In the following list I always see difficulty values like 2^-12 How to calculate the actual difficulty from it?
First of all, actual difficulty is defined as the number
d such that the probability of a random hash being valid is
1/(d*2^32). In other words, the hash needs to have at least
32 + log_2(d) zeros in front of it to be valid. (This is not exactly the case if
d is not a power of 2, but you get the general idea.)
In that article, a difficulty of
2^-12 means that the probability of a random share being valid is
1/(2^-12 * 2^32) = 2^-20. Note that typically, pool difficulty, is measured in multiples of
2^-16 for reasons discussed in the article, so a difficulty of
2^-12 would be a pool difficulty of
2^4 = 16.
These figures represent the difficulty at which the respective mining pool accepts a hash as a share of the mining pool's work. The actual network difficulty cannot be calculated from these figures.
While Bitcoin uses
SHA256 as the hashing algorithm, Litecoin uses
Scrypt which is much more complex.
Therefore, a Litecoin difficulty of 1 represented a much greater amount of work than a Bitcoin difficulty of 1. If Litecoin mining pool operators would have accepted only hashes that fulfilled a difficulty of 1 or greater as valid shares, they would have only accepted valid blocks as shares, essentially reducing their contributors to solo miners. In order to estimate the work done by their contributers, they allowed hashes that fulfilled a much lower difficulty as shares, as seen in the linked website.