Wednesday, March 14, 2007

Panoramic Stitching

Creating one, large image file from a combination of several smaller files has always intrigued me. I guess, because of the hours that have been spent shooting with a dedicated panoramic camera....and usually wishing that there was a way to get just a little more width....or sometimes a skosh more height to the image. But, there was no way. We're stuck with the format that's pre-determined by the amount of film being exposed. And, with these cameras, if you change lenses to get the width needed...there's always a proportionate increase in the image height. No way to customize the framing. Still, better than simply shooting with a super wide lens, but not perfect.

Fast forward a couple years, and let me tell you about some pretty cool stuff. Computerized image stitching has been around for awhile, but man, has it gotten better, recently. I've been an experimenter with several of the various software answers over the past few years. But, by far, the easiest way to combine several photos into one large horizontal pano is with the Arcsoft Panorama Maker... version 4. I'd been using 3.0 for a couple years, but, the latest edition appears to be bullet-proof.

In a nutshell, here are some tips on ways to get the best results. And, keep in mind, if you don't follow all of the basic may not ruin the composite image....but, you'll be creating a LOT of work for yourself in post production.

First, choose an appropriate scene that lends itself to the long-skinny format. City skylines, mountain ranges and bridges all are good subjects.

Get your camera AND tripod level...not just right to left...but plumb. This is a crucial starting point. If the camera is tilted backwards or forward it'll create a keystone effect on the vertical lines.

Take a meter reading on the brightest area in the scene....put the camera on manual metering, and lock this reading in. This will be the shutter speed and aperture setting for EACH of the frames.

Set the focus for the appropriate depth of field desired, using the hyper-focal point, if there's an extreme need for depth of field. Keep the camera's focusing system set on manual, and don't shift the focus point. Slight shifts in focusing will change the image size between frames, and cause problems overlapping.
Below, are the six images which make up the final extreme horizontal, shown above.

If you're shooting JPEG's, carefully determine the correct pre-set white balance to use....and again, this needs to remain a constant throughout the entire series of exposures.

Double check to make sure there's no Polarizing filter in front of the lens. For single exposures, they are a great tool...but, for panoramas, they will render the sky differently in each shot. Big problem.

That pretty much wraps up all the pre-flight set-up. Now, make a test exposure....again. The lighting conditions may have changed since the first test...and give the histogram a careful look, for any subtle signs of over-exposure.

Now, we're ready to shoot! Finally.

Keep in mind that what makes this whole technique work, is that there is an overlap in each consecutive image exposed. And, it's this area of repeated, same information, that the software locks in on, and uses, to create each seamless stitched area. So, it's necessary to overlap each shot, to provide sufficient information for the software to lock-in on at least 3 EXACT points to achieve this triangulation. For a perfectly smooth effect, 30% to 50% overlap seems to be best.

I like to run through a simulated shoot....pan...shoot....pan.....shoot.... pan..... test- series, before actually exposing the finals. A cable release is necessary to prevent camera movement. Along with mirror lock-up, if you have it. Work quickly..... 'cause if there's any change of light levels between the first and last frame, there will be a similar variation in the densities of each image....more problems in post.

For a series like this one of the Oakland Bay Bridge in San Francisco....the total time taken to complete the entire series was about 30 seconds. After about 4 minutes of prep.

Now, for the easy part. And if you've been careful in your pre-shooting REALLY is easy. Simply open the Software program, choose the images to be stitched, and let er rip!

In almost every case it nails the compositing on the first try! Truly amazing!!

That pretty much covers the BASICS of stitching....there's a lot more, and I'll get to it next time.

Some technical info on the bridge photo: Shot with the Fuji S-3 with the ISO at 100. White balance set for Cloudy. Shutter speed 3 seconds at f/8. File format / large JPEG...I also shot a series of RAW files. Focal length of the lens / 35mm. Tripod / Manfrotto Neo-Tech with the hydrostatic ball-head. This was taken about 45 minutes before sunrise in early February of this year. And a special thanx to my good friend, Bill Rugh for navigating us through the pre-dawn darkness. He drove, I slept.

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