The boolean option preview is used by a frontend to inform the backend when image acquisition should be optimized for speed, rather than quality (``preview mode''). When set to SANE_TRUE, preview mode is in effect, when set to SANE_FALSE image acquisition should proceed in normal quality mode. The setting of this option must not affect any other option. That is, as far as the other options are concerned, the preview mode is completely side effect free. A backend can assume that the frontend will take care of appropriately setting the scan resolution for preview mode (through option resolution). A backend is free to override the resolution value with its own choice for preview mode, but it is advised to leave this choice to the frontend wherever possible.
This option is not mandatory, but if a backend does support it, it must implement it in a manner consistent with the above definition.