Photogrammetry with handheld DSLR - use of round flash?

Jonathan85

Hello, one should do photogrammetry on tripod probably for best results, but tripod isnt always available/one doesnt want to bother with in terrain/outdoors. But if you dont have "steady" hand, you can get yourself in trouble, you have to use quite high shutter speed to handheld it, also take into account you might not have a good lighting conditions to begin with - e.g. a cloudy day - perfect for photogrammetry - good ambient even lighting, BUT you might not get ENOUGH light for handhelf camera.
So... could one use a flash for photogrammetry? I have seen some round flash for DSLR that provided quite good even "cloudy day like" lighting (which you need for photogrammetry).
Did anybody tryed that? Did it work good? How much does the flash add lighting to the object you are scanning?
Also does anybody know the correct name of the "circular/round" flash that you place "around" your objective?

Thanks

AndySC

That'll be a Ring Flash. Light originating from all around the lens gives softer and less directional shadows than a normal flash firing from one side of the lens.
The ammount and intensity of the light depend on how bright the flash is, how bright it is compared to the ambient light, and how far away you are from the subject. The closer you are the more obvious the light falloff will be between closer and further parts of your subject.
As far as I know photogrammetry needs consistent lighting on parts of the model in different photographs in the set.  Using flash attached to the camera will result in different exposures on a given part of the object which will cause problems when the software tries blending them.
If you need extra light, try off camera lighting/flash that stays in the same position between all the photos. This could be a couple of desk lamps for smaller objects inside, or full Soft-boxes for diffuse light with off camera flash.

gnoop

Just don't close diaphragm beyond f5, contrary to what  some photogrammetry advisers  suggests.    Tiny holes  gives you less crisp results  due  to light diffraction.   So f22 in fact gives you essentially softer image.

  Use mirror less smaller sized cameras  and optic . Smaller the size of a lens  lesser DOF effect is an issue, wider you could open the diaphragm .    Look at tiny phone optics and how they incapable to produce any fancy bokeh at all.

That of course would have a price of a  smaller sensor and lesser image resolution  but the plus is  Reality Capture wouldn't need lots of RAM  and wouldn't take forever to calculate with not so many pixels to digest.  Anyway they are going to restrict resolution to 24mpix  in few days ( for cheap Steam version)

Also I usually shot with -3  underexposure. Pictures just look in a bit of a linear style. RC is totally ok with that, better at least than overexposed whites.

In a word you could cope without flash light or tripod in cloudy condition unless it's some Nordic country with low sun all the time

AlecMoody

"Just don't close diaphragm beyond f5, contrary to what  some photogrammetry advisers  suggests.    Tiny holes  gives you less crisp results  due  to light diffraction.   So f22 in fact gives you essentially softer image. "

That's technically true but practically it's bad advice. The effects of diffraction aren't that dramatic and you will be better off shooting a few stops past the peak resolution aperture. If you shoot f/5.6 your going to have a much narrower depth of field- You will have a nice slice of resolution but more of the photo is going to be poorly resolved and thrown out by the photogrammetry software. I tend to shoot around f/9 - f/11 and that seems to be a sweet spot for balancing amount of the subject in focus without losing a lot of resolving power. I'm shooting on a sigma 50mm 1.4 ART with a nikon d800E and have no issues with sharpness.

gnoop

AlecMoody said:

That's technically true but practically it's bad advice. The effects of diffraction aren't that dramatic and you will be better off shooting a few stops past the peak resolution aperture. If you shoot f/5.6 your going to have a much narrower depth of field- You will have a nice slice of resolution but more of the photo is going to be poorly resolved and thrown out by the photogrammetry software. I tend to shoot around f/9 - f/11 and that seems to be a sweet spot for balancing amount of the subject in focus without losing a lot of resolving power. I'm shooting on a sigma 50mm 1.4 ART with a nikon d800E and have no issues with sharpness.

In good lighting condition I totally agree.  But we speak about shooting outdoor  in cloudy conditions , often in winter time since it' when the sky is gray most of a time.    Lack of light is a known issue.

So opening you aperture is an obvious way  if you already came to 1/160 sec .  There is nothing wrong with it.    Some small sized  and cheap lenses  still doing not that short of DOF .   My old Olympus e-pen  for example. 

Besides some subjects just doesn't need much of a DOF and need as much crispness as your camera able to produce . If you shot for textures for example.

Now I shot with Sigma Foveon matrix camera with in-built optic . It produces amazing detalisation and crispness way beyond typical Bayer matrix cameras,  sort of a pixel art. It results in fewer artifacts and lower geometry noise in RC meshes. So I believe pixel to pixel crispness and resolution is a big  deal actually.  

But since the camera shoots at 100 iso only really  the  lack of light is an issue I have to deal with all the time.   

m4dcow

Check out this artstation blog link https://www.artstation.com/shrogg/blog/1r0Q/scanning-large-exterior-objects-with-flat-albedo
He uses that godox AR400 ring flash, it is a lot more powerful than the little macro ring flashes so he probably gets a lot more coverage. Not exactly cheap though with it costing $500.