Originally posted by tux08902 How much of a difference in perspective is 21mm vs. 24mm?
Insignificant. In fact, use your zoom and try 21mm and 24mm setting. The difference is only 1.14X and not of any practical importance.
My rule of thumb: space wide angle primes at about 1.5-1.75X, telephoto primes at about 2-3X (over 35mm on APS-C).
For example, if you get DA21 you should be looking at something about 1.5X longer. FA31, FA35/2 or new DA35 Macro would fit the bill. Then with those focal lengths you are hitting normal range on APS-C, so next step is 2X and options are DA70, FA77 or even FA*85.
So go with something like DA21 <-1.67X-> FA35 <-2X-> DA70.
And consider FA20/2.8 as well (if you can find it).