<tl;dr> Where an index is desired on a mysql column of base36 encoded sha1 hashes, I recommend ADD INDEX (sha1column(10)). Shorter indexes will be sufficient in many cases, but this is still provides a >2/3 space savings while covering a huge (2^51.43) space.
Isn't it better to store BIGINT containing part of (binary) sha1 and use index on numeric column?
AJF/WarX