Make simple syrup: Combine sugar with 1/2 cup water in a small saucepan. Bring to a boil, stirring occasionally to dissolve sugar. Remove from heat and let cool completely.
Purée raspberries with 1/4 cup water in a food processor or blender. Strain through a fine-mesh strainer into a large, clean bowl, pressing on the seeds with the back of a ladle to get all the usable raspberry purée. You should have about 3 1/2 cups.
Add 3/4 cup simple syrup, the lemon juice, and salt. The mixture should be a bit too sweet; once it's frozen, it will lose some of its sweetness. Add more simple syrup if necessary.
Freeze mixture in an ice cream maker according to the manufacturer's instructions. Eat right away or, for a firmer sorbet, freeze about 4 hours.