because there is a limit to the correction of chromatic aberration with 3 lenses.


How about the PENTAX M 120 mm F2.8 and the apple iPhone 15 Pro Max 120 mm F2.8, which is probably the world's first and last comparison?

The PENTAX's ability to correct aberrations is also a factor, but the iPhone's performance, which deviated from optical theory, was a little painful.

However, since the iPhone is expected to incorporate the latest image processing technology with AI-like processing, it is easy to predict that it will show off image quality that breaks this handicap.

I can't look forward to the future release and actual shooting.

This concludes the analysis of this lens, but is the result of this analysis reasonable? Why don't you actually take a picture with your own hands and verify it?

Let me conclude by wishing you a chance to meet one of the fates of your life.

LENS Review Jin Takayama

Sample Picture

For a collection of examples, see the individual article.

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

-Apple, PENTAX