Processing with Long Exposure Stacker

There are several ways to open image files with Long Exposure Stacker.

• Launch Long Exposure Stacker and it will present a file selection dialog.

• If you have Long Exposure Stacker running, in the menu bar select File -> Open (or ⌘ O) and a file selection dialog will appear.

• Select your image files in the finder and select “Open With -> Long Exposure Stacker.”

There are three types of files that can be supplied to Long Exposure Stacker. Only light frames are required.

1. light frames. These are your normal exposures. Any number of light frames can be supplied. At least 2 light frames must be supplied. The more images you supply the more the motion will be blurred and the greater the reduction in noise.

2. dark frames. These are images captured with the lens cap on. They are used to reduce fixed pattern noise. Any number of dark frames can be supplied. Dark frames are not likely to make much difference for brightly lit shots, but they may be useful for twilight and night shots. For Long Exposure Stacker 1.5 and later I recommend at least 10 dark frames, although some cameras with sensors that have very low noise might see good results with as few as 5. If you are using very high ISO settings or have a camera with a noisy sensor you may need more dark frames. Dark frames can be processed into master dark frames. The single master dark frame can be supplied with the light frames saving some time and memory. A single master dark frame can also be a convenient way of sharing a single set of dark frames with multiple sets of light frames that were taken at about the same time. For version 1.4 of Long Exposure Stacker and earlier I recommend against using dark frames. These versions did traditional dark frame subtraction that requires a very large number of dark frames, at least 5x the number of light frames. Also, these versions could not make a master dark frame. See Preparing Master Dark Frames.

3. master flat-field frame. At most one master flat frame is allowed. Flat-field frames primarily correct for lens vignetting, but may also compensate for other lens and sensor defects. If you are happy with the vignetting correction provided by the image editing tools you use after stacking then flat-field frames may not be useful to you. See Preparing Master Flat Frames.

No matter how you are selecting your files, select all the files that you want to process into a single image. Long Exposure Stacker automatically recognizes light frames, dark frames and flat-field frames. If Long Exposure Stacker is uncertain about the frame types for the files it has read, or if there is a problem with the numbers of frames of each type, it will present a table showing all the frames that it read, along with the type of frame it thinks each frame is, and allow the user to make corrections (see Image Classification Panel).

Here are some suggestions on how to organize your files to make them easier to manage.

the images have been read and classified, the dark and flat-field frames are applied to the light frames. Then the user is free to do any of three things.

Alignment (Optional)

The alignment option can compensate for camera motion between images. If your camera moved slightly because you were hand-holding or because your tripod sank into the sand on a beach, you should be able to get Long Exposure Stacker to align the images. You can open the “Alignment” control by clicking on it. You have to paint areas of the image that have not moved. For landscapes, large rocks and distant mountains are good things to paint. Things that move such as water, plants swaying in the wind, and clouds in the sky should not be painted. Painting large areas of blue sky does not help and might hurt. You can switch between painting and erasing with the buttons or the P (paint) and E (erase) keys. You can change the size of the brush with the slider or the [ and ] keys. You can paint or erase a straight line between the last place painted or erased and the current cursor position by holding the shift key down while pressing the mouse button. For the alignment to work, the Alignment panel must be open, and the composite button must be labeled “Align and Composite” for the alignment to work. If you change your mind and choose not to align, then close the Alignment panel and the mask will be hidden and the composite button will revert to “Composite”.

Exclude Images (Optional)

If you want, you can exclude individual images from the composition. You can open the “Exclude Images” control by clicking on it. To include or exclude a specific image, first make it the current image using the menu of images at the bottom of the the viewing controls, or the Prev and Next buttons, or the forward and back arrow keys on the keyboard. Then you can use the buttons to include or exclude the current image. You can hold the option (or alt) key down and the buttons change to Exclude All and Include All. This is a quick way to include or exclude all images before making changes to individual images.

Composite (or Align and Composite)

When you ready to composite your final image, click on “Composite,” or “Align and Composite” if you are using the Alignment option.

When Long Exposure Stacker is finished compositing (or aligning and compositing), you can view the results of each of six composition algorithms.

Mean Take the mean (average). This often gives the smoothest looking result.

Mean-outliers removed Remove outliers, then take the mean (average) of the remaining values. This often gives a very smooth looking result with the benefit of hiding fast moving objects in the scene.

Median Take the 50th percentile (median). This can give very good results if you are more interested noise reduction or alignment than in blurring motion.

Median-outliers removed Remove outliers, then take the 50th percentile (median) of the remaining values. This can give very good results if you are more interested in noise reduction or alignment than in blurring motion and have fast moving objects moving through your scene that you would like removed.

Max Take the maximum value at each pixel. This is useful when you want to preserve transient lights such as car lights on a distant roadway or airplanes in a night sky.

Min Take the minimum value at each pixel.

When you select one of the composition algorithms, you are shown the result produced with than algorithm. You can save that result by clicking on the Save button or with ⌘ S. If you want, you can select a different algorithm and save the result of it as well. For example, it you are creating an image with lights from cars snaking up a road on distant hill, you might save the result from Max for the lights on the road and Median to get a low-noise version of the rest of the image, and then blend the two images in an image editing tool.