Older cameras have 1/25, 1/50, 1/100 and 1/200 but they also tend to have 1/10 which is obviously not half of 1/25 and if they go faster than 1/200 then 1/300 is often the next speed or they sometimes jump from 1/100 to a top speed of 1/250

http://www.kl-riess.dk/compur.01.jpg


Steve.