Preparing Images for Long Exposure Stacker

For versions of Long Exposure Stacker before version 1.5, it was necessary to first convert all RAW images to a format that Long Exposure Stacker could read. With Long Exposure Stacker version 1.5 and later it is possible to open RAW files, for example, .CR2 files from Canon cameras and .NEF files from Nikon cameras. For many users, opening the RAW files will be the quickest and easiest way to get started. If you want to open RAW files directly, you can ignore the rest of this page, and go to Processing with Long Exposure Stacker.

Details of RAW processing in Long Exposure Stacker are discussed here.

There are reasons you might prefer to use an external RAW converter rather than have Long Exposure Stacker read RAW files directly:

• more control is available over the conversion parameters;

• better results might be achieved for some cameras with an external RAW converter;

• TIFFs load much faster than RAW files, so if you anticipate reloading some images many times, first converting to TIFF could save time;

• you may have to alter the images in some way, for example painting out defects in the images before stacking.

There are reasons why you might want to use RAW files directly:

• new users often have difficulty finding and overriding all the defaults that must be overridden in some RAW converters;

• you can save time compared to first loading the images into a RAW converter and then exporting them as temporary files;

• you reduce the risk of errors, such as not synchronizing modifications across all files, or applying inappropriate corrections such as lens distortion correction, which adds artifacts that are enhanced by stacking;

• you avoid overly “helpful” RAW converters (for example Adobe Lightroom always applies lens distortion correction for most mirrorless cameras, but lens distortion correction adds artifacts that are enhanced by stacking, and must never be applied before stacking);

• when you use RAW files directly, Long Exposure Stacker will automatically synchronize orientation and color balance between dark and light frames.

If you want to use a RAW converter some instructions and guidelines follow.

The goal of RAW conversion for Long Exposure Stacker is to produce a set of TIFF files with 16 bits of data per sample (the default is often 8 bits per sample), all the images processed identically, and with only the needed changes. In most cases the biggest challenges are turning off all the processing features that you do not want, and synchronizing the settings across all of your light frames and, if you are using them, dark frames.

Typically, for each shot, you will have a set of exposures possibly including both light and dark frames. Load your images into the RAW converter you prefer to use. (See below for some notes on issues with specific RAW converters).

In your RAW converter, make some basic adjustments. I think the most useful adjustments are as follows:

reduce the contrast—If I am not using dark frames, I might reduce the contrast a little. The objective is to avoid losing details either in the dark areas or in the highlights during stacking. You will darken the dark areas and make other tone adjustments after you have processed the images with Long Exposure Stacker. It is not necessary to reduce contrast when dark frames are used.

adjust the exposure if needed—try to leave a little room at both ends of the histogram.

turn off sharpening and noise reduction—In Lightroom this is under "Detail". Some level of sharpening and noise reduction is usually on by default. Applying sharpening or noise reduction to the original images before processing with Long Exposure Stacker may reduce the quality of the final result. I have been told by a reliable source that a small amount of color noise reduction before stacking, with more after stacking, can improve the final result compared to turning color noise reduction completely off before stacking. I have not verified this myself, but will be trying it.

apply lens chromatic aberration corrections if you want—Turn off all other lens corrections; in particular, lens distortion correction must be turned off.

In general, the changes you can make during RAW conversion are:

• adjust exposure

• adjust contrast

• adjust white balance

• turn on chromatic aberration correction

• apply a little noise reduction (a little, not a lot).

In general, all other exposure or color adjustment should be at their neutral settings and all other correction should be turned off. Unless you are absolutely sure about an adjustment or correction, turn it off or set it to the neutral setting. Some common things that you absolutely must not do:

• do not apply lens distortion corrections (this is often on by default--turn it off)

• do not resize the images

• do not crop the images

• do not straighten the horizon

Your primary goal at this point is adjust the tonal range of the images so that the bright areas are not blown out and the darker areas are a bit brighter than you want them in the final image, and nothing more. It is imperative that you do the minimum possible. Be sure that identical changes are applied to all images—both the light frames and the dark frames. Pay particular attention to white balance. For example, in Lightroom, if the white balance is set to “automatic” and you “sync” the settings across all of the images, the “automatic” setting is copied, not the specific temperature and tint. This means that the color balance is not synced. I always leave my camera in auto white balance and in Lightroom change white balance to "Custom" in the image that I am using as the master for syncing.

Once you have these basic corrections synced across all the images, you can go through the individual images and make adjustments to specific images. Mostly this means painting out any major issues in individual images. There is no need to paint out anything that appears at a specific place in only one image like an airplane track. Airplanes appear in a different place in each image. Long Exposure Stacker will remove these things as part of its processing. If you have something that sits in one place for several but not all frames, it is best to paint it out. Long Exposure Stacker will make any errors in your painting less visible as part of the noise reduction process.

When you are done, export your images as 16-bit TIFF files. In most RAW converters the default is 8 bits, so be sure to change this. It is important that the exported files have 16 bits of data as Long Exposure Stacker needs all the details it can get in the dark areas. During export also pay attention to the color profile you are using. I use ProPhoto RGB. I recommend against sRGB, which is often the default.

Most image processing tools allow saving transparency data. For example, Lightroom has a checkbox labelled "Save Transparency" in the "File Settings" pane. It is best to not save transparency as the transparency data requires more file space. Most image processing tools default to not saving transparency.

When saving images for processing with Long Exposure Stacker it is best to include all metadata in the images. Long Exposure Stacker will copy this information into the files it writes which will make final processing and digital asset management more convenient than it would be if the metadata is stripped.

Once you have your 16-bit TIFF files you can process them with Long Exposure Stacker.

Some notes on specific RAW converters

Lightroom and ACR (Adobe Camera RAW): These are very popular but have a fatal flaw for owners of some mirrorless cameras—they always apply lens distortion correction. This is optional for most cameras, but for some modern mirrorless cameras it cannot be turned off in Lightroom. Lens distortion correction adds artifacts that are enhanced by stacking and must be turned off. Thus, if you are using a modern mirrorless camera you may not be able to use Adobe products for RAW conversion. There may be a work around for this, but I have not tried it because I do not have one of the affected mirrorless cameras. Load your images into Lightroom and export as .DNG files. The .DNG files should have the original image data without any modification. Then open the .DNG files with LES. LES will treat the .DNG files as RAW files. This workaround is useful for Lightroom users who have cameras that are not yet supported by the Apple RAW library that LES uses.

Luminar tends to occasionally change the size of images slightly, and not consistently within a set of images. Until this is fixed, Luminar cannot be used as a RAW converter for Long Exposure Stacker.

ON1 appears to work fine as a RAW converter for Long Exposure Stacker.

CaptureOne appears to work fine as a RAW converter for Long Exposure Stacker. I am told that you can turn off lens distortion correction for all cameras, but that it might be some work to find how to do it.

Photos. The Photos app that comes with your Mac can be used as a RAW converter. It is possible that it cannot be used with macOS 11 and later as it appears to strip essential metadata on export, but on earlier versions of macOS it worked well. To synchronize changes across a set of images, first edit one image, then Image->Copy Settings (or ⇧⌘ C) to copy the settings, then select all the images you want to synchronize and Image->Paste Settings (or ⇧⌘ V).