Monday, October 06, 2008

Utility of a global shutter

The most notable party trick performed by the new Nikon D90 is the ability to capture short high definition video clips.

Unfortunately, the HD video mode, while perfectly functional for certain types of cinematography has a 'wobble' problem. Different parts of the image frame are captured at a slightly different time. If the camera is panning quickly, vertical lines become slanted. If the camera pans forwards and backwards a jelly like effect is created.

The same effect is in fact present in some consumer level camcorders and some are better than others at hiding it. But the only way to eliminate the wobble completely is to implement a global electronic shutter in the image sensor.

The drawback to having a global shutter is cost: an extra transistor or two per image cell. If the image sensor is pushing the limits of the manufacturing process this reduces the number of megapixels that can be squeezed in.

Video is currently an 'experimental' feature on DSLRs. Early results suggest that it is going to be a major success but at the point video mode is not driving DSLR design to the extent that a 10MP camera with good video would outsell a 12MP camera with occasional wobble issues.

But what if a global shutter could be of advantage to still photography?

One obvious advantage of a global shutter is the elimination of a component that is costly to make and ha a limited lifespan. The D300 shutter is 'only' rated for 150,000 actuations. In the film days nobody cared because 150,000 actuations would cost in the region of $50,000 in film. Today a professional photographer can easily take that number of pictures in a year and spend less than $200 to store the results.

Replacing a mechanical shutter with a global electronic shutter also means that a DSLR can finally achieve the type of flash sync speeds that were previously only available with medium format cameras with in-lens shutters.

And as the global shutter is a purely solid state device it should be possible to achieve even higher shutter speeds than 1/8000th. Traditionally, flash was used to 'stop motion'. What if the same effect could be achieved with available light?

Going solid state also means going silent, at least if the mirror is locked up, that is.

A 12MP camera with a mechanical shutter may still outsell a 10MP camera with a solid state shutter. But what about 24MP versus 20MP? Unlike some I do see a real value to going to higher resolution sensors. But it is clearly a case of diminishing returns.

Going a stage further and designing the sensor to allow a sequence of images to be captured in a short time allows for even more interesting effects. Need high dynamic range? Why not take a bracket of shots at different gain (i.e. ISO) settings and compose the results in the same RAW file?

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