What good are filters on a drone with a variable aperture camera?
Three main reasons for this, no different than a traditional DSLR lens that has much greater aperture range (often out to F22 or F32).
1) Filters aren't limited to ND filters, something like a polarizer can be very useful depending on the scenario. The ability to use filters if/when needed is
always an advantage.
2) Diffraction starts to degrade image quality rather early on due to the sensors being so small and the resolutions being fairly high, so you never want to be using too small of an aperture for typical usage cases. For example, on the
M2P, with a 20MP 1" sensor, diffraction starts to degrade the image beyond F4. Essentially what that means is the information intended to a single pixel starts to spill over onto neighboring pixels beyond F4. By F8 this gets quite bad and by F11 in my opinion the image is so bad it's unusable, so you would only use such a small aperture if you had no other option.
Keep in mind F11 on a 1" sensor is the equivalent of using F30 on a full frame camera, and anyone familiar with traditional DSLR photography knows how much worse the image looks at apertures that tiny. Again in the case of the
M2P, F4 is sharper than F2.8, so if you wanted the highest possible image quality, you would always shoot at F4, with a tiny bit of leeway to F2.8 or F5.6 if absolutely necessary. Beyond that, the image degradation is very noticeable.
On the
M3 with the rumored 20MP M43 sensor, it will become diffraction limited after F6.7 and the image quality will begin to degrade noticeably, so the sweet spot will very likely be F4.0-F5.6 (will depend on the specific lens design, but it cannot be above F6.7).
So, as you can see, you can get away with fewer ND filters, but you absolutely will still need them if you are at all serious about video or want to do long-exposure photography. For example if you are comfortable with the image quality you are getting at F2.8, F4.0, and F5,6, setting your exposure based on a ND8 filter at F4 and changing the aperture within that range will give you the equivalent of a ND4 and ND16 without having to land the aircraft and swap filters. You always want to be using base ISO until you have no other choice but to raise it, so that should be considered a constant.
3) Using wide apertures in bright scenarios, usually to control depth of field. This is less important for drone applications though because the sensors are so small that DOF is usually around 3 feet to infinity at typical usage distances. But on a traditional camera, say you're on a beach shooting stills of a model at F1.4 in the middle of the day, you are going to need a ND filter in most instances to avoid exceeding the shutter speed limit of the camera, and if you are shooting video, you absolutely will need a very strong ND filter for that.