Why did they ever make smaller than full-frame sensors?

Question

You will occasionally encounter articles about how awesome full-frame cameras are. Lot of that is probably over-enthusiasm over a new piece of equipment or simple marketing, but it seems to me that at least these things are true:

• Sensor with a large area captures more light
• Sensor with large individual pixels would have less noise
• Larger sensor can fit much more pixels

The full-frame cameras are much more expensive. This is weird to me, since I had the impression that making electronics smaller is always harder, since you need more precise equipment.

That must've been even more important in the dawn of digital single lens cameras, many years ago.

So why was the decision made to make sensors smaller than is the film originally used in the cameras? AFAIK some lenses made for film cameras still work with some DSLRs, so why make the sensor different from the film?

Note that I'm interested more about history of the initial decision (since film frame size was the status quo, and DLSRs were expensive anyway), than the price difference.

Answer 1

Making large semiconductor devices with no, or only a very small number, of defects is very hard. Smaller ones are much less demanding to make.

In particular the yield – the proportion of the ones you make which are usable – for semiconductors drops as you try and make them larger. If the yield is low, then you have to make a lot of devices for each good one, and this means that the cost per device becomes very high: possibly higher than the market will bear. Smaller sensors, with the resulting higher yields, are then strongly preferred.

Here's a way of understanding the yield curve. Let's say that the chance of a defect per unit area in a process is c, and that such a defect will kill any device which is made out of that bit of semiconductor. There are other models for defects in devices but this is quite a good one.

If we want to make a device which has an area A then the chance of it not having a defect is (1 - c)^A. So if A is 1 then the chance is (1 - c) and it gets smaller (since (1 - c) is less than one) as A gets larger.

The chance of a device of area A not having a defect is the yield: it's the proportion of good devices of area A we get. (In fact the yield may be lower, because there may be other things that can go wrong).

If we know the yield y_A for decives of some some area A, then we can work out c: c = 1 - y_A^1/A (you get this by taking logs of both sides and rearranging). Equivalently we can compute the yield for any other area a as y = y_A^a/A.

So now, let's say that when we make 24x36mm (full-frame) sensors we get a yield of 10%: 90% of the devices we make are no good. Manufacturers are shy about saying what their yields are, but this is not implausibly low. This is equivalent to saying that c, the chance of a defect per mm² is approximately 0.0027.

And now we can compute the yields for other areas: in fact we can just plot the yield curve against area:

In this plot I've marked the expected yields for sensors of various less-than-full-frame sizes if the full-frame yield is 10% (these may be approximate, as APS-C can mean various things, for instance). As you can see smaller sensors get much higher yields.

Over time, as manufacturing processes improve, this yield curve flattens, and the yields for big sensors improve. As this happens, larger sensors drop in price to the point where the market will bear their cost.

Answer 2

The first mainstream applications for electronic image sensors (be it Image-Orthicons, Vidicons, Plumbicons, or CCDs, or CMOS active pixel sensors, be it analog-electronic or digital workflows) were in video, not in still images.

Video followed form factors similar to movie film. In movie film, 35mm (equivalent to full frame still) or even 70mm were exotically large formats only used for actual (cinematic) movie production due to significant costs.

Also, the resolution demands for most video applications used to be much smaller - if pre-HD home televisions (maximum resolution 625 lines of maybe a 1000 pixels each) were the major target, high resolution capabilities were not necessary.

Also, in the non-cinema moving image world the demands on lenses appear to be different - much more expectations on lens speed and zoom range, much less on image quality. This can be done far more cost effective with lens designs that only have to service a small image circle.

Digital still cameras existed several years before interchangeable lens cameras became plausible, and these used tiny sensors first that were very likely designed for or based on designs for video.

APS-C sized sensors were HUGE compared to a normal digital camera sensor when early DSLRs were introduced; the few early full frame DSLRs (think Kodak DCS) and their sensors were extremely expensive, probably because there was very little design experience in making economical sensors in that size.

Image sensors are very coarse in actual structure compared to what CPUs or memory chips even in the 1990s used - for example, a common CPU for late 1990s desktop computers used 250nm feature size, which is quite smaller than what would even be physically useful on a visible-light imaging sensor. Today, 14nm (!!) is about state of the art.

The necessity to avoid large die sizes per part, regardless of the structure sizes, as already explained in other posts, has not changed much.

Answer 3

Big sensors cost more than small sensors for more-or-less the same reason that big TVs cost more than small TVs. Compare a 30-inch TV and a 60-inch TV (about 75cm and 150cm, if you prefer). Miniaturization is no problem — we could make all of the parts of the 30-inch TV way smaller without running into any difficulty. The 30-inch TV costs less to make than the 60-inch TV because it uses less materials and requires less work to finish. And the 60-inch TV will have a higher defect rate — 4 times the area means much higher the chances that something goes wrong somewhere on the screen, creating a dead pixel. Because customers hate dead pixels, a panel that has more than one or two (or maybe even more than zero) gets scrapped, or sold as part of a lower-cost product. The production costs for defective units get rolled into the price of the acceptable units that are sold, so the bigger you go, the more expensive things get.

