I have a Nikon bellows unit and would like to pair it up with a 50mm adapted lens. Do I need to acquire a macro lens or will a standard lens work? Is a 50mm okay. I just would like to do macro. The camera I have is a Fuji X-Pro 1.
As a previous owner of a Nikon PB-6 bellows unit, I remember from the manual that Nikon did recommend using macro lenses with the bellows unit. I don't see why a non-macro lens would not work with a bellows unit, but you will certainly be able to get even closer to your subject with a macro lens (if that matters for the type of macro shots you have planned). Also, macro lenses are usually specifically designed for close-up photography and will likely give you better results in this scenario.
An important issue to consider when using a bellows unit is the presence of an aperture ring on the lens. Unless the bellows unit specifically provides a feature to change the aperture on the lens (typically a lever close to where you mount the lens on the bellows unit), you will not be able to stop down the aperture on your lens. You really want to stop down the lens, because with a bellows unit, the depth-of-field (DOF) of a wide-open lens will be tiny.
To be on the safe side, I would choose a lens with a dedicated aperture ring, allowing you to set the aperture manually. It can be a manual focus lens or an autofocus lens, just the aperture ring needs to be there. Nikon "G" series AF-S lenses do not have an aperture ring like the older "D" series AF/AF-S lenses do.
I was using an older 60 mm AF-D Nikkor micro lens with my bellows, but I also had the 70-180 mm Nikkor micro zoom lens mounted on it. With the larger focal lengths, you get higher magnification. It really depends on your intentions.
Any mountable lens will work just fine on your bellows, provided you can change the aperture somehow. But since you have a Nikon bellows unit, I would go for a used Nikon manual focus micro lens (50 mm or longer), they should be available for a reasonable price and they are all equipped with an aperture ring.
About close-up photography: Most any lens that mounts on your bellows will work. However there are caveats. The bellows and its cousin, extension rings, allows the camera lens to be relocated forward of its normal position. Such lash-up allow extreme close-up photography. A problem develops in that the f/numbers engraved on the lens barrel are now incorrect. As an example, if the lens is racked forward to obtain “unity” (life size), the inaccuracy is two f/stops (4X reduction in light energy transversing the lens). This boo-boo is called “bellows factor”. Simply stated, when using extension rings or bellows, the photographer must make exposure adjustments based on the now increased magnification.
One of the big advantages of the macro lens is, the bellows factor problem is attended to by the design of the lens. Bellows factor compensation can be difficult however in the modern camera eads exposure through the lens, and can automatically set exposure regardless. That’s true if the camera body and the lens remain interconnected. Sorry to report that many extension rings and bellows do not maintain body to lens linking. If this is the case, you become the exposure adjuster.
Another point, the standard camera lens is optimized to take pictures of objects at a distance of about 1 meter (yard) thru to infinity ∞. The standard lens will be slightly compromised as to acuity if tasked to do close-up work. The micro lens is optimized for close-up work and it maintains the accuracy of the f/numbers and it is interconnected to the camera body affording full automation.
To improve the acuity of the standard lens when doing close-up work, we often mount it backward. This slightly improves acuity as the rear of the lens is optimized to work a flat field and most close-up subjects are even, nearly level. This revering of the lens prevents coupling the lens to the body. It does add some extension thus a little more magnification is obtained.
My advice – For now procure a macro lens and get your feet wet before moving on to the more difficult. Now you can move on and procure extension rings (best choice) and then a bellows.