Focus not sharp when using ND filter on Mini 3 Pro

[badshah]

Greetings Mini 3 Pro flyers!
Today I noticed that my M3P wouldn't set focus whether I tapped on the screen or half pressed the camera button. It would just keep infinity focus (I suppose). For objects closer to the camera they would momentarily come into very sharp focus and then again blur out. Only the farthest parts were in focus. Of course, I could manually adjust the focus and set it to what i wanted. But when in AF, and touching on an area of the screen wouldn't bring that object into focus.

I suspected that the camera wasn't functioning well. Then I thought of removing the ND filter (which at that time was only an ND4, not anything very dense) and lo and behold, the focus started working as expected. Put the ND filter on and indeed that's what was messing up the focus.

Has anyone experienced similar behavior and if so, do you know why? BTW, I am using Freewell filters so the quality is expected to be good. Would like to hear from others because a lot has been said about ND filters and how great they are, must have accessory, etc. But if they prevent a sharp picture then in my view they are questionable.

EDIT - In case anyone's wondering, no smudges on the glass, both filter and lens, are spotless. Never touched with a finger!

 

[SpinItUp]

have you smudged the lens filter? (clean it with a lens cloth)

 

[badshah]

have you smudged the lens filter? (clean it with a lens cloth)

I should have stated that, shouldn't I? No smudges on this one, a finger never touched the glass - EVER!

 

[mtsvn926]

I have the same issue, did you ever get it fixed? CPL filters work fine, ND filters cause autofocus issues.

 

[badshah]

I have the same issue, did you ever get it fixed? CPL filters work fine, ND filters cause autofocus issues.

Yes, I got it fixed! How? Returned the ND filters. ;-) Decided I didn't need them for now. I am more of a still photography guy and didn't want to deal with out of focus shots. Albeit, I did find that the solution was to manually set it to infinity before taking off. But that wasn't something I preferred.

 

[mtsvn926]

Yes, I got it fixed! How? Returned the ND filters. ;-) Decided I didn't need them for now. I am more of a still photography guy and didn't want to deal with out of focus shots. Albeit, I did find that the solution was to manually set it to infinity before taking off. But that wasn't something I preferred.

I'm wondering if that's what everyone on YouTube does in their videos while using ND filters? Manually set focus to infinity? Hmmm..... Or is it a defect for specific drones....

 

[CanadaDrone]

As with all drones, you should be setting focus manually and never touching it again for 99% of scenarios. Once you're up in the air, your depth of field is from about 3 feet to infinity thanks to both the tiny sensor size used and the distance from your subject (the scenery). This is the case for the overwhelming majority of situations people fly drones in.

If the drone goes into focus and then loses focus (i.e. hunting), that's because there isn't enough light for the AF to function properly through the ND filter. This happens on traditional cameras too, and it's worse as the ND filters get stronger. The Mini 3 Pro uses contrast detect autofocus which is the most rudimentary kind of autofocus and an absolute nightmare for video capture - it is basically a guess and check algorithm for contrast, meaning it "hunts" by design. If you have too strong of a ND filter on the camera, it won't be able to autofocus properly. The same thing will happen even on a $7,000 DSLR or mirrorless camera - you either need to focus manually, or set focus before putting the ND filter on because the camera cannot autofocus through a strong ND.

Your ND filters were almost certainly just fine, you just weren't setting manual focus. You also don't necessarily want to set it straight to infinity, you want to use the focus peaking feature and set focus accordingly. Usually the sweet spot is slightly back from infinity, at least it is on every DJI drone I've used.

 

[Bed]

Same issue with large objectif for dji mini 2. Af works jus for pictures not for videos

 

[4wd]

I've never heard anyone state that ND filters mean you must set focus manually but I suppose it would work.
Never actually used the MF on mine with NDs on or not.