REDUCING THE NEED FOR FILTERS
If you have or are considering a full spectrum converted camera, you might be concerned about the spiraling need for filters - Here are a few tricks to cut down on the filters you might need...
Limiting lenses to one filter sizes
Limiting your wavelengths - see above [can combine with trick 1]
Some cameras can add filters inside the mount [can combine with trick 2]
Adapters with slot-in filters, SLR lenses to mirrorless [combine with trick 2]
Use step-up filter rings, only buy largest filter you need
Filter holder systems
Below I will explain each of these options in a little more detail...
1). - Limiting Filter Size
Restricting the lenses you use based on their filter size can be tricky. You could restrict your IR photography to a single multi-purpose zoom, but unfortunately most zoom lenses have middling to poor IR performance (hotspot). A set of prime lenses that share the same filter size can be done, depending on which lens mount you're using. Adapting to older manual focus SLR lenses to a mirrorless camera can provide much better IR performance and some of these are more likely to share the same filter size. The best combination I have found for this is Konica AR lenses. 90% of them use the 55mm filter thread and a similar percentage have absolutely stunning IR performance and many are reasonably priced.
Here are some examples of lenses that share filter sizes:
49mm - Pentax PK | Minolta MD | Olympus OM | Takumar M42 | Sony FE
52mm - Canon FD | Canon EF | Nikon F
55mm - Konica AR
2). - Limiting Wavelengths
Restricting the wavelengths that you shoot is potentially a little easier. It's easy to search for example images from each common wavelength and choose the ones you like. For me this was to choose one pure infrared, b&w, not influenced by visible light (which I settled on 830nm (B+W 093), one color IR (simple & cheap red filter) and one for visible light (hot mirror). Any effect in between the IR filters could be done in processing from the lower wavelength IR (590nm / red filter) IMO, so these 3 filters cover all bases.
3). - Internal Filters
There are a few companies that offer internal filters (Astronomik, Cyclops Optics etc.). These are easier to find for mirrorless cameras due to there being no swinging mirror to get in the way, but they do exist for DSLRs as well. The benefits here are clear. Only one filter is needed for each wavelength. It would work for every lens, even ones that don't have a filter thread (like super wide-angles) and there would be no aberration issues associated with external filters (like ghosting). There are some unfortunate down-sides however. They're fiddly to change and can cause vignetting on longer focal lengths, so it's not quite the magic bullet. It will be a great option for anyone who likes the freedom associated with a full spectrum conversion, but doesn't need to switch wavelengths often or buy a lot of filters.
4). - Adapter With Filter Holders
Similar to the internal filters - this option has most of the same benefits while being easier to change filters, but it also comes with some annoying down sides. Firstly you will need a mirrorless camera. Sony has more options here, but the Canon EOS R is the only one to offer an official adapter. Both would require you to use Canon EF lenses. Although you would get exif transfer and auto-focus this would of course work better on the Canon R. There are some dumb adapters for Sony, but just don't get the Canon EF version (like I did) as the lenses will not work properly. Unfortunately most options don't allow standard filters to be used and just like clip-in filters - switching them increases the sensor's exposure to dust (even on the Canon R).
5). - Step-Up Filter Adapters & One Large Filter
This is the most commonly used trick. A set of step-up filter adapters will not cost much and will allow you to use a single large filter for most of your lenses (not super wide-angles). Unlike options 3 & 4 this doesn't avoid ghosting issues associated with flat external filters in harsh lighting conditions. Other down-sides for this method are that it makes using lens hoods difficult. You could buy a large screw-in hood for the size of your large filter (same for a lens cap), but there's no easy way to know if it will vignetting on wider lenses or not. Switching lenses and step-up rings can be fiddly and like using a filter holder system, this adds unnecessary bulk to your camera setup.
6). - Filter Holders Systems
Like many landscape photographers you could also use a filter holder system. This adds the most bulk to your system (especially if you want it to cover super wide-angle lenses) and large plate filters will be expensive, but if you already need this system it could make sense to use it for your wavelength choice as well. Unlike options 3 & 4 this doesn't avoid ghosting issues associated with flat external filters in harsh lighting conditions.
WHAT'S THE BEST OPTION?..
There is no easy answer to this. It will differ for each person (depending on wavelengths & lens options), so I wanted to lay out the options here for everyone to see. I would have loved to choose option 3 or 4 to avoid the need for external filters and improve image quality, but that didn't work out. Ultimately I like the option of switching filters quickly (see below), so... I have chosen a combination of option 1 & 2, which the below image illustrates.
MAGNETIC FILTER SYSTEM
Options 1 & 2 (for limiting filter sizes and wavelengths) can be combined with another system that makes a full spectrum camera considerably more friendly to use - a magnetic filter system. I don't want this to sound like an advert, but the only one I could find is the 'Manfrotto Xune'.