FAQ
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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

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. Instead of capturing the entire frame at once, information is read from each row of the frame one after the other, top to bottom. The whole process takes up to 1/30 of a second on most cameras. This creates a jelly-like wobbling effect in recorded video when the camera is handheld or moves a lot.

  In a given sensor, this rolling shutter happens equally regardless of the shutter speed, though with slower shutter speeds it may be less noticeable in subject movement due to the extra motion blur.

  CMOS sensors capable of higher shutter speeds than 30 frames per second (and not just through repeating frames) will exhibit less streaking. Hypothetically, if you had a CMOS sensor capable of 48 frames per second (and not just through repeating frames) this would have about the same amount of rolling shutter effect as a 24fps film camera, which has an actual rolling shutter.

  CCD does not suffer from the rolling shutter effect.

• Noise / quality in general

  They'll both perform similarly. 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 while it was once inferior, it seems all the best sensors (for still cameras) in the last year or so tend to be CMOS ones.

  For very small sensors such as in compact cameras and cellphones, CMOS sensors traditionally have poorer sensitivity, a result of making the pixels so small relative to the size of the circuitry on them. However, most new tiny CMOS sensors employ microlenses, and in the future will employ back illumination, which makes up for the sensitivity.

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.

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