How would a curved sensor change camera and lens design?

Question

I've read that lenses are designed to project an image onto a flat surface and that as image sensors grow larger, lenses will grow correspondingly larger and by some additional factor. Flat imaging was clearly important in the early days of photography where an image was projected onto the actual display medium as with Daguerreotypes.

Additionally, flat image projection certainly simplified film photography, where images could be conveniently stored on a rolled-up, long flat strip.

But why are we still projecting images onto flat surfaces if neither the storage medium, or the processing requires it?

Wouldn't lens design be vastly simplified if the image were projected onto a curved sensor? Shutters faster and more efficient if they were located at a fixed point? Is the problem with CCD design and sensor manufacturing? Are manufacturers pursuing this at all? Or are we destined to luggage-sized cameras.

Answer 1

Digital image sensors used in modern cameras are essentially silicon chips:

Sensor dies are produced in large batches on silicon wafers. The wafers are cut into many pieces with each piece housing a single sensor die. The larger the sensor die size, the lower number of sensors per wafer.

The wafers are sawn with diamond saws from silicon ingots, which renders them flat. I can only imagine that trying to saw or otherwise cut a surface curved in two dimensions would be very hard or impossible. Consequently, the "natural state" of a digital sensor is flat.

Image of two such wafers

(Image by Hebbe at German Wikipedia, via Wikimedia Commons.)

However, the raw wafers undergo considerable "post-processing" like polishing which conceivably could be used to grind one surface into a concave shape; but of course the succeeding processes currently are expecting a plane surface, so one must expect all kinds of costly adaptations and problems down the road.

That said, Canon has filed a patent for a curved sensor. I suppose that if the chip is thin enough it is flexible enough for the purpose.

Answer 2

Curved film planes were very common in low-end roll film cameras and even 35 mm simple cameras.

This is because they improve the image quality of a lens with a spherical focal surface, as is the case with the simplest lenses used in consumer cameras back in the day -- the meniscus. Meniscus lenses were common as far back as wet plate days (1850s on), and were popular because they were cheap. Kodak sold the Vest Pocket Kodak with an achromatic meniscus, but did what was necessary to use a flat film plane: installed the lens concave to the world, behind the aperture and shutter.

Later, for marketing reasons, meniscus lenses were often installed the other way around, convex to the world, which allows placing the lens in front of shutter and aperture and avoids "the lens has fallen off!" conversations at the camera store. Unfortunately, that way 'round a meniscus has a curved focal surface; this was compensated, especially in rectangular frame formats, by constraining the film surface to a cylinder which minimized the error, mostly confining defocus visible on the contact prints of the day to the extreme corners of the frame.

This "optimization" for lens cost is very unlikely to appear in digital cameras, however, because it would be offset by the costs associated with fabricating a non-planar sensor surface. Designing lenses with flat focal plane is old news; the Cooke triplet, Periskop, Rapid Rectilinear, and Tessar all have flat focal plane, and predate the introduction of roll film as we know it in 1901.

If you want to look it over, however, try to find one of the "plastic fantastic" 35 mm cameras that have been sold over the past fifty or so years. Ilford and Lomography sell them now as "reusable" (as opposed to disposable) cameras, at prices around US$30 to US$50, and cheaper examples can be had direct from China. All of these have simple meniscus lenses and curved film planes.

Answer 3

It's not like there isn't any research being done in this direction:

Origami silicon optoelectronics for hemispherical electronic eye systems

...reshaping conventional planar sensor systems into hemispherical formats would empower visual recordings with features that are beyond what state-of-the-art cameras can see, such as infinite depth of field, wider view angle, and lower aberrations

These guys have taken rudimentary silicon imaging technology and folded them into curved shapes, mimicking insect eyes:

Admittedly, their results are fairly primitive but the paper is already over 4 years old. Modern CCDs also had humble beginnings!

Answer 4

Or are we destined to luggage-sized cameras. If you consider what can be done with mobile phone cameras, we're really not.

Using that sort of technology and a small sensor we can already build a fixed focal length, fixed aperture, variable focus camera only about 1cm thick. Applying this to a dedicated camera, with other advances from the last couple of decades, we should be able to get the image quality of an SLR from say 20 years ago in a small compact.

But then you hit an ergonomic limit. Cameras have to be held steady, and the controls have to be operable with human hands. Compacts without viewfinders, just like phones, have to be held tens of cm in front of your face, which makes them prone to shake. Viewfinders take up space, and good ones as on SLRs take up a fair bit.

So there's not much demand to make cameras even smaller than we already can. If you want small, just use the phone you're probably already carrying. If you want good quality, you can do pretty well with something that will fit in a pocket. Bulky cameras get you versatility, including better performance over more metrics in a wider range of situations.

Curved sensors are of interest in astronomy. In space-based telescopes they could improve the quality in a given payload.