|Extension TIFF Tag SubIFDs|
TIFF Tag Reference
TIFF Tag SubIFDs
Offset to child IFDs.
Each value is an offset (from the beginning of the TIFF file, as always) to a child IFD. Child images provide extra information for the parent image - such as a subsampled version of the parent image.
TIFF data type is Long or 13, IFD. The IFD type is identical to LONG, except that it is only used to point to other valid IFDs.