The lumia 730 has basic HDR.. the difference being that you wouldn't be able to open the picture you took and manually set the brightness afterwards. With the 640 you do have that option..
In daylight, with a bit of work, you can get quite decent pictures with the 640 and stock windows camera. Haven't owned a 730 so can't comment, but I think the carl zeis optics always make a difference. Unless you're really into pictures, you probably wouldn't notice.
If you're a big selfie person or at least use the front cam, it's best to go with the 730 over the 640.. The specs in this case aren't wrong lol
07-06-2016 10:40 AM