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It is possible to edit RAW ("undeveloped") photos in Adobe Camera Raw but what if I open the photo in Photoshop's main windows and then apply the Camera Raw Filter instead? Do I still have all the data from the original undeveloped shot to work with? Or does something get lost and it's better to edit in ACR first before opening in the main editing window?

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Everything you do to a raw file is recorded as a set of instructions on how to interpret the original raw image data, which does not change (assuming the file doesn't suffer random data corruption in whatever storage medium it is held, or whenever it is copied to another storage medium) as long as the original file is retained.

Those instruction sets are generally proprietary to whatever application was used to create them. Work you do in Adobe products will not be applied if you later open the raw image file in other raw conversion applications, such as Capture One or DxO PhotoLab (formerly Dxo Optics Pro). Similarly, any work you do in Capture One will not be reflected when the raw image file is later opened using Adobe Camera Raw.

In the case of Adobe products, Adobe Camera Raw is used to interpret raw image files when worked with in other Adobe applications such as Adobe Photoshop or Adobe Lightroom. So whether you open the file in Adobe Camera Raw or in Adobe Photoshop, the raw conversion engine under the hood is the same: ACR. The interface is all that is different with regard to how raw files are handled in Photoshop. The additional functions/options of Photoshop that are not included in ACR are applied to the image information after it has been converted from raw image data to a raster image by ACR working in the background, and those instructions are also added to the file (or a sidecar file containing the instructions) without altering any of the original raw image data.

Changing the way an image looks on your screen using ACR, LR, or PS doesn't alter the original file at all until you "save" it. It just changes what you see on the screen by changing the way the raw image information is processed/interpreted. When you "save" your work, the instructions are added to either the image file itself or to a separate sidecar file associated with the image file. (There are options within ACR and all the other Adobe products that use ACR to interpret raw data that allows the end user to select whether the instruction set is appended to the main file or contained in a sidecar file.) In either case, none of the original image data is replaced. The instructions on how the original data is to be interpreted are added and used again the next time the file is opened using an Adobe product. If you "save" your changes, make additional changes and then "save" it again, the latest instruction set will overwrite the previous instructions with regard to the particular changes you made.

One analogy is that of a culinary dish. The raw image data is like the list of ingredients. The instruction set is like the various instructions on how to prepare those ingredients. One can take the same ingredients and prepare them differently and get two different results. One can even use only some (but not all) of the ingredients on the list to get various results. One might have one set of instructions for preparing the dish in a conventional oven, another set of instructions for preparing the dish using a microwave oven, and yet another set of instructions for using an open fire and a dutch oven. Changing the instructions on how to prepare them does not change the initial set of ingredients we start with. It does change, to one degree or another, what we end up eating.

However, a raw image file usually contains more information than just the image data collected from the sensor. Much of this information is stored in a portion of the file known as the EXIF information. Much of the EXIF info is standardized, but there are provisions for what are known as "maker notes" to also be included in the EXIF information. These "maker notes" allow each camera manufacturer to include whatever non-standardized information they desire in the EXIF info. For the most part, Adobe products ignore most of the maker notes section of the EXIF info, but other raw conversion applications, particularly those created by the camera manufacturer like Nikon's ViewNX or Canon's Digital Photo Professional, can and often do use some of that information when interpreting the raw image data.

When images are exported using Adobe products, much of the maker notes section included in the original raw file is not included in the EXIF info of the new file created by Adobe products. It's still there in the original raw image file unless the original is overwritten by the new exported file. Again, there are options from within Adobe's products to leave the original file and create another exported file or to replace the original file with the exported file.

For example, if one converts a Nikon .NEF file to .DNG using Adobe's convertor, the maker notes are stripped from the EXIF information and not included in the resulting DNG. If one only plans to ever use Adobe products this is no big deal, because all Adobe products ignore the information that is stripped. But if one later wants to use another raw conversion application that does use the maker notes info, that information is gone unless one still has the original .NEF as well as the .DNG made from it.

