Friday, March 20, 2009

Taking Panoramic Landscapes

I love panoramas. There's something very appealing about their shape. It's probably because we see the world more in these dimensions than the near square format of standard film/sensor frames. It might also explain the upsurge in the popularity of widescreen TVs!


Panoramas have a reputation of being hard to take. There are dedicated panorama cameras available but unless you've got at least a thousand dollars to spare, you probably can't afford one! But you can take panoramas with any kind of camera.

All a panorama is, is a sequence of images where you turn slightly for each different frame. In the old days, before PCs and the likes of Photoshop were around, you'd take your prints (there wasn't much point in shooting panoramas on slide film, for obvious reasons), lay them out on a table and position them over each other where they overlapped. A bit of sticky tape held them together. [As a side note, this technique was used by NASA to build up mosaic pictures of the planets and satellites their spaceprobes visited, up till the late '70s/early 80s when computers were introduced to make the process less laborious].

Now that PCs and image manipulation packages are easy to come by, high-quality panoramas can now be created by anyone. If you're shooting slide or negative film, you will need to have your images scanned before you do anything else.

DIY Panoramas

The idea behind taking panoramas with SLR cameras is that the camera is rotated around its nodal point during each successive exposure. What's the Nodal Point? It's the point inside your camera where the light rays converge and flip over. It's different for different focal lengths (on zoom lenses) and for different prime lenses (fixed focal length lenses like a standard 50mm lens). It's important to rotate about this point to eliminate image mismatches due to changes in parallax. Parallax is the apparent shift of an object against a background due to a change in observer position.

Just to be clear, the Nodal Point is not the same as the film/sensor plane. Generally, for most SLR cameras and lenses, the Nodal Point is located somewhere towards the center of the lens barrel and lies in front of the image/sensor plane.

The Problem With Parallax

Parallax is easily demonstrated by a simple experiment. Hold up your finger about 1 foot in front of your face and alternately open and close your left and right eyes. You'll notice that your finger shifts left and right with respect to the background depending on which eye is open. Try another experiment: With your finger still raised, close one eye and turn your head from side to side. Notice how your finger moves with respect to the background. This relative movement is due to the fact that you're not rotating your head around your eye's nodal point, which is somewhere in the center of your eyeball. Instead, you're rotating about your spine which is several inches to the rear and off to one side. It is this relative side-to-side motion that we try to eliminate when setting up a camera for panoramas.

Now, if you consider a camera held up to your face - it will suffer even greater parallax errors as it's farther from your spine (the point of rotation of your head) than your eye. It's surprisingly common for people to take panoramas in this fashion and then find the individual pictures don't match up.

So use a tripod and rotate the camera on the tripod. The parallax errors will be significantly smaller but there will still be some error involved. However, the images will match up better than with the head rotation method.

Mechanical Contraptions

What perfectionists strive for is to have the camera rotate about the nodal point. There are brackets and contraptions available that will let you offset your camera from the tripod's axis of rotation and with a little experimentation and trial and error, you can position your camera so that its nodal point is directly over the axis of rotation of the bracket. Getting this spot-on means your images should line up perfectly.

A few months ago I bought such a bracket - the Kaidan Kiwi. This comes in two halves which produce an L-shaped bracket. Its instruction manual explains how to set it up and find the nodal point for your camera and lens. However, you have to get your tripod perfectly level before using it, otherwise you end up with a curved panorama rather than a straight one.
I've had good success using this bracket, but it is large and heavy and certainly a bit too cumbersome to be carrying on long walks or while away on vacation.

AutoStitch To The Rescue
Then I recently came across a free bit of software called AutoStitch. Written by a couple of students at the University of Columbia, this takes all of the heartache out of creating panoramas. All you do is select the size of the final image and tell it what images you want it to stitch. It then goes off and produces your panorama.

It really is that simple. Unless successive images are radically different in exposure (i.e. one image to too light or dark compared to another), it seamlessly blends them. It performs all the warping of the images necessary to get them to align (other software I've used can cause ghosting in the overlap areas where it hasn't quite aligned the images). It also aligns multiple rows of images rather than just a single strip.

Even better, it doesn't require you to set up your camera to rotate about its nodal point. When I was in Crete last year, I tried shooting a few panoramas with my Canon EOS 300D held up to my eye (I didn't have a tripod with me). When I got home, I tried stitching the pictures together using various bits of software (including software dedicated to stitching images together) and didn't get satisfactory results. I knew, though, that it was because I'd swivelled the camera about my spine. But I tried these images with AutoStitch and they came out perfectly.

I went walking up the Wicklow mountains in Ireland no too long ago and up to a high point called Djouce which offers a view over the rolling hills south of Dublin. As an experiment, I shot 8 frames while rotating my head about the scene (camera to eye as per normal). I wanted to see if the Crete photos were a fluke as the panoramas from there were composed of, at most, 3 frames each (sometimes 2).

Conclusion

What can I say? I plugged 8 frames into AutoStitch and after a bit of time processing the images, it produced a perfect panorama with no ghosting I could see in the overlap reasons. I like software like that. It may only do one thing but it does that one thing very well.
Give AutoStitch a try. It's free and, so far, it produces the best panoramic results of all the panorama/stitching software I've tried.

One thing to remember when taking panoramas is that the exposures of each frame should be the same. So if you make your first exposure at f/8 and 1/125 of a second, take them all using those settings. Yes, you will have to put your camera into manual mode. Otherwise, you run the risk of having radically different exposures for your images. For example, if you're panning over a landscape that contains water, like a lake, any sunlight reflected off the water may make your camera take a shorter exposure than for the other frames in your sequence. Setting your camera to manual mode will prevent that.

About the Author
Gary Nugent is a software engineer by profession and has been in the business for over 20 years. Photography has been a hobby for an even longer period of time and he's now even more passionate about it since making the switch to using a digital SLR camera. You'll find more tips and techniques at Great Landscape Photography: http://www.great-landscape-photography.com


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3 komentar:

SBL said...

Quite interesting and informative post. Thanks.

Regards,
360 Panorama creation

Lina said...

Bikin Guestbook dong, bro. Biar gampang komennya.
Thax ya...

Xotri said...

wah perlu blajar photography nih gw... gak mudeng waktu baca yg beginian... thanks artikelnya... moga sukses....
termasuk adsensenya.. he...he...

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