Tuesday, February 3, 2009

What's wrong with electronic shutters?

The Micro Four Thirds Standard just eliminated one moving part the mirror so what's about the other one, the shutter? The Panasonic G1 still has a mechanical shutter. What's wrong with electronic shutters?

I found two competent sounding answers at photo.net

Joseph Wisniewski says:
Nicholas, Nikon made six DSLRs with electronic shutter that you describe. D1, D1X, D1H, D70, D50, and D40. The reason these cameras still have a mechanical shutter is that the electronic shutter designs have a few quirks that make a mechanical shutter necessary:

"Read out", the transfer of data from the sensor to the camera's processor, has to happen in darkness. The sensor transfers data by "shifting" it from one pixel to another, like a "bucket brigade". If there's light on it at this time, that light will add to the moving image, resulting in a "smear".
If there were no mechanical shutter, the sensor would be exposed to lots of light for an extended interval before the exposure. This would overload cells, and as the overload spreads across the sensor, you get a phenomenon called "blooming". So, the sensor is kept in the dark, and the light that does fall on it in the time the mechanical curtains open is "cleared" by added circuitry that drains charge from the sensor. This draining process is not good enough to cope with the massive overload you'd get without a physical shutter.
Right now, no camera on the market has this system. The reason it is not more popular is that it requires extra circuitry (protected charge shifting, clearing, etc) that takes silicon area away from the "light sensing" parts of the sensor, and from other useful circuitry such as blooming protection. So, sensors without electronic shutter circuitry simply have better image quality: better dynamic range, better low light sensitivity, and less blooming.

and Eike Welk says
Video cameras have no mechanical shutter for each image. The technology used is fully electronic.

CMOS sensors have something called a 'rolling shutter'. Which means one row of pixels is digitized at a time, and one (other) row of pixels is cleared at the same instant. http://en.wikipedia.org/wiki/Rolling_shutter

This readout of one row, while other rows still collect light, leads to artifacts when the camera or objects in the image move. Those artifacts are frequently called 'jello' effect in discussions about the Canon 5D Mark II and Nikon D90.

CCD sensors in video cameras have light insensitive regions to store the image information ('interline' or 'frame transfer' architectures). The transfer of charge from the light sensitive regions to the storage regions is fast. Therefore CCD cameras can have real electronic shutters. http://en.wikipedia.org/wiki/Charge-coupled_device#Architecture