This is a performance analysis and review article of the Apple iPhone 15 TelePhoto Lens 120mm F2.8.
In this article, we analyze the structure of the small telephoto lens in the Apple smartphone iPhone.
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 from the patent information and the actual production example of the telephoto lens for the Apple iPhone, and analyze the lens performance by simulation.
Please enjoy the special information that can only be read on this blog in the world.
Overview
At present (2023), the focal length (angle of view) of smartphone cameras, represented by the iPhone, is expanding due to the so-called multi-lens system.
In short, it is equipped with multiple cameras and lenses, and the focal length (angle of view) can be changed like a zoom lens by switching between these and supplementing the image.
First of all, the standard lenses for smartphones in the early days started with a focal length of about 35 mm (converted value), but they have gradually become wide-angle (short-focus) and have a focal length of over 26 mm.
Representative examples of the standard lenses of this smartphone have been analyzed in the past, so please refer to the following.
In addition, the second camera of the smartphone will be equipped with an ultra-wide focal length of around 13 mm.
The ultra-wide angle is probably because the wide-angle lens can be designed to be thin, and if you can shoot with a wide angle of view, you can cut out (digital zoom).
And in recent years, the third camera in the smartphone will be equipped with the long-awaited "telephoto lens".
Now let's take a look at the specifications of the telephoto lens installed in the iPhone in recent years.
- iPhone 12 Pro : 65mm F2.0
- iPhone 13 Pro : 77mm F1.8
- iPhone 14 Pro : 77mm F2.8
- iPhone 15 Pro Max : 120mm F2.8This article
*Figures are full-size equivalents.
The focal length of a telephoto lens is different from that of a general camera, and the telephoto lens of a smartphone is longer than the standard lens (around 26 mm) installed together with it.
The 65 mm f / 2.0 spec, which was installed in the beginning, is in the category of standard lens or medium telephoto lens for general cameras.
However, improvements have been made year by year, and the iPhone 15 has finally achieved a longer focal length of 120 mm, reaching a range where there is little discomfort even if it is called a telephoto lens.
Now, we will analyze what kind of technology and mechanism are behind the realization of a longer focal length for the 120 mm f / 2.8 telephoto lens installed in the iPhone 15 Pro Max.
Document Survey
Apple iPhone lenses are mainly developed by Asian manufacturers such as Lagan Precision in Taiwan, and the specifications of the lenses are not disclosed, so it is usually difficult to estimate from patents.
However, there are very few cases in which Apple itself applies for a patent for a lens, and we were able to find a related patent, which is rare.
I would expect that Largan has deals with other smartphone manufacturers, so I think Apple itself wants to apply for patents and hold the rights to novel features to prevent them from being leaked to other companies.
Now, looking at the related document US 2022/0091373 that was found, a plurality of examples of patterns are described, and roughly speaking, there were three lens configurations and four lens configurations.
At present, it is not clear which is closer to the actual product, but since it seems that the three sheet configuration is recommended in terms of the number of embodiments and variations, the design data will be reproduced below assuming that the three sheet configuration of Embodiment 1 has been commercialized.
Since the detailed design is probably carried out by external organizations such as Lagan Precision, I think it is appropriate to regard this data as "this mechanism as a basic design".
In this blog, the scale is adjusted according to the size of the image sensor so that the graph can be compared regardless of the size of the image sensor.
From an image point of view, it is easy to understand if you think that if this lens is designed for full size.
It should be noted that the final image quality of a system with a small image sensor is not the same even if the amount of optical aberration is the same, due to the influence of stretching and noise.
In addition, it is said that smartphone cameras make full use of various synthesis technologies and image processing, so you need to be careful when you see the performance.
Notes!
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
Above is the optical path diagram of the apple iPhone 15 Pro Max 120 mm f / 2.8.
The optical system is composed of three lenses in three groups, and the first lens uses ultra-low dispersion glass, which is effective in correcting axial chromatic aberration that tends to occur in telephoto lenses.
This is called a Super ED lens by NIKON, and is said to be as effective as fluorite.
The second and third lenses are aspherical lenses made of plastic that are effective in correcting spherical aberrations and field curvature.
After the lens, the most distinctive feature is the "long square" glass block on the image sensor side of the lens.
This block corresponds to the fold in the optical path that apple calls a "tetraprism."
