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The photo sensors of some recent smartphones are approaching sizes so far reserved for system cameras with interchangeable lenses. For example, the Google Pixel 7 has a 25 mm equivalent f/1.9 main lens. The diagonal of the associated sensor is 12.3 mm, hence the crop factor is 3.5. I do not know the weight of this lens, but it’s hard to imagine that it’s more than a few grams, given that the entire phone weighs a bit less than 200 g.

The Micro Four Thirds system features a crop factor of 2. Let’s consider a lens for that system that is generally well-regarded and equivalent to the above one: the Olympus M.Zuiko Digital ED 12mm f/2 lens has a focal length equivalent to 24 mm and weighs 130 g.

Optical lens designs can be in principle scaled: Doubling the linear size of a lens doubles all the linear measures and maintains all the angles. Naturally, the weight increases as the third power of the linear size.

Hence, in theory, it is possible to scale up the design of the Pixel 7 lens such that it reaches the crop factor 2 of Micro Four Thirds. It’s linear size would increase by a factor of 1.75. If we add 10% to this because Olympus cameras need a larger image circle due to sensor shift stabilization we arrive at 1.9. This means a roughly seven-fold weight increase: the enlarged lens would still weigh not more than 35 g - three times less than the supposedly similar Olympus lens.

Even if one takes into account that the Olympus lens has a metal enclosure, one gets the impression that the smartphone lens design is much lighter and much more compact.

Why is it so and does this mean that there is still a lot of potential for making system camera lenses lighter and smaller? In particular, is it the small flange focal distance (as found in phone cameras) that allows to make lenses so much smaller? If this was the case, this would be a systematic disadvantage of system cameras, since even in mirrorless system cameras a certain flange distance is needed to protect the sensor when changing lenses.

If, as it seems, the spectacular compactness of smartphone lenses is made possible by unorthodox approaches like extensive software correction or extensive use of extreme achromatic elements, do you think that this is a viable path forward for system camera lenses? Or are the associated R&D costs too high for the low volume system camera lens market? It could also be that manufacturing of bigger versions of such lenses becomes radically more difficult.

Or, perhaps the increased bulk of system camera lenses is justified by better optical performance?

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  • \$\begingroup\$ What is the material of the lens (glass or plastic)? How many groups & elements? Do you include the mount and barrel in the 130g weight of the MFT lens? \$\endgroup\$
    – xenoid
    Nov 5, 2022 at 17:45
  • \$\begingroup\$ You've pretty much listed all of the factors involved. What is your question? \$\endgroup\$
    – Michael C
    Nov 6, 2022 at 0:27
  • \$\begingroup\$ If you want to ask and answer your own question, the appropriate way to do that in the SE network is to ask the question as a question only, then write an answer as an answer. \$\endgroup\$
    – Michael C
    Nov 6, 2022 at 0:28
  • \$\begingroup\$ Thanks. I clarified my question and also fixed the crop factor calculation (I wrongly assumed that the diameter of full frame sensors is 35 mm. \$\endgroup\$ Nov 6, 2022 at 22:55
  • \$\begingroup\$ You've still pretty much listed all of the factors involved. What is your question? \$\endgroup\$
    – Michael C
    Nov 7, 2022 at 3:06

2 Answers 2

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Wide angle lenses for system cameras (going back to the Contax and Leica of the 1930s) were often of retrofocus design, so that a very wide lens could mount on a flange of fixed distance from (then) the film plane. This became critical once SLRs were common -- you can make a 35 mm lens for a Contax or Leica rangefinder mount with a deeply protruding rear element group and still clear everything inside the camera, but once you have a moving reflex mirror between the flange and the focal plane shutter, you have a hard limit on how far the lens elements can protrude into the mirror box.

Now, fast forward 90 years -- even in 2022 almost no smart phones feature interchangeable lenses (two years ago, there'd have been no "almost" there), and they have software internal to the "camera" to correct for certain aberrations (like pincushion or barrel distortion), which can simplify the lens design -- especially for wide angle lenses. Rear elements can be as close to the sensor as they need to be -- so there's no need for all the extra elements needed just to make a lens mounted 41-45 mm from the sensor act like it has a 25 mm focal length.

Eliminating those requirements of system lenses (all of which are pretty recently descended from film camera system lenses) means the lens designs can be simplified which, in turn, lets a lens with the same focal length and aperture be physically smaller.

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Looking at the specifications of the Google Pixel 7, the sensor size is 1/1.31 inches. This nomenclature still valid today, stems from the 1950’s. It’s the diameter of a vacuum tube sensor called a Vidcon Tube. We are not going to change the industry --- but this translated to a rectangular sensor for the Google Pixel 7 that measures 7.2mm height by 9.6mm length (some minor variability expected). The diagonal measure of this 4:3 ratio rectangle is 12mm.

It is industry practice to mount a "normal" not wide-angle and not telephoto on a camera for everyday use. Thus "normal" for the Pixel 7 would be a 12mm lens. The realm of wide-angle is 70% of "normal" and shorter = 12 X 0.7 = 8.4mm. Telephoto = 2X "normal" and longer.

Given the angle of view delivered by a lens, we can easily calculate what the focal length must be. In this case, the angle of view given by the specifications is 114⁰. It is industry practice to publish only the diagonal angle of view. The other two are vertical 55.1⁰ and horizontal 69.6⁰. I calculate actual focal length to be 6.9mm In conclusion, I find that the lens f/1.9 with focal length 6.9mm is what I would suspect for a moderately wide-angle lens match for this sensor.

As to the crop factor. This value derives a magnification factor. In other words, the gold standard is the 35mm image frame size used by most all 35mm film cameras otherwise called a Full Frame or FX size. We find the diagonal measures of the diagonals of the two frame sizes to be compared.

The FX measures 24mm height by 36mm length and the diagonal is 43.3mm. The Google Pixel 7 measures 7.2mm height by 9.6mm length with a diagonal of 12mm. The crop (magnification) factor is 43.3 ÷ 12 = 3.6. This tells me, the tiny image generated by the Pixel 7 must be magnified 3.6X to match the size of the venerable 35mm full frame.

Again, I find nothing unusual about the Pixel 7 lash-up. Finding the equivalent focal length if we are working with an FX camera. Crop (magnification) factor multiplied by actual focal length yield equivalent. Thus 6.9 X 3.6 = 25mm (equivalent to a 25mm lens mounted on an FX camera. Wide-angle for the FX is 35mm or shorter.

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  • \$\begingroup\$ Indeed, the diagonal of the Pixel's sensor (the one that I asked about) is 12 mm and thus the crop factor is about 3.5. This is already stated in the question. The question is about how the associated lens can be so small and light. \$\endgroup\$ Nov 9, 2022 at 19:43
  • \$\begingroup\$ The original poster does not know the focal length of the lens. I believe it has a focal length of 6.9mm. The question is, can a 6.9mm project a circle of good definition that inscribes a 12mm diameter? \$\endgroup\$ Nov 10, 2022 at 4:38

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