It is fairly easy to acquire a collection of lenses from 90mm and up that can be used on both 4x5 and 5x7. It is, almost, possible to do the same for 4x5 and 8x10 but shorter lenses than 200mm that cover 8x10 are usually expensive, somewhat rare and oftentimes quite heavy. But this is an extreme example.
Originally Posted by jfdupuis
You will find that several lenses of different focal length can be used on both 4x5 and 8x10, primarily when it comes to longer focal lengths.
I would follow the link Soeren provided above. After some reading you will be able to make a more informed decision on which lenses to get.