It is not smart to store this long optical system in that thin smartphone, so we fold the optical path to make it thin.
The actual bent optical path is also shown below.
When viewed from the side, it is shaped like a diamond, but since it is called a "tetraprism", it reflects light four times inside the prism and guides the light to the image sensor.
By making good use of reflection in this way, the distance (thickness) from the top of the lens on the subject side to the image sensor is greatly reduced.
This folding allows it to be stored smartly in a smartphone.
If you've never seen the iPhone 15 telephoto lens before, it may look like a "very unusual optical system."
In general, only lenses for single-lens reflex cameras are disclosed, so it is reasonable to think so.
However, the bending optical system using this prism is "so-called mature technology" from the point of view of an optical designer.
Such an optical system is called a reflecting optical system or a bending optical system, and it is not very rare.
For example, the Fujicolor QuickSnap Telephoto is a folding optical system that uses two mirrors and was already put into practical use in the 1990s.
The above diagram shows the optical path of the Fujicolor QuickSnap Telephoto. It looks a little different because it reflects twice using a mirror, but the basic concept is the same.
In addition, the MINOLTA Dimage X, which was launched in the 2000s during the rise of the compact digital camera, is famous for being the world's first foldable zoom optical system using a prism.
To give you more examples, the pentaprism in the SLR finder mechanism is more complex and mysterious.
When you look at the cross section of the pentaprism, it reflects three times, but it also switches the left and right directions at the top of the sky.
However, the scary thing about the iPhone 15 telephoto lens is that it mass-produces this ultra-small tetraprism.
It is an amazing manufacturing technology because such a small part is finished with ultra-high precision and installed in a product that is manufactured in ultra-large quantities, such as a smartphone.
I don't know the exact number of the difference between the production of camera lenses and smartphones, but 100 times is not enough.
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. When you look at the d-line (yellow), which is the reference ray, there is a slight bulge on the plus side, but it can be said that it is doing well as a three lens configuration.
The axial chromatic aberration, which represents the color bleed in the center of the screen, uses a Super-ED lens, and although it does quite well as a three lens configuration, it still has a bit of a rough amount left.
Field Curvature
The field curvature of the flatness index for the entire screen is sufficient up to an image height of about 1.4 mm in the middle of the screen, but insufficient correction is noticeable in the periphery.
Distortion
The distortion aberration, which is an index of the distortion of the entire screen, is almost zero because it is difficult to occur due to the focal length specifications because it is a lens in the medium telephoto range.
Lateral Chromatic Aberration (Magnification Chromatic Aberration)
The lateral chromatic aberrations of the color bleeding indicator for the entire screen is not very small, but it is enough and well organized.
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 column tangential direction, the coma aberration (asymmetry) becomes strong from around the image height of 1.5 mm in the middle of the screen, and the halo (inclination) becomes tight beyond the image height of 2.3 mm around the screen, and the influence of field curvature seems to be quite strong.
In the sagittal direction of the right column, Fno is modest at F2.8, so it doesn't look so bad.
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.
Due to the influence of axial chromatic aberration, even the center of the upper screen in the graph is a bit large.
Spot Scale 0.1 (Detail)
This spot diagram has been scaled and enlarged.
This scale is a display for modern ultra-high-performance lenses, so it is a bit of a harsh evaluation to apply to a three lens configuration like this.
MTF
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 with an open aperture, the mountain is low as a whole and it is painful.
The reason is that the number of components is small due to miniaturization, and it is difficult to correct the axial chromatic aberration.
Conclusion
The apple iPhone 15 Pro Max 120 mm f / 2.8 has a 120 mm equivalent optical system that fits perfectly in the size of a smartphone thanks to its warm, new and reflective optical system technology. However, with only three lenses, the optical performance was a bit painful.
In addition, as I explained at the beginning, the detailed composition of the lens is not disclosed, so the actual performance may be improved more.
At the time of writing, it is not possible to verify it because it has not been released yet, but smartphone cameras are far ahead of general cameras in image processing and complementary technology, and it is predicted that even a slightly painful performance will be dramatically corrected by utilizing the latest AI technology.
Of course, a company like Apple would optimize for that.
I'm sure they will show you the amazing real-life performance.
I'm looking forward to having a comprehensive evaluation of the whole system.
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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.