Rafal has the right idea. For most printers, basic-exposure times less than 10 seconds invites extreme difficulty in controlling nuance. If you have a 4.8 second exposure, how can you possibly control a 1.5 second 'burn"? if you stop down only one stop, the basic exposure is just 9.6 seconds (an even number!) and that 1.5 second burn becomes 3 seconds - MUCH easier to control. Whats the rush? :>)
Originally Posted by Rafal Lukawiecki
For most people, a basic exposure of 12-20 seconds offers relatively short exposures and much more control.
Don't worry about odd numbers - stick with the even ones if you like. there's really no difference between 14.8 and 15.0. However there is a BIG difference between 4.8 and 5.3 or 6.0! Stop down and you'll be happier!