Mobius non-fish-eye lens conversion

patcoyle
Sat, 01 Aug 2015 18:32:58 +0000

What I want to do

Explore alternative non-fish-eye lens options for use with Mobius Infragram plant cam or standard visible imagery Mobius. The "flat" images are preferred for mapping in MapKnitter or other auto-stitching programs. This continues a few notes to explore smaller kites for mapping with Mobius Infragram plant cam. See prior note and note and note and note.

This work relies heavily on Chris Fastie and other's work in developing the Mobius Infragram plant cam.

My attempt and results

This initial option is attractive, compared to use of software for lens correction (such Photoshop, PTLens, or opensource options like as Gimp).

The following photos show lens focal length results for range of lenses. I found this site very helpful for overview and links to sources for lenses, filters and other items.

This YouTube shows how to modify the mobius with alternative lenses. His results with "flat" lenses look okay for stitching. He also has links to lenses and IR filters.

The following video shows lens change process:

After removing lens from standard visible Mobius, I tried this set of lenses with focal lengths from 16, 12, 8. 6, 3.6 to 2.8mm. I used the AV out cable to display output from camera (no IR filter) on TV and screwed the liens in until focused, then tightened set screw.

16mm

12mm

8mm

6mm

3.6mm

2.8mm

I then converted the Mobius Infragram plant cam. I removed the standard lens. Rather than try to reuse the circular piece of Wratten 25 filter material on each lens, I placed a piece of the filter material in the lens mount directly in front of the sensor. I have ordered a lens holder with IR filter so can take similar approach to the visible camera as well.This allows changing lenses without having to fit the filter into each lens.

Based on review of the images, I chose the 8mm focal length lens for the first attempt.

The following video shows lens change process:

I used a few images to make this MapKnitter map of Livermore CA: Mobius with 8mm lens.

Images from first flight of Public Lab Infragram plant cam Mobius modified with 8mm "flat" non-fish-eye lens, from Prism Stowaway Delta with bottle-top rig.
http://mapknitter.org/embed/livermore-ca-mobius-with-8mm-lens
Questions and next steps

Try the visible camera with IR filter in lens holder. Explore other lenses.

Why I'm interested

I'm interested in using small kites with the Mobius plant cam and visible unit. Together, they make for a compact solution. As noted in prior posts, very portable, even if not "pocket-sized" aerial photo mapping tools have a place in our tool kits. I think this platform is undrutilized for mapping.

The Public Lab Kickstarter: Infragram: the Infrared Photography Project back in 2013 had almost 300 people pledge for this option, so there is a good sized set of potential users. I also hear that the unit continues to sell from the PL kit store. However, I don't see many user mapping results.

As Chris Fastie noted, "It’s a great idea to use a more normal lens on the Mobius for mapping. While you can correct for the distortion of the standard wide angle lens, you can never correct for the fact that objects at the edges of each photo will be photographed from an oblique angle, not from above. It also seems that part of the distortion correction procedure is to crop the image, which throws away lots of resolution."