  • One small clarification: DNG convertor have option to include original RAW file in resulting DNG so you can restore the original RAW even if you do not keep it as separate file – Romeo Ninov Dec 16 '19 at 19:31
  • It is my understanding that it still strips most of the "maker notes" section of the EXIF info, though it does preserve the actual data from the sensor. – Michael C Dec 16 '19 at 19:33
  • Just checked and (for Canon CR2) DNG Convertor restore 100% original file (checked with sha1 hash) – Romeo Ninov Dec 16 '19 at 19:40
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    That's good to know. However, the vast majority of folks who convert various raw formats to DNG usually do not select the option to include all of the original file because of file size considerations. One of the benefits touted by Adobe when they introduced the DNG format was that it reduced file sizes and that is why many folks decided to use it instead of keeping their actual raw files as they came out of their cameras. – Michael C Dec 16 '19 at 19:45
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The following edits are non-destructive when applied to a raw file:

  • any edit in Lightroom
  • any edit in Camera Raw
  • any edit in Photshop using adjustment layers

Almost all other edits in Photoshop (filters, image adjustments, etc.) are destructive. If you overwrite the original file with these edits, your original data are lost. Fortunately, you can’t save to a raw file format (it wouldn’t be raw anymore), so you can’t overwrite a raw file (but you can overwrite a jpeg or any other file format and your original data will be lost).

A good practice when working in Photoshop is to open a raw, edit it in Camera Raw and then edit your image and save it as a psd (PhotoShop Document).

In Lightroom and Camera Raw, your edits are not stored in the raw file, but in the (Lightroom) library or in separate (xmp) files, which just contain the edits to the original (raw) image. When opening such a file, Lightroom/Photoshop reads your image and re-applies those edits.

  • In PS you can't save the edited file to RAW. You can save it to image file. Independently if you do destructive or nondestructive edits – Romeo Ninov Dec 16 '19 at 19:03
  • @RomeoNinov thanks. Wasn’t sure if PS could do that or not. Corrected the answer accordingly. – agtoever Dec 16 '19 at 19:15
  • Has Adobe removed the option to append the development instruction set to the original raw file in the most recent versions of ACR/PS/LR? It was there at one time, though not too many folks used it because it meant needing to rewrite the entire file each time it was saved and that increased the risk of data corruption as well as took longer. With newer OSs, though, files can be amended with additional information without having to rewrite the entire file. – Michael C Dec 16 '19 at 19:30
  • @MichaelC, last LR have option to add date/time of change to RAW. And can add development settings only in JPEG, TIFF, PSD and PNG. Not sure about PS/ACR – Romeo Ninov Dec 16 '19 at 19:51
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I all the cases you have untouched RAW file. All the changes (lossless) are stored in special area of the file (when file is DNG), stored in side file (XMP) or in special catalog (in Lightroom for example).

And you can get back untouched RAW file just by delete this side file. Or by reset settings you done.

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The camera raw filter inside PS is not using the original raw data, nor the raw edits file (xmp).

You can verify this by editing the image in ACR before opening it in Photoshop and then applying the camera raw filter.

If the filter was using the raw data/xmp ACR would open the image with the original ACR edits shown (slider positions); in the same manner that LR displays a copy, and how ACR opens a raw file with an associated xmp. But instead the filter opens the image with the previous ACR edits applied, as well as any (destructive) edits that were made to the layer inside PS (all sliders/edits at zero).

The xmp file is created by ACR when you originally open/edit the file. Edits made w/in photoshop are made using that modified data. And that includes edits made with the camera raw filter... i.e. those PS filter edits are not added to the xmp because they are not raw file edits (instructions).

In other words, ACR is destructive in terms of the image/data used w/in PS. If your original ACR edits are bad you will not be able to restore the image unless you start over from the original raw file... in which case it will open in ACR with your previous "bad edits" showing, and you can adjust them to a new positions as desired.

I.e. if you apply a +5 exposure in ACR when you first open the raw file, the camera raw filter will subsequently open the image with the exposure set to zero, and a -5 adjustment will not restore it to the original raw state. To get back to the original raw state with the exposure slider at +5 (resettable) you will need to start from the raw file again.

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