ROLLING SHUT­TER, WHAT IT IS AND HOW TO AVOID IT

HWM (Singapore) - - Front Page - by Mar­cusWong

The abil­ity to cap­ture video with our dig­i­tal cam­eras is noth­ing new, but does bring along a unique set of prob­lems that af­fect your video, com­monly known as the “Rolling Shut­ter” ef­fect. We look at th­ese prob­lems and how to min­i­mize their ef­fects. When we think of cam­eras and shut­ters, we gen­er­ally think of the tra­di­tional DSLR shut­ter that closes quickly to pre­vent ex­cess light from hit­ting the film, thus giv­ing you a prop­erly ex­posed im­age.

With the ad­vent of dig­i­tal though, imag­ing sen­sors have re­placed film, and this brings with it a dif­fer­ent set of ad­van­tages and dis­ad­van­tages - some of which are more ob­vi­ous when we start to take video.

CMOS OR CCD?

There are two main types of sen­sors used: CMOS and CCD, each with a dif­fer­ent ap­proach to the way they col­lect light.

CMOS sen­sors gen­er­ally use what is called a Rolling Shut­ter method that reads the ex­po­sure in­for­ma­tion line by line. This means in­for­ma­tion is read line-by-line down the sen­sor through the du­ra­tion of ex­po­sure. When the en­tire sen­sor has gath­ered enough light, the pixel-rows again ter­mi­nate in suc­ces­sion, hence giv­ing the ap­pear­ance of “rolling”.

CCD sen­sors on the other hand, tend to use a Global Shut­ter method, in which the en­tire sen­sor is ac­ti­vated and de­ac­ti­vated to­gether. There’s no phys­i­cal shut­ter that cov­ers and un­cov­ers the sen­sors, and the en­tire process is driven by tim­ing.

Both sen­sor types suf­fer from their own types of ar­ti­facts. CCD sen­sors tend to suf­fer from ver­ti­cal smear­ing from bright light sources, while CMOS sen­sors are prone to skew, wob­ble, and par­tial ex­po­sure be­cause of the way the sen­sor col­lects data.

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