The same considerations apply to image sensors. Even the smallest sensors on prosumer cameras have features that are huge compared to what semiconductor technology is capable of, so the cost of miniaturization isn't a major factor. Compact cameras and cell phones normally use far smaller sensors, and even budget phones normally have two cameras, with fancier ones having three or four! For reasonable sizes, smaller costs less, not more. The defect issue also comes into play. The bigger you make the sensor, the more likely you will have a defect that requires you to scrap the whole thing, and the more money (in materials) you will lose when you do scrap it. That drives cost up with size, dramatically beyond a certain point.

The largest-format digital camera you can get as of this writing has a whopping 9"x11" sensor (that's more than 8 times the diagonal of a "full frame" sensor, or more than 64 times the area), and it only has 12 megapixels so obviously miniaturization isn't an issue — those pixels are huge. It retails for over $100,000.

Answer 4

Because you specifically asked about history...

I'd suggest: size, weight, & cost.

All those considerations were equally true in the pre-digital (ie film) days. A popular film format was the 110 size. See: https://en.wikipedia.org/wiki/110_film

The 110 film was cheaper, the cameras were cheaper, and many of the cameras were a lot smaller and lighter than the smallest 35mm film compacts. They could fit very easily in a small pocket. Of course those same constraints exist today with digital cameras, as others have pointed out. So it's not just small and big image sensors today; it was also small and big film formats back then as well.

Answer 5

Long before digital, people sought to produce smaller film formats to address manufacturing, usability, and other cost-benefit issues, which are described in other answers.

What is now known as "full frame" was once known as "miniature". If not for miniature and sub-miniature formats, we'd have to carry around cameras like this:

Answer 6

Apart from what has already been mentioned, there is a particularly good reason for making smaller sensors for DSLRs; It makes it easier to design cheaper and lighter lenses for the rapidly growing consumer market. But still of a high quality.

When you make the sensor smaller, you can also make the mirror smaller, and then you can decrease the distance from the rear element in the lens to the sensor (what is known as the flange distance).

Decreasing the flange distance makes it easier to design lenses; wide-angle lenses in particular benefit from the smaller flange distance. An f/2.8 wide angle zoom lens for a full frame camera can be quite costly.

Today, as mirrorless are becoming more popular, the flange distance problem is eliminated.

However, the smaller sensor still means that the lens only has to project the image to a smaller area, requiring a smaller diameter of the lens, contributing to smaller costs in the lenses too.

BTW, to my knowledge (which could be wrong), the sensor is not even close to being the most expensive component a DSLR. The light meters (there are many) are far more expensive.

I thought I had read this from a respectable source, but trying to search for a source to confirm this fact ended up with nothing; so I am probably mistaken here.

Answer 7

Smaller sensors have higher production yields, and the electronics to process are lower cost.

Double the sensor, and roughly square the processing power needed.

The reality is that DX sensors are often higher resolution and greater dynamic range than films they are replacing.

Answer 8

Separate answer, since it is unrelated to the other:

While full frame sensors offer much benefit to the enthusiast, artistic, and professional photographer, they also introduce drawbacks that are in many cases really unwanted by the casual user - and in some cases even by the professional artist or reporter for certain tasks:

• The maximum depth of field reachable is more actually more limited. Extremely slow apertures are needed for extreme depth of field, leading to problems like bad lowlight handling and sensor dirt visibility.

• lenses will be more bulky, heavy, and expensive.

• ... especially when it comes to long focal lengths to have long reach.

• Image stabilization will be more difficult due to the need for larger movements to compensate shake.

• Some target groups will prefer images that have the high depth of field, everything in focus, hard tonality style that they are used to from mobile device cameras.

Answer 9

Well, let me put it this way. Here is a photograph with a small sensor camera (1/2.3"), crop factor 5.6, and an APS-C class sensor (crop factor 1.66, slightly smaller than APS-C) in their maximum zoom position (which the big camera reaches only by using an 1.7× tele converter). The small camera has 3 times the effective focal length (600mm) of the large camera (200mm).

Here are the same cameras ready to pack away:

If you try for birds and closeup shots of small objects, the longer zoom range of the small sensor camera will beat the comparatively short range of the large sensor. Now today's sensors have larger resolutions than the 10MP of the old camera here, but even a 40MP sensor just buys you a factor of 2 in focal length when cropping to the same number of pixels.

The image quality of the larger sensor is quite better but that does not buy you a lot when the image size is that of a stamp.