When deciding on a fountain pen to start your journey with, you should keep in mind that specific filling systems might be better for your individual needs than others. Do you want the highest possible variety in ink colours? Are you okay with tinkering and trouble-shooting? Do you need a high ink capacity without the need for constant refill? All these questions can (and should) influence your decision-making process.
Cartridges
In countries in which elementary school students still use fountain pens, cartridges are the most popular ink delivery method. Cartridges are plastic containers that are inserted into a fountain pen without too much hassle. Once empty, simply remove the cartridge and insert a new one.
Many ink colors are only available in glass bottles, not as pre-filled cartridges. If you are dead-set on using cartridges (and, by the way, creating waste that could be avoided), you can use an ink syringe to inject bottled ink into an empty cartridge.
Some fountain pen manufacturers use proprietary cartridges. Figuring out whether your pen of choice takes “standard international” cartridges should be your first step if you want to use cartridges.
Converters
Pens that take cartridges also take converters (or “cartridge converters”). Made from more durable plastic and, in some cases, some metal parts, converters are inherently less wasteful and intended to be refilled. Using a converter, your pen can also be refilled without removing the converter: just dip your pen in ink and turn (or push) the handle of your converter.
Like cartridges, converters can be proprietary and not compatible with some pens. Any pen that takes a cartridge also takes a converter, and vice-versa. But not any pen takes any converter.
Piston-Filling, Vacuum-Filling
Piston-filling pens are often on the higher end of the price range. They don’t take cartridges or converters; instead, a piston mechanism is built into the pen. The pen is filled by dipping the nib and twisting the back, thus sucking ink into the pen’s body. Since a large portion of the body is used as ink storage, this gives you a higher ink capacity than cartridges or converters—but it also means you can’t carry a spare cartridge in case your pen runs out of ink mid-day.
Vacuum-filling is essentially the same thing, except that the filling mechanism doesn’t work by twisting the handle, but by pushing it down. Vacuum-fillers feature a plunger that creates a vacuum inside the pen. When the plunger is released, ink is sucked in. Vaccum-fillers are generally safe for travelling on an aeroplane precisely because of the plunger closing the connection between barrel and nib. Other fountain pens can leak due to the pressure fluctuations occurring whilst flying.
Eyedropper Filling
Some pens do not feature a filling mechanism per se. They don’t take cartridges or converters, nor do they have a piston or a vacuum system. Using an eyedropper, the whole barrel can be filled with ink, giving you unrivalled ink capacity—at a price. The threads securing the nib section to the barrel must be absolutely airtight to prevent leaking, which means there’s usually some tinkering with silicone grease involved, especially when converting a cartridge or converter pen into an eyedropper, which is definitely a possibility. Due to possible leakage if you don’t know what you’re doing, this method is best left to experienced users.