Kevin Merchant Photography Articles

What this article is not - it is not an exhaustive attempt to describe all the ins and outs of panoramic stitching. This article is about how I go about the process myself without making it too complicated (in my opinion).

What is a panorama? It generally refers to one dimension being considerably longer than the other. Traditionally, a panorama has been much wider than it is high, but it can also be tall and narrow. From very early on in the history of photography, panoramas were made using various techniques including stitching single images together to form a single final panorama print and using specialty cameras that captured the image on a single piece of film.

For a long time, I had admired the possibilities of panoramas with the specialty film cameras but never took the plunge due to the expense. And, I never pursued stitching images together in the early digital darkroom days of scanning film. In the early days of digital photography, I started seeing panoramas being created from digital capture. There were discussions about creating panoramas by "stitching" together a series of single images captured with a digital camera. Still, not having taken the plunge for a digital SLR, I didn't pay much attention. Well, when I bought my first digital SLR camera, the Canon EOS 20D, I quickly started seeing possibilities to make higher resolution images in the panorama format and dove right in.

Why make a panorama? Why not? Panoramic possibilities abound. This is not to say that everything you see deserves to be captured as a panorama, but many times an extreme wide angle view tells a different story than something a wide angle lens can capture in a single image. For example, on a recent trip to Alaska, I had before me the wide sweeping Alaska Mountain Range with Mt. McKinley and numerous other peaks. It could not all be captured in a single image, so I decided to make a panoramic series. The result, from a 21 image series, is this horizontal strip of mountains including very little sky and very little river. A normal wide angle image would most likely have included both sky and foreground objects. This image emphasizes the mountain range.

When to shoot a panoramic? There are different situations that I have found where a panoramic is more effective than a typical aspect ratio image. Many have a common thread - boring foreground material and uninteresting sky for what amounts to a wide angle shot. Sometimes a lot of blankness either in the foreground or the sky calls out for elimination. Sometimes the contrast range is too wide for the camera to capture, forcing you to choose the most important elements of the scene. Sometimes the subject simply cries out for panoramic treatment such as a waterfall or a mountain range.

The trick is recognizing when the scene before you is right. I make no claim to being an expert. I still come home from shoots and ask myself why I didn't recognize an opportunity after reviewing the shots I did make. With time and experience looking for those opportunities your eye does become trained to spot more of those special moments.

How do I make a panoramic stitch? The technique, typically referred to as panoramic stitching, is literally sewing together (in the digital sense) a series of images shot in sequence using an appropriate photo editing program. You take a series of overlapping images by panning the camera with each shutter release. With the resulting set of images, they can be overlapped into a single image, by aligning the common portions and blending so that a seamless transition is created between each image.

Photoshop CS2, which was my tool of choice until CS3 was released, has an automated tool to perform just such a task. However, I found that its ability to blend the images for seamless transitions was sometimes less than perfect. As a result, I didn't do that much with the panoramic series shots that I made. When Photoshop CS3 was released, the game changed. I found that for the most part, the transitions were not noticeable at all, except in cases where there was movement in the subject, such as ocean waves. In these cases, Photoshop needed a little help. But, by and large, I was now able to make very high resolution panoramic images, even with the EOS 20D.

When contrast range is such that a lot of the foreground or sky is too dark, you can find a narrow band of interesting light that will make a great panoramic subject. This often happens in early morning and late afternoon light. I ran across this situation recently at a sunrise shoot at Mesa Arch in Canyonlands National Park. The eastern clouds had this amazing sunrise light, but the canyon was still in darkness and much of the sky was still dark. While it lasted, I frantically shot series after series.

How do you go about making a panoramic series? First you have to realize that you are not going to see the resulting image through the viewfinder of the camera or the camera's LCD screen. You have to visualize the important elements in the scene. Depending on the situation, a zoom lens may be the best choice for adjusting the focal length to "crop" the scene for the panoramic series. It is best to get a rough focal length setting and then scan the portion of the scene you are selecting. Once you have done this you are ready to proceed with other equipment setup.

First, you need to be using a sturdy tripod, preferably one with a bubble level. A tripod will not only ensure that each image is as sharp as possible, but it will help to keep the camera aligned between shots maximizing the amount of digital data you are able to keep. When using a tripod, you need to make sure that the platform (usually the tripod head) is level to the world. This will ensure that as you pan, the camera tracks the desired portion of the subject. This is where a built in bubble level comes in handy on the tripod. Secondly, the camera itself needs to be leveled on the tripod head. This will help give the least amount of lens distortion that is encountered near the edges of each frame, particularly at wide angle focal lengths. Here, I recommend a double bubble level that fits in the flash hot shoe of the camera body. Actually, I recommend using one of these levels whether shooting panoramas or not, in order to keep the camera level to the horizon.

For the most precise leveling, you need to take into account both the horizontal and vertical planes of the imaging sensor. This may be accomplished with a double bubble level. Being level in the vertical plane forces the horizon to be smack dab in the center of the image. In many cases this works just fine and other times it doesn't. However, I have found that as long as I don't deviate too far from the vertical plane and I'm not too close to the foreground, there is no problem in being off in the vertical plane, particularly when using telephoto focal lengths.

