Reposted from a WUWT discussion as a reference:
The Sunspot Counting Methods
1) Wolf learned that Schwabe did not count ‘small spots and grey pores’. In order to be compatible with Schwabe [so Wolf could use Schwabe’s counts on days when Wolf did not make an observation], Wolf decided also not to count small spots and grey pores. This was before Wolf realized that a k-factor on the formula R = k (10G+S) was needed, and was the way to ensure compatibility.
2) Wolf did not ‘design’ a threshold in his method. He knew quite well that it was silly to throw away spots just because they were small, especially if they defined a group. But he became victim of the desire to be compatible, and when he realized a threshold was dumb it was too late.
3) Around 1875 Wolf found [from the geomagnetic data supplied by Sciaparelli] that Schwabe after all [even after that Wolf had unfortunately adopted Schwabe’s method] was counting about 25% too, and summarily increased all 1849 values [which he had published in 1861] by those 25%.
4) From the mid 1860s Wolf was traveling so much [he was by then director of the Swiss Geodetic Survey] that he stopped using the 80mm X64 telescope altogether and switched to a much smaller 37mm X20 hand-held telescope that he could take with him on travel. With this small telescope there was no longer any need to omit small spots and grey pores, because they could simply not be seen anyway, so the question of a threshold is now moot. Wolf decided by comparison with his larger [standard] telescope that he got a compatible yearly average relative number by multiplying the one derived from the small telescope by a factor of 1.5. This did not carry over to daily of monthly means, because zero times 1.5 is still zero.
5) Wolfer correctly surmised that valuable information was thrown away by omitting spots, so decided to count everything he could see. Every serious observer since then has accepted the wisdom in this. This, of course, means that a k-factor less than 1 must be applied to be compatible with Wolf’s values [after 1976]. Over a 17-yr period of both low and high solar activity Wolfer [or rather Wolf] adopted a k-factor of 0.6.
6) Later observers have simply adopted that same k-factor [as it can never be measured again].
7) Waldmeier introduced a new classification of groups, using letters A, B, …, J, which was an evolutionary sequence from A, an emerging group of small spots without penumbra and without the typical bi-polar structure, e.g. a single small spot, through B, small spots still without penumbra but with a clear bi-polar structure, to C, etc where the spots grow larger and have penumbra. A and B groups make up almost half of all groups and could not be seen with Wolf’s 37mm telescope [as we can verify today as the telescope still exists] and were presumable the ones he did not count with the 80mm [although we don’t really know what he counted]
8) Wolfer’s k-factor of 0.6 was not made by comparison with Wolf’s count on the 80mm [as it should have been], but by comparing in this way: if Wolf had a sunspot number [using the 37mm] of 100, then that was first multiplied by 1.5, yielding 150 which was then divided by Wolfer’s count of 250 to result in 150/250 = 0.6. If we break it down into groups and spots, then Wolf’s R=100 comes from typically G=8 and S=20, while Wolfer’s R=250 comes from G=15 and S=100. The difference, 7, between 8 and 15 reflects nicely that A and B groups almost half of all groups [seen by Wolfer]. The real difference between Wolf and Wolfer is that Wolfer sees 5 times as many spots as Wolf, commensurate with the fact that the modern sunspot number is made up mostly of small spots.
9) The Waldmeier weighting increases on average the number of spots, S, by 44%.
10) The Waldmeier classification increases on average the number of groups, G, by perhaps 10% [this requires a full and careful – but difficult – analysis].