What's the difference between CCD and CMOS image sensors?

Question

I keep reading articles about CCD vs CMOS image sensors. What is the difference between these two types? What exactly do these sensors do in terms of photography?

Is a CCD-based camera going to be able to compete in the future? If I buy one, can I count on using it for some years or would it be better to upgrade to a camera with a CMOS based sensor?

Answer 1

Both technologies serve the same purpose: to sample and record how much light hit each pixel. They just work differently to achieve that goal. The pixels on a CCD contain no active circuitry, just a small "capacitive bin" which passively stores a charge until it can be shifted along to the next bin, and eventually off the sensor and read by circuitry then. A CMOS sensor is basically a sensor on a big integrated circuit, and it includes a small active circuit including transistors within every pixel so each pixel is capable of actively measuring and maintaining the charge hitting it, rather than just passively holding the charge until it is shifted off for reading.

Both have strengths and weaknesses - some of the top ones involve video mode (or live view mode).

• Vertical streaking

  In live view or video mode, CCD sensors exhibit vertical streaking, where bright points of light in the frame, even at the edge, can create a vertical bright line from the top to the bottom of the frame. This is caused by current from a single pixel "overflowing" and leaking throughout the whole row. Note that professional video cameras which use CCD sensors (and cost thousands of dollars) have circuitry to minimise this. Also, when used for stills ie not in live view/video mode, CCDs operate in a different mode which isn't susceptible to vertical streaking.

  CMOS sensors don't exhibit streaking at all, as each pixel has its own circuitry isolated from other pixels.

• Rolling shutter

  CMOS sensors exhibit a rolling shutter effect in live view or video mode, or any time they are not using a physical, mechanical shutter. Instead of capturing the entire frame at once, information is read from each row of the frame one after the other. The time this takes varies between cameras, but 1/30s or 1/60s would be typical durations for a full sensor readout (at full resolution). This creates a jelly-like wobbling effect in recorded video when the camera is handheld or moves a lot, or even in stills, when using electronic shutter (for fully silent operation).

  CMOS sensors specifically designed to allow high frame rate video capture (such as 120fps or more) will exhibit less rolling shutter effect. Additionally, running a sensor at lower than full resolution (eg recording 1080p on a sensor instead of 4K) can switch the sensor to a faster readout mode and therefore have less rolling shutter effect.

  CCD does not suffer from the rolling shutter effect.

• Noise / quality in general

  While there used to be a quality trade-off in CMOS, this is negligible now and may even have reversed. Certainly for large sensors (DX, 4/3, FF) there is no practical difference apart from just individual differences due to the design of the sensor. CMOS technology is moving quickly and image quality has been improving, especially with small sensors such as those used in smartphones.

  For very small sensors such as in compact cameras and smartphones, CMOS sensors used to have poorer sensitivity, a result of making the pixels so small relative to the size of the circuitry on them. However, improvements to fabrication processes, and a new technology called "Back Side Illumination" (BSI) have countered this.

Professional still cameras are increasingly using CMOS sensors these days, and the CMOS sensors you'll find in them are at least equal in performance to their CCD cousins. It so happens that CMOS technology is moving quickly at the moment and many of the best sensors these days are CMOS. Unless shooting video, there's no reason to pick a camera based on whether it has a CCD or CMOS sensor.

Answer 2

Don't worry about the sensor technology, it's probably the least important thing to consider when deciding on your equipment set. It'd be like thinking of whether Kodak or Fuji black and white film is "best", without considering the camera you're going to use it in, the lenses you're going to use, or your skills as a photographer.

Think glass, not sensor.

Answer 3

CCDs can have "electronic shutters"; they can be electronically "turned off" before the mechanical shutter closes.

With this feature, you can achieve higher flash sync speeds. For instance, the Nikon D70s and its electronically-shuttered CCD can sync at 1/500s.

CMOS sensors typically can't do this, so they're limited to how fast the mechanical shutter can close. The Nikon D90, for instance, has a max flash sync speed of 1/250s.

Answer 4

The working principle is the same in both systems.

Light makes electrons in silicon 'jiggle about' and the silicon is etched in a way that the jiggling makes those electrons move in the same direction. This process is the same as occurs in solar panels.

When the image is 'read' from the sensor, each pixel has the charge measured (how this happens differs between the two) using an analogue>digital (AD) converter and those values represent the light levels which make up the image.

What splits CCD and CMOS are that the materials and construction differ. That has a knock-on effect on how they're used in practice in photography. CMOS sensors can be baked at just about any chip foundry where CCD's require a tailored VLSI process that can only make CCD chips.

Both systems have traits which give them some on-paper advantage. Excepting some specific tasks (astrophotography for example) it's hard to say that currently either one is actually better than the other. CMOS sensors are cheaper/easier to produce, benefit more readily from other advances in chip making, allow for readouts to occur in parallel and use less power. CCD leaves more area available for the photosite and better noise characteristics but must read line-by-line which slows processing. Currently the chip advances mean CMOS has the edge in photography today and is likely to for the time being.

Answer 5

There are more differences between CMOS and CCD. CMOS sensors are much cheaper then CCD sensors.
It is a lot cheaper to produce CMOS sensor, then produce dificult to make CCD sensor.
CMOS sensor consume less power then CCD sensor which is good for your battery life, and overheating.
Also, you can integrate lot more functions in single CMOS chip, which enables manufacturers to reduce number of chips in their cameras. For example, image capturing and processing can be integrated in one chip, which reduces the costs.