What do you guys think about the variable aperture? Some guys told not to use anything different than f4. 5 or so because everything else is NOT sharp enough.
The reason people say that (if they understand what they're saying anyway) is because you can mathematically calculate when diffraction starts to affect the image, which degrades image quality. The simplest way to put it is that when diffraction occurs, information intended for a single pixel starts to spill over to other pixels, and the image softens (i.e. image quality gets worse). This occurs on all sensor/lens combinations of all types and is not unique to drones. Now, if you're being told to use a specific aperture because of a common misconception that "all lenses are best at X aperture", that doesn't make any sense.
On a 20MP 4/3 sensor, diffraction occurs somewhere between F5.6 and F8, so you actually have quite a bit of leeway there.
On a 20 MP 1" sensor (
M2P) this occurs after F4, so you have fewer options before image quality takes a hit.
Everyone's tolerance/threshold for this is different - some may notice it very early, others may not notice until the aperture gets quite small, like F8-F11. I am personally very sensitive to it and it's very obvious to me, but YMMV. If you simply want to be sure you're always getting the maximum possible image quality out of your drone, then you would shoot below the diffraction limit.
Depth of field is not a concern at all unless you are flying incredibly close to your subjects, at typical distances, from about 3feet to infinity will be in focus, so there is no need to adjust the aperture with DOF in mind like you would on a full frame DSLR for example. The sensors in these drones are small enough and typical distances are large enough that it is virtually a non issue. By far the largest benefit of the adjustable aperture (a variable aperture is something different) is that you don't have to land the drone every time the light changes to swap ND filters - for example if you're flying with a ND8 at F4, if it gets darker you can use F2.8 without having to land and change to a ND4, or if it gets brighter you can use F5.6 without having to land and change to a ND16.
As for autofocus - there is almost no reason to ever use it on a drone. You are far better off setting your manual focus once with the focus peaking feature and then never touching it again, rather than risking the drone focus hunting and ruining the entire shot. The sole exception would be if you were shooting a moving subject very, very close up. Personally I haven't changed the manual focus setting on my drone in ~4 years and everything has been absolutely perfect.
A variable aperture is one that changes with the focal length, or when you zoom in/out. Examples of this would be the
M2Z lens or a 100-400 F4.5-5.6 lens in the DSLR world - it's F4.5 at 100mm and by 400mm it is F5.6 (probably much earlier in fact). This is a cheaper way to make lenses and helps keep size down, but you pay for it in maximum aperture.