| Digital cameras usually have both an electronic and a | | | | before starting integrating the incident light, but one |
| more traditional mechanical shutter. In this article we | | | | at a time, in sequence. Each row is reset so that the |
| are going to see how an electronic shutter is realized | | | | time over which it will integrate the light will be |
| in CMOS images sensors. | | | | exactly that selected by the user for the shutter |
| In CMOS image sensors the electric charge created in | | | | speed. In the example above, each row is reset 1 |
| each pixel by incoming photons is read by an amplifier | | | | 125s before being readout. So here is what happens. |
| within the pixel itself. This analog information is then | | | | The first row is reset and, after 1/125s, read out. It |
| read out one row at a time. The time required for | | | | takes about 50 microseconds (1/20000s) to read out |
| the reading operation of a whole frame is quite long. | | | | a row. So, 50 microseconds after the first row has |
| For instance, in a 3 megapixel sensor it takes about | | | | been reset, the second row is reset, too. After |
| 82ms. This time corresponds to 1/12s. Compared to a | | | | further 50µs the third row is reset and so on. 1 |
| typical shutter speed (1/15s - 1/1000s), such 1/12s is | | | | 125s after the first row has been reset, it is read out. |
| a very long time. So it is not feasible to reset all the | | | | 50µs after that, the second row will be read |
| rows (to clear out any accumulated signal), letting | | | | out, too; after still other 50µs it will be the turn |
| them integrate the incident photons over the set-up | | | | of the third row and so on. In this manner, all rows |
| exposure time and then read them all out. This can | | | | are exposed exactly for 1/125s but will have an |
| not be done as the time required for the readout is | | | | offset between each other in the time they are |
| not negligible compared to the exposure time. As an | | | | actually exposed. This "rolling" mechanism gives the |
| example, let us assume we selected an exposure | | | | name to this kind of shutter: "electronic rolling |
| time of 1/125s. After this amount of time, we start | | | | shutter". It can give exposure times from as low as |
| reading the frame. The first row is read after exactly | | | | 50µs up to an arbitrary amount of time (e.g. 1 |
| 1/125s. While the first row is being read, all other | | | | minute). |
| rows keep transforming incoming radiations into | | | | The same rolling shutter can be used for shooting |
| electrons, thus effectively making the exposure | | | | movies, which require 25-30 images per second. |
| longer. The last row will be read after 1/125+1/12s = | | | | CMOS imagers usually also have a global shutter |
| 1/11s. So its real exposure time will not be the set-up | | | | release mode to reset all the rows simultaneously to |
| 1/125s but unacceptably longer (1/11s). | | | | be combined with a mechanical shutter. However, |
| An electronic shutter should work differently. The | | | | electronic shutters are much more reliable than |
| smart idea is resetting all the rows not together | | | | mechanical ones and don't require maintenance. |