Why aren't square CMOS sensors more popular by now? [duplicate]

Question

For anyone who has ever delved into the world of Medium Format Photography, they will undoubtedly understand some of the benefits of using a square format over a rectangle format when creating photos.

Aesthetic considerations of the image aside, it's just mechanically obvious that a square fits inside of the image-circle produced by the camera's lens a lot better than a rectangle does. In other words, you get more bang for your buck in terms of information/light passing through the lens and onto the sensor/film. Capisci?

Another point to consider is that for some shorter lenses with more distorted properties, the symmetry of the image seems a lot more 'balanced' when going square. If you have ever had a go using a 30mm Fisheye on a Hasselblad V system you will know exactly what I'm talking about. When compared to 35mm / Full-Frame Sensor with a 15mm Fisheye it's far superior in creating a more pleasing image, especially of buildings or architecture. Anything where long parallel lines converge now becomes easier to frame up, appearing less squashed.

As sensor technology becomes cheaper wouldn't it make sense for all the leading camera manufacturers to begin moving towards reclaiming some of that wasted image-circle? (especially as lenses cost so much too). Imagine if you had the option to select what size sensor you wanted to use before you shot the photo?

This topic has been on my mind since about 2007 when I first bought a DSLR, back in the era of the crop sensor and DX Fisheye lenses. Around the same time I had begun to experiment with 120 film. I'm just asking questions a decade later as to why the CMOS & CCD technology didn't move in a larger, squarer direction?

If someone can provide a logical reason to cling to the 35mm format in the digital age I'm more than happy to listen. Thanks

Answer 1

Aesthetic considerations of the image aside, it's just mechanically obvious that a square fits inside of the image-circle produced by the camera's lens a lot better than a rectangle does.

The short answer to all of this is that this premise seems logical, but is actually incorrect. It's true that a square is the most area, but that's only the best use of space if you want a square in end.

To put it in concrete terms, a 35mm frame of 36×24mm has a diagonal of about 43.3mm — so we need a circle of that diameter. With our handy high school geometry, we can figure out that the largest square that can fit in that same circle is about 30.6mm on a side. (See Would it be possible to make a 36×36 mm "full frame" sensor? for some example diagrams showing how this works.)

So, if a wider frame is desired, 36mm is reduced to 30.6mm — almost 20% less. That's not "a lot better" at all for that case. If you look at What historic reasons are there for common aspect ratios? — or other aspect-ratio questions, you'll generally see that while square is interesting and popular, there's a historic preference for somewhat-off-square composition.

For almost all of these other ratios, a 3:2 sensor is better — even with almost-square 5:4, that comes to 30×24mm, which is close enough to not make a big difference.

Of course, this could be solved by making a much larger sensor, exceeding the size of the image circle — the corners would vignette but you could crop however you want. But this would be a lot more expensive — the sensor would have to be bigger and so would everything around it. In the case of DSLRs, there would need to be a lot more clearance for the mirror. The shutter would have farther to travel. Overall, the market has basically judged this to be not worth it.