Multiple Exposure Techniques

2013.11.23

Now that CMOS sensors are incredibly common, even on ultra-compact cameras, many digital cameras can quickly capture multiple frames to produce a single image which cannot be captured in a single shot. Even if the camera cannot do it, software exists to combine multiple images together in different ways.

An exposure is what happens in the camera from the time the shutter opens to the time it closes. Most images are produced from a single exposure which takes a slice of time and dynamic-range and captures it. By taking multiple exposures and combining them, one can produce an image which shows more than what an exposure can capture.

Multiple images can be combined in a number of ways:

• Panorama Stitching - This produces an image with a wider field-of-view than any individual image can, up to 360-degrees actually.
• Exposure Fusion - This averages out pixels from different exposures to produce directly a low-dynamic-range image, so there is not need to do the tone-mapping like for HDR images. Tone-mapping is the delicate operation where, without a subtle hand, you end up with the types of images you are talking about.
• HDR Blending - This takes images from multiple exposures to produce an image which stores more dynamic-range than what the camera can normally capture. These images are often used in 3D rendering to make simulated reflections more realistic.
• Focus Stacking - Takes images focused at different distances to produce and image with an impossibly deep depth-of-field. Often this is used for macro photography to show entire subjects in focus.
• Image Stacking - This is one most often used with astrophotography. This creates a long exposure by adding up short ones. You would need to take 525 consecutive exposures and the Image Stacking software will blend them into one.
• Multi-Frame Noise-Reduction - Averages a number of images to produce one which is less noisy. This can also be used for astrophotography to reduce the noise in long low-light exposures.
• Multiple-Exposure - Overlays multiple images to artistic effects. This is what layer-blending in Adobe Photoshop essentially does. It shows images as if they were super-imposed.

Note that you can combine techniques which requires even more images. For example, to produce an image which captures a wide dynamic-range and infinitely deep depth-of-field, one may use 5 shot brackets at each focus distance. With 5 such distances, one would need 5 x 5 = 25 shots.

Some artists take this to the extreme by using different combinations of multi-exposure techniques for foreground, middle-ground and background elements and then blending the results together which is yet another similar technique. Images combined from hundres of exposures have been produced!

• Digital Photography