Alley in Pisa, Italy

by Lars Kotthoff

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This is not about HDR, exposure fusion, or any kind of bracketing (exposure, focus, ISO, what have you).

There's an excellent iOS app called Cortex Camera that takes a couple of dozen photos in a burst and then merges them to produce a low-noise photo -- one that has so little noise that you would normally consider it beyond the iDevice's capability. This doesn't need a tripod.

Is there similar functionality in Lightroom or another app? I find that when I shoot low-light on my NEX, I prefer using a relatively low ISO (400 or less) with a tripod. I was wondering if I could skip the tripod and instead take a burst of a couple of dozen photos, and then merge them in software?

EDIT: Based on the comments below, some clarifications are due:

  1. I don't have Photoshop. I do have Lightroom, so LR plugins are okay, as is standalone software.

  2. I use a Mac, so Windows software doesn't work for me.

  3. I don't want to use command-line tools.

  4. I'm okay with both free stuff ($0) and cheap software (like $20, not like $100).

  5. I am not looking for an onerous, multi-step process with multiple decisions to make and different things to try, but more like a 20-second one: drag the source images in, press Fuse, and press Save.

  6. I require the software to work without a tripod or a remote shutter release, since if I have to carry these with me, I might as well use a long exposure and be done with it.

  7. This means that the software should handle the camera moving a bit between shots (both horizontally and vertically) and rotating a tiny bit. It can't assume perfectly aligned shots -- that's not useful for me.

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3 Answers 3

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The technique used by Cortex Camera is called Median Blending. Astrophotographers have been using this concept for years to combine multiple photos of dim objects in the night sky to reduce the noise and increase contrast and color. It is often referred to as image stacking in which hundreds of images of the same piece of sky are overlaid and the values for each pixel are set at the median value for that pixel from all of the combined images. But astrophotography is far from the only type of imaging that benefits from Median Blending.

Most full featured imaging applications, such as Photoshop and the GIMP, include the ability to do image stacking. There are also some cameras such as the Sony A-mount and E-mount cameras that can do this in-camera. Depending on exactly which model NEX you have, your camera may already include this feature. It is called Twilight Handheld Mode. The camera captures a series of photos in rapid succession (maybe 5 photos) and then merges them to create a low-noise JPEG. As with anything done in-camera, you give up the finer control of a post-processing application for the convenience and time savings of letting the camera make many decisions for you.

Although it works fairly well without a tripod, for the ultimate in resolution you will still want to stabilize the camera. When the camera moves slightly from one frame to the next, then everything in the image will shift a certain number of pixels. If the shift is purely horizontal and/or vertical, there isn't much loss in terms of absolute resolution. This is rarely the case. When the shift from one photo to the next is diagonal, or worse yet rotational, then there is not a one-to-one correspondence of pixels from one frame to the next. The use of a Bayer filter to mask different pixels with different colors also comes into play when the camera shifts between exposure. This reduces the effectiveness of using the median value of each pixel by a miniscule amount in much the same way that using a lens correction profile to correct the distortion created by a lens' design will reduce the absolute resolution of an image.

Here are some links to articles and discussions on the subject:
http://petapixel.com/2013/05/29/a-look-at-reducing-noise-in-photographs-using-median-blending/
An expanded version of the same article:
http://blog.patdavid.net/2013/05/noise-removal-in-photos-with-median_6.html
http://www.digitalphotopro.com/technique/revolution/reduce-noise-with-multiple-shots.html
http://diglloyd.com/articles/LensAndCameraIssues/NoiseAndMultipleExposures.html
http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm
http://photography-on-the.net/forum/showthread.php?t=1221282

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Thanks, Michael. Most of these articles seem to describe how to do this in Photoshop (which I don't have), or describe the general technique, which I already know (but I didn't know the term "median blending" -- thanks). But what software would you recommend? LR/Enfuse? –  Kartick Vaddadi Nov 26 '13 at 6:08
    
BTW, I also need the software to compensate for diagonal displacement and rotation. If I have to use a tripod, I might as well use a longer exposure -- I won't need median blending / image stacking in that case. –  Kartick Vaddadi Nov 26 '13 at 6:09
    
Multiple short exposures using median blending will be much less noisy than a single long exposure, even though you need a tripod for both. Read the diglloyd.com article for why the pixel displacement reduces overall image quality both in terms of noise and resolution. –  Michael Clark Nov 26 '13 at 6:55
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The first link lists three open source programs that may be useful to you. The open source GIMP has the ability to do image stacking. Additionally, deepskystacker.free.fr/english/index.html is free but the GUI is definitely intended for stacking very large numbers of dark sky images. Image stacker (tawbaware.com/imgstack.htm) is feature limited unless you register it for $17US. –  Michael Clark Nov 26 '13 at 7:13
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@Rmano That's the same basic article that was presented in the petapixel link in my answer. When I saw it while writing the answer I didn't realize how much additional material was in this one compared to the other. I think I'll add it as well. Thanks! –  Michael Clark Dec 3 '13 at 15:44
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There is a way to do this in Photoshop (I'm using CS5 Extended)

  1. Shoot multiple exposures.

  2. In Photoshop, go to File > Scripts > Load Files into Stack.

  3. Click Browse and select the exposures to be used in the Stack and check Attempt to Automatically Align Source Images and Create Smart Object after Loading Layers. This will create a single Smart Object from the multiple exposures. Double-clicking on this Smart Object will allow you to see the layers separately.

  4. Go to Layer > Smart Objects > Image Stack Mode > Median to blend the separate exposures. You’ll see that the noise is reduced substantially.

  5. Optionally, compare Image Stack Mode > Mean. This works best for exposures containing no movement. (aka on a tripod)

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Unfortunately, I don't use Photoshop. Is there any other software you'd recommend? Thanks. –  Kartick Vaddadi Nov 26 '13 at 6:08
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Yes. Averaging exposures is similar to what you'll see in movies. If you stop on one frame, the grain is visible and often dust spots and scratches. When you're viewing at 24FPS, those aren't as noticeable because the subject is constant, but the grain has a randomness to it.

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