You have the Phase One DF+, so you pretty much can use any Mamiya, Phase One lens out there, plus with adapter, (and you will need to check with a dealer) Hasselblad lenses.
Mamiya, Manual Focus lenses, mostly will have the aperture ring on them, so you may have to manually stop down depending on the age of the lens. These lenses are still very good in the most part, and can be found used for bargain prices.
Phase One AF, and Mamiya AF, more modern optics for the most part. Usually have the D letter on them (meaning Digital optimized) Many of these lenses will have a clutch to allow you to drop to manual focus, none will have a manual aperture ring (that I am aware of).
Examples of these, Mamiya 28mm F4.5, Mamiya 35mm F3.5, 55mm F2.8 75-150 (I believe F 4.5) Wides will show a bit soft towards the corners especially on 60MP backs or higher. The vast majority of these lenses are branded both in a Phase One Digital and Mamiya brand. There is no difference in them, so if you can find a used Mamiya 28mm F4.5 odds are it will list for less than the equivalent Phase One 28mm F 4.5 lens.
Phase One LS (Leaf Shutter) Made by Schneider. In some cases, these lenses just have a leaf shutter added like the 28mm LS, most of the rest, 55mm 110mm 240mm are all new designs and excellent optics, but expensive at the same time. If you don't need the Leaf shutter, you might consider the older Phase One Mamiya stuff. I believe the LS 75-150 is also the same optically as the old AF 75-150, just has the leaf shutter added.
Remember, with the DF+, you still fire the focal plane shutter, it's not disabled with the leaf shutter lenses, so you have a chance of getting some vibration at certain shutter speeds.
Everything I have read on the LS glass is that they are optically as good as most of the Tech camera lenses, however you don't have all the other options with them you get with a tech camera body.
The newest lenses in the lineup is the 40-80 zoom. Very expensive, but seems to have a very good optical design.
As you are just starting out, if you are in the US, contact a dealer about a demo of some of these lenses, also at least one dealer I know of in the US used to sell very good quality used Phase One, and Mamiya lenses, Capture Integration in Atlanta.
You can find out a bit more info on the main Mamiya website, take a bit of digging to get to the specs, Phase One's website also eventually allows you to dig to the specs.
Paul