There is another parameter that comes into play when shooting panoramic images, especially where accuracy is critical. It is known as the lens entrance pupil (incorrectly identified by many as the lens nodal point) - the virtual aperture through which light that enters the lens system also exits the lens system onto the digital sensor. This parameter has to do with achieving the least amount of lens distortion (parallax error) while panning the camera. It requires two adjustments - 1) the optical axis of the lens must be centered over the center of the panning point and 2) the camera/lens must be shifted, forwards or backwards, so that the entrance pupil is over the center of the panning point. Sound complicated? Yeah, for me, too. But here is the good news - I have found that for the subjects I shoot, this has not been critical. For that reason, I won't go into any more details here. My advice is to not worry about this step. But should you have an interest in addressing this problem, there are specialty manufacturers (Really Right Stuff is my choice) who have equipment to address it.

Now that you have the camera leveled on the tripod you are ready to deal with camera settings. You need to be using manual focus on the camera. If the focus is changed between sequential images, you will have slight variations in image size which may cause problems when it comes time to do the stitching. I also recommend manual exposure settings as this gives the best results when it comes time to blend overlapping images. Otherwise you end up with very noticeable seams due to lighter or darker images. It is best to pan the camera through the scene to determine the brightest part and set your exposure accordingly.

You may have to decide whether highlights or shadows are important and let the other fall where it may. In either case, it is best to make a test exposure of the brightest or darkest portion and verify with the histogram that you are getting the best exposure possible. For highlights, you don't want the histogram to go past the right hand margin and for shadows you don't want the histogram to go past the left hand margin. More can be said on this, but that is a topic for another discussion.

In most cases it is not advisable to use a polarizing filter due to the variation in polarization angle as the camera pans. This will cause each image through the series to either be slightly darker or lighter depending on the angle of the camera relative sun.

I highly recommend that you shoot RAW images as opposed to JPEG. At one time there was the issue of both limited memory card space and the high cost of memory cards. With greater memory card capacity and storage prices dropping (both memory cards and hard drives), there's really no reason not to be shooting everything in RAW mode these days. If you don't, you will need to set the White Balance to a custom setting. Otherwise, your JPEG images will have a variance due to the auto white balance changing as you pan the camera for sequential images. Again, this has to do with blending the images seamlessly.

One final thought about framing a panorama. Be sure not to crop too tightly or you may be faced with losing part of an important element when you stitch the images together. This is due to how level the camera is both horizontally and vertically and how far the panning center is from the lens entrance pupil.

With everything set up, it's time to shoot. You need to have overlap in the images. I have seen suggestions of around 30% overlap. I tend to be more conservative and make it about 50%. This means I end up with a few more shots per series. Begin at either the left or right most part of the scene you have selected; it doesn't matter which one you choose but be sure to not crop tightly, lest you lose a portion in the stitching process. After you have made an exposure, pick some detail near the vertical center of the frame and pan the camera until that detail is near the edge of the frame. This will give 50% overlap. Repeat until you have reached the other side of the scene.

That is pretty much the field work side of a panorama. Now it's time for the fun part. Once you have your images loaded on the computer, there are two ways to perform the panoramic stitching in Photoshop CS3.

Using Bridge, from the menu select - Tools > Photoshop > Photomerge

Using Photoshop, from the menu select - File > Automate > Photomerge

Either way brings up the Photomerge tool. Then Browse to select the image files. A shortcut in Bridge is to first select the images and then open the Photomerge tool.

Use the Auto setting (default) and click OK. Depending on how fast your computer is this may take awhile, so sit back while the computer does the work. This is a memory intensive operation. Depending on how many images are being stitched, on a Windows XP system, I have had Photoshop quit in the process because of a lack of memory. However, on a 64 bit version of Windows Vista, I have not had any problems. When Photoshop did fail to stitch all of the images together, I simply broke up the process into smaller pieces and then merged the resulting pieces. This has only rarely happened.

When the process is done, you have a somewhat odd looking image that has a layer with a layer mask for each image included in the series. Save this file as a PSD file to preserve the layers and all that work the computer has just performed. Next, you will want to flatten the image, but before you do this it is a good idea to check the seams for any glaring artifacts resulting from the Photomerge process. This is particularly true when dealing with something that is moving during the exposure process, such as ocean waves. In these cases, you will have to make manual corrections. If you don't flatten the image, you will find that when it needs to be rotated to make it 'level', the seams start to show up. But, if you first flatten it, this is not a problem as you will be dealing with a single layer. Also, flattening the image gives you a much smaller file to work with.

After you have 'leveled' the image, it is time to crop it so that you have nice clean borders. Once you have done your cropping, you are ready to complete the task of processing the image - spot removal, contrast, saturation, or whatever the image calls for.

Your result is a panoramic image of much higher resolution than had you simply cropped a single image to the same aspect ratio. You now have the possibility of making prints at fairly large sizes. You may be challenged to find a printer that is up to the task. Unless you have a photo printer that handles roll feed paper, you will not be able to make very large prints, but there are printing services available that can handle this task.

• Really Right Stuff