Fast TV Mode

Where the camera movements are very fast, we risk having blurred images.

Indeed, motion blur is a problem that blunders most of the photos taken from other missions that have used point-and-shoot cameras.

In normal conditions this problem would be mitigated by using the Shutter Priority mode (Tv Mode): we specify a short shutter time and the camera adjusts aperture accordingly.

In our case we just cannot do that.

First of all, we do not have a Shutter Priority mode: we will have to emulate it through software, by manually calculating the exposure.

Second, the SD1100 does not have an iris to adjust aperture; our lens is always at an aperture of F/2.8, and we just have a ND filter (a grey filter) that the camera uses when it thinks there's too much light, dimming the luminosity to an equivalent of F/8.1

This means that we don't have a way to fine-tune the quantity of light: it's either a little or a lot.

To compensate exposure, I have developed a sort-of TV Mode, that I have called Fast TV mode. In this mode (programmed in LUA) you specify a desired shutter time (for example 1/2.000"), the camera meters the luminosity and then tries to set the shutter time to the desired value while adjusting the ISO setting in order to have a correct exposure.

I am saying "tries to set" because if the calculated ISO goes too low (below the minimum ISO 50 values supported by the camera) or too high (above a certain limit that you can set to avoid high-ISO noise); the camera, after adjusting the ISO value to its maximum/minimum value will also adjust the shutter time in order to expose correctly.

The shortest shutter time available on a standard SD1100 is 1/1.500". CHDK allows forcing this value to much shorter times (down to 1/10.000" and below) but I have found un-practical to go under the 1/4.000 as with low values the sensor somehow loses linearity and my exposure calculation routines would give bad results.