From fairest creatures we desire increase,
That thereby beauty's rose might never die,
But as the riper should by time decease,
His tender heir might bear his memory:
But thou contracted to thine own bright eyes,
Feed'st thy light's flame with self-substantial fuel,
Making a famine where abundance lies,
Thy self thy foe, to thy sweet self too cruel:
Thou that art now the world's fresh ornament,
And only herald to the gaudy spring,
Within thine own bud buriest thy content,
The meter of this line changes from the traditional   iambic pentameter  with the  spondaic  “thy sweet self too cruel”. 
 There is an emphatic rhythm about this line, with the elongated vowels and slow pace, creating a dramatic climax that ends with a  caesura , or pause, formed by the colon. 
 There isn’t a clear  volta  or “turn” in this sonnet, though perhaps it comes here, after the colon.
Sonnet 1