Aplanat

[Steve Clark]

Is there a difference in a " Rapid Aplanat" and an "Extra Rapid Aplanat", or was that just turn of the century marketing?

[Ole Tjugen]

There's even a difference between "Aplanat" and "Aplanat", depending on who made it!

but in general, an "Extra Rapid Aplanat" would be something like f:5 to f:6, while a "Rapid Aplanat" would be f:6 to f:7. Just "plain Aplanat" tends to be f:7.2 to f:7.7.

It doesn't look like much of a difference, but it was important at the time (especially in marketing).