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Stacking and Stitching: An Overview of Multi-Shot Techniques

Cameras are tools. They can do a great job of recording what the world around us looks like, but they are far from perfect. There exist techniques for combining more than one press of the shutter to help overcome some of the limitations that exist in traditional single-shot photography. Some of these I've talked about at length already, and some of them I haven't. Here's an overview.

HDR Stacking
High Dynamic Range (HDR) imaging is all the rage these days and I've written about it before. If you're not familiar though, the idea is to shoot multiple images of the same scene at different exposures and then merge them to form a composite image that exceeds the brightness range that your camera is natively capable of. Each shot should differ from the others only in terms of shutter speed. Varying exposure by means of aperture isn't recommended since doing so would have an effect on depth of field that generally would be more apparent than a comparable change of exposure time.

True HDR imaging works by combining all the source frames into a 32-bit per pixel image that accurately places everything relative to everything else even if that composite image exceeds the ability of computer monitors and printers to render. At least all the details are in there. The trick then is to intelligently down-sample (or "tone map") that seemingly unusable image into something that can be used. There are also programs for merging images shot in the same way directly to a standard image bypassing entirely the actual high dynamic range image format, but these often don't work as well when the total brightness range to be merged is too great.

Focus Stacking
I've spent the past several weeks discussing the concept of focus stacking. No matter what subject you may be trying to shoot, there are limits to how much depth of field you can capture. If you happen to be shooting macro, there are severe limits. But if you shoot a series of image of a subject with each one focused slightly further into the scene you can later merge them to create a composite image that makes use of the portion of each that is in focus. What you end up with is an image that captures your subject with a greater depth of field than any one shot would have given you.

Shooting for focus stacking can be done either by changing the lens focus ring slightly between frames or by moving the entire camera setup forward or back on a focus rail pointed toward your subject. Both have advantages and disadvantages. There are software programs and hardware setups that can simplify either approach but you will likely need some practice to master either.

Panoramas and Stitching
The standard two-by-three ratio of a camera frame is obviously an arbitrary one. Some subjects are naturally much wider or taller than that. After shooting a series of images with the camera panned slightly between each one, you can merge the results to form a single image that covers the entire area. Of course you could instead just use a wider focal length lens to get the entire subject in a single shot and then crop the sides later, but the resolution wouldn't be as good since you would be throwing away some of what your camera captured. Even with today's high resolution sensors you can likely tell the difference if you look closely enough.

And the same thing holds true even if your subject does fit the standard aspect ratio of your camera. By panning both horizontally and vertically you can stitch together multiple rows and columns of source images to form a larger composite image with greater resolution than possible in a single image. Nick Risinger, a photographer from here in the Seattle area recently published a 5-gigapixel image (that's five-thousand megapixels) of the entire night sky by combining 37,440 digital exposures. You can read the story of how he did it on his website, but the results are stunning. His site has an interactive, zoomable version of the result. Well worth a look.

I don't know Rick but I do have friends who routinely create great panoramas. I have to admit though I've never really done much of this myself. It's not that I don't want to, it's just that I'm usually busy shooting what I am shooting. It's not until later that it occurs to me that something would have made a great panorama subject.

Noise Reduction Stacking
In the early days of digital photography, long exposures meant coping with digital noise. Although sensor technology has improved since then, with long enough exposure durations noise can still be a problem with most any camera. Rather than shoot one long exposure though, it is possible to shoot a number of shorter ones and merge the resultant frames digitally to build up a result that yields a similar result but with less noise.

This works based on the fact that such noise is, to a degree, random. If you take multiple exposures and average them, noise in one will be largely cancelled out by the lack of noise at that point in the others. Only actual image detail will be constant in every frame. The results from such techniques compare quite favorably when compared against the results of software noise reduction filters. Random noise is effectively reduced without sacrificing image detail so long as the subject isn't moving. When your subject is moving and you want to record the motion blur the results can be good too. Flowing streams or even star trails can be rendered the way you expect if you have enough exposures spaced closely enough together.

Stacking for noise reduction doesn't require any changes to camera setup between frames but the other techniques I've discussed all do. Each requires a degree of planning and organization to keep track of what you are doing. It's best to work methodically and always shoot a series of shots in the same order. Whether it is focus, exposure or camera position, each frame needs to overlap the ones before it and after it by enough to provide a margin for safety and to give the software room to do its magic.

You may be familiar with some of these techniques already, but hopefully this will give you some food for thought.

Date posted: May 15, 2011


Copyright © 2011 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: Focus Stacking: Getting the Shots in the First Place Return to archives menu Next tip: Stacking and Stitching: So is All This Real Photography?

Related articles:
Photoshop HDR 32-bit Format: The Dawn of a New Era?
What the Heck is HDR Tone Mapping?
Focus Stacking: Breaking the Depth of Field Barrier
Stacking and Stitching: So is All This Real Photography?

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