No, the brightness depends on what percentage of the time each column is on.
Let's consider some precise numbers. Suppose you switch from column to column every millisecond. Column 1 is on for 1 millisecond, then column 2 is on for 1 millisecond, etc.
It takes 8 milliseconds to go over all 8 columns. Then column 1 (or any column) is on for 1/8 of the total time.
Now suppose you switch columns every 10 milliseconds. Column 1 is on for 10 milliseconds, then column 2 for 10 milliseconds, etc.
Now it takes 80 milliseconds to go over all 8 columns, and column 1 is on for 10 milliseconds, so again, 1/8 of the total time.
Obviously you don't want to switch too slow else you will see the LED's flicker.
If you switch too fast then you're wasting power in the switching transistors and there is no point because the human eye can't really see flicker faster than 70 Hz or so (so 10ms would be a good switching time to start with).
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