Camera manual settings vs automatic

I’m making orthomaps with a P4 and WebODM. I’ve been told it’s better to fly with manual camera settings. Fly up to your mapping altitude, use the autofocus to set the focal length then switch back to manual focus so the focus length doesn’t move when flying over trees etc. Set F5.6, ISO 100 for sunny, 200 for cloudy days, then use shutter speed to adjust exposure.

My problem is clouds. In my most recent batch of photos a lot of them are perfect, but many are overexposed due to the sun appearing briefly between the clouds.

Would it be better to set f-stop, ISO and and shutter speed to automatic, and just set focus manually? How does everyone else set their cameras?

2 Likes

Oof… That’s a tough one.

What camera system are you using?

Are you sure that F5.6 is the sharpest? Typically, it is whatever F-Stop is roughly in the middle of the lens’ range, but that isn’t always the case. You’re also trading a lot of light gathering (and gaining some diffraction) by stopping down a lot.

How is your camera’s ISO performance? Roughly ISO-invariant? Or really noisy with higher ISO? Maybe lock ISO and let the camera use aperture control for exposure? At the height you’re flying, you shouldn’t have much to worry about with the focal plane changing with the aperture.

2 Likes

If taking the images while the drone is moving, you need to make sure that shutter speed doesn’t get too long, and introduce motion blur, so fixing the f/stop could be risky. I set my M2P to autofocus, and I sometimes see that the effective focal length can change slightly between images, but that doesn’t seem to be a problem when creating orthophotos. It is noticeable when taking images every 2 seconds, and when the drone slows right down at the end of a run to turn, 2 photos can be taken from almost the same location, but if a focus adjustment has occurred between them, then the image scale is noticeably slightly different.
I also use auto exposure, as I don’t want problems due to over or underexposure when lighting conditions change. However, I continually monitor the shutter speed and histogram, and sometimes adjust ISO between 100 and 200 on the fly, to avoid the exposure becoming too long, or the aperture getting too close to wide open, as the image quality falls off significantly around the edges of the frame.

2 Likes

Using the standard Phantom 4 camera. I don’t really know much about photography, learning slowly.

Okay, so what I’m hearing is auto-exposure/focus is okay. That’s different to the other information I’ve received.

1 Like

Yes, but you need to keep an eye on it to avoid any issues as mentioned above. It certainly worked well for the >23000 images I took on a recent job, patchy cloud would have made for plenty of unusable photos. However, I did pause a few flights part way through for a few minutes to allow particularly dark clouds to pass, avoiding exposures that would have resulted in motion blur.

2 Likes

Auto-Focus can be tricky, but I honestly think a lot of that fear is a result of the early days of sUAS photogrammetry with hacked Canon SX Point&Shoots with CHDK/MagicLantern velcro’d onto the bottom of some cobbled together monstrosity…

Modern AF hardware and firmware is honestly super freaking impressive. Unless you’ve observed your platform struggling to keep focus (and you’re not confusing that for motion blur from long exposures), I would not worry about AF at all.

To be honest, the only times I could see modern AF getting really tripped up would be when imaging stuff that frankly would not be likely to reconstruct anyway (highly spectral low-feature things like flat water expanses during high sun, bright snow, etc).

That being said… It is yet another fragile mechanical system that can misbehave. If you can easily focus-lock it and your subject-sensor distance isn’t going to change over the course of the flight, I don’t see how locking it to MF would hurt.

1 Like

Yup, agree with all the above. There’s two different principles at play here and for different purposes:

Fixing F-stop and ISO will help keep your shots as even as possible across the scene, making the texturing easier, and in theory make the orthophoto better/smoother/more even. That said, the texturing algorithm that ODM uses is fantastic and can handle compensating for all sorts of dynamic changes to ISO, F-Stop, even flying on different days.

Focal length constancy is a different issue: there are two reasons one might do this, but probably the most important is making sure that there’s no change to camera calibration characteristics during flight. For this reason, if you are doing large scale 3D or elevation model work, fixing this should help. But under most circumstances it isn’t a bias you’ll notice. Only the hydrologists and surveyors will notice the sins of autofocus. :smiley:

3 Likes

Great explanation. Thanks.

I’m working for a surveyor and the area we work is often cloudy so I might go to just fixing the focus and leaving everything else automatic. We’re only doing orthomaps currently but working up to DEMs.

3 Likes