In the case of a flatbed scanner, you call up the preview via the Preview Scan button in the bar above the preview. For the preview, the entire flatbed is scanned in low resolution and is shown in the preview window. If you want to zoom into a photo, SilverFast scans this photo again in higher resolution. In order to prevent the additional scan, you can increase the resolution for the preview so that it is high enough to zoom into the pictures on your scanner without rescanning.
In Ai Studio, a high-resolution preview can be activated for flatbed scanners under OSX via the menu SilverFast 9 > Settings > General and under Windows via the menu Image > Settings > General. This is then performed with a multiple of the selected preview resolution. This takes a slightly longer initially, but subsequently saves a great deal of time. Without getting too far ahead of ourselves, it is important to point out that both the automatic image optimization and the frame search IFF+ (Intelligent Frame Finding) work all the more precisely the more data is available. If you want to use the high-resolution preview, you must define the scan resolution before the preview scan. See the chapter "Scan Dimensions" for more about resolution.