Lens Calibration

glh · Mar 13, 2017

I have a question about correcting lens distortion. I have a Fujifilm point-and-shoot, so not high-grade equipment. I took a picture of the horizon, and it was noticeably convex. Even more so when I shrunk the horizontal direction in... I think I pasted it into a word processor, actually. I had also taken the picture with the camera upside-down, and in that the horizon looked noticeably concave. So I seem to have what a helpful graphic called a "curved horizon" distortion. And I don't know what else.

At any rate, I had imagined using the camera for measurements, where the points in the image are a faithful representation of distances and angles. So not just tweaking for aesthetics. And I wonder if there is a way to calibrate the lens and set up a filter to correct images. I imagine something like taking a picture of a reference grid and loading the image into some kind of software that will set up a filter that makes the lines straight and uniformly spaced.

My background is that I know a little about optics and film photography, comfortable with math, but I know very little about digital cameras and, except for some of the simplest things in GIMP and drawing programs, virtually nothing about software related to photography and image processing. But it seems like there is amazing stuff out there, and I hope this is a solved problem.

photo1x1.com · Mar 13, 2017

Hi there and welcome to the forum.
The Software Adobe Lightroom has options for lens correction. If you want you can upload the file and some of us could try to software on your image. There is also a free trial version if you want to test yourself.
It works pretty well, but it can not correct everything.
If you decide to upload, at best use the file as is out of camera - so lightroom would usually automatically find out what camera and lens it was. If not, you would have to tell us the exact camera.

zombiesniper · Mar 13, 2017

glh · Mar 13, 2017

To be honest, I was actually hoping the curvature of the earth would be visible in about a 70 degree field from 480 feet high. So I don't just want the horizon straightened out. I'm thinking of the image as data rather than as art, so I'm looking for an actual calibration. (To continue being honest, I haven't mathed out a model of it yet, so I don't know if the curvature SHOULD be visible. It was more of a try-it-and-see attempt. Taking pictures from an observation deck is more fun.)

zombiesniper · Mar 13, 2017

What you can do is take a photo with the camera of a known flat/straight object. Use the video to figure out the correction amount needed to bring to straight. Use those correction values when taking the horizon photo. If it remains a bit curved then you can see the curvature of the earth. If not, you can't.

glh · Mar 13, 2017

zombiesniper said:
What you can do is take a photo with the camera of a known flat/straight object. Use the video to figure out the correction amount needed to bring to straight. Use those correction values when taking the horizon photo. If it remains a bit curved then you can see the curvature of the earth. If not, you can't.

Thank you. I'm a little uncomfortable using my eyes for that, but it was my eyes that detected the distortion, so maybe they'll do fine. I did actually open an image and follow along with the video. But the horizon is curved in the same direction both at the top of the frame and at the bottom (I did top and bottom, rightside up and upside down). Does GIMP have a suitable filter? New ones can be downloaded?

zombiesniper · Mar 13, 2017

Unsure as I don't use GIMP.

glh · Mar 13, 2017

photo1x1.com said:
Hi there and welcome to the forum.
The Software Adobe Lightroom has options for lens correction. If you want you can upload the file and some of us could try to software on your image. There is also a free trial version if you want to test yourself.
It works pretty well, but it can not correct everything.
If you decide to upload, at best use the file as is out of camera - so lightroom would usually automatically find out what camera and lens it was. If not, you would have to tell us the exact camera.

Exact camera? Would it already have it in there? Mine is a Fujifilm Finepix JX, I suppose, 16 megapixels.

glh · Mar 13, 2017

zombiesniper said:
Unsure as I don't use GIMP.

You've been helpful, anyway. Now I have a plan and something specific to look for, which puts me ahead of where I was an hour ago. Thanks!

zombiesniper · Mar 13, 2017

Cannot find Fujifilm Finepix JX in lightroom.
Lightroom lists Finepix X100 as the only finepix.

astroNikon · Mar 13, 2017

glh said:
To be honest, I was actually hoping the curvature of the earth would be visible in about a 70 degree field from 480 feet high. So I don't just want the horizon straightened out. I'm thinking of the image as data rather than as art, so I'm looking for an actual calibration. (To continue being honest, I haven't mathed out a model of it yet, so I don't know if the curvature SHOULD be visible. It was more of a try-it-and-see attempt. Taking pictures from an observation deck is more fun.)

Unfortunately you are not high enough to see the curvature of the earth.
The local ground is normally not flat as small hills, buildings, trees, etc get in the way.

If you were on the ocean you could watch (with big binoculars, etc) a boat's hull disappear before the mast in the distance though.

Otherwise about 35,000 feet you can definitely see the earth's curvature.

glh · Mar 13, 2017

astroNikon said:
glh said:

To be honest, I was actually hoping the curvature of the earth would be visible in about a 70 degree field from 480 feet high. So I don't just want the horizon straightened out. I'm thinking of the image as data rather than as art, so I'm looking for an actual calibration. (To continue being honest, I haven't mathed out a model of it yet, so I don't know if the curvature SHOULD be visible. It was more of a try-it-and-see attempt. Taking pictures from an observation deck is more fun.)

Click to expand...

Unfortunately you are not high enough to see the curvature of the earth.
The local ground is normally not flat as small hills, buildings, trees, etc get in the way.

If you were on the ocean you could watch (with big binoculars, etc) a boat's hull disappear before the mast in the distance though.

Otherwise about 35,000 feet you can definitely see the earth's curvature.

I've been coming to realize that. In fact, I found a page that discusses exactly the problems with lens distortion in that application. His solution was to put the horizon in the middle of the lens, and take pictures from a student rocket launch that went 82 km high. 82 km! Good gravy!

There's another method due to Al-Biruni, a 10th century mathematician, that involves finding the angle to the horizon from a mountaintop. He did better than 1% with an astrolabe and a mountain in Pakistan. On that 480 foot high observation deck I'd have to measure an angle of less than 0.4 degrees. Not with my equipment!

But I would still like a calibrated lens. If I take a picture of a ship, for instance, and knew the dimensions of the ship, I could figure the distance and visible portion.