Working on JPEG saved from RAW is this the same as working on RAW ?

Question

I have a few questions about the RAW format. It may be silly but I love that some explanations on the subject.

First of all, I know that RAW has many advantages over JPEG for post-production (in my case with Lightroom)

For example, if my camera records the RAW + JPEG, RAW image will be better quality than the JPEG image recorded by the camera.

My question is as follows:

Is this really the same thing if I work directly on the RAW file or if I work on a JPEG file recorded with a RAW Lightroom? (With Lightroom, working on JPEG saved from RAW is this the same as working on RAW ?)

Because I've noticed that with identical settings, I get to have the same result on a RAW file and a JPEG file recorded with a RAW Lightroom.

I really find it weird. This means that I can just save JPEG from RAW and doing post production with Photoshop or the other and return to Lightroom with JPG without losing data RAW ...?

In short :

Test 1

• Open JPEG saved from Camera in Lightroom
• Save in JPEG
• = Bad result

Test 2

• Open RAW saved from Camera in Lightroom
• Apply filters
• Save in JPEG with Lightroom
• = Good result

Test 3

• Open RAW saved from Camera in Lightroom,
• No filters
• Save in JPEG with Lightroom
• Re-open the JPEG with Lightroom
• Apply same filters as Test 2
• = Same result as Test 2 or not ?

Answer 1

No, it does not matter where your JPEG comes from. Editing a JPEG degrades its quality.

The reason that JPEG is not suited to editing has to do with the way the data is saved in JPEGs, not whether the camera produced the JPEG or Lightroom did. I suggest you read the many excellent answers on this site that explain why raw is better than JPEG for editing.

In your test, you found that the quality did not degrade when editing the JPEG. I suspect that one of two things is the problem:

1. Your edit is not substantial enough and you are not looking close enough. A very few number of edits can be made without quality loss (some scaling and rotation by 90/180 degrees) and many more type of edits can be made with only a bit of information loss.

   Example: You crop the image. Most of the time this will be a lossy edit. But the quality loss will be small and you might not notice it. However, then you also change brightness a bit, then you rotate by a few degrees. All these small losses accumulate and in the end when you print or look at the image on a high-resolution screen you can see the difference.

2. Lightroom is behaving differently to what you expect. Lightroom is first and foremost an image database. It keeps track of your images and also what edits you did to them. It's misleading to think that you "open" an image, edit it, and then "save" it as JPEG. You only "import" and "export" the image.

   Are you sure that when you exported the image as JPEG, you did not include it in the Catalog, then later imported the image to the catalog? On export there is a small check box that let's you "include the exported image in the catalog". That means Lightroom knows that the exported image is just one version of the original raw. If you now edit the image some more, you don't do so on the JPEG data but on the raw with some edits already applied.

Edit

In response to one comment below: When I talk about "editing a JPEG degrades its quality", I mean the most common scenario where a user opens a JPEG in her favourite editing programm, does something to it (i.e. rotate by 3 degrees and crop to fit) and then saves as a JPEG again.

• There is no information loss when opening a JPEG. At this stage a decompression occurs and the JPEG data are converted into a RGB format that can be displayed on the screen. Yes, JPEG uses a lossy compression but this loss occurs when compressing not when decompressing.

• The information loss occurs at the stage of the edit (for a large subset of edits), this can be mitigated by upscaling, or moving to more than 8bit channel representation. (Note that a rotation would also cause a loss when done in raw, the difference is that in raw Lightroom always starts from the raw so losses of various edits don't accumulate.)

• There is also information loss when compressing/saving the edited image as JPEG. From the two sources of loss, I suspect that compressing causes more loss than editing but this test has many variables, so can't be answered here. Have a look here for a good visualisation to what does occur.

• The comment suggests that editing a JPEG can be done without quality loss when saving in a lossless format like PNG. This is only sometimes true (see point above) but more importantly, it is also irrelevant: If the reader knows a bit about the intricacies of JPEG, the question can be answered immediately. If, however, the reader does not know these details about JPEG then she is unlikely to save in PNG.

Practical relevance: If you edit in JPEG and only do one save you only lose a bit of quality. If you do so, it's up to you to decide if you can live with that. Most of the time a single edit will not be noticeable. But I wrote that as part of my original answer anyway.

Answer 2

This is well answered by Unapiedra, but I thought I might add a bit more about RAW itself.

A RAW file, whatever format it comes in (NEF on nikon) is a big stack of data.

It's not even just one image: it actually contains a small jpeg "preview" which is what you saw on the back of your camera, and what some editing programs (like Aperture) make the loading process look faster.

The important part of the RAW file is the brightness information for each of your pixels. If you were to render this data directly to an image file (which can be done using command-line tools) you wouldn't see anything—it would likely be nearly black. Scale that data to a reasonable brightness, and you'll get a black-and-white image of the photo, covered in a tiled grid.

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The camera sensor only detects brightness, not color, so color must be extrapolated later. The RAW file is the sensor data, before any of this extrapolation.

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That grid is due to your camera's bayer filter, and that's the magic to your camera's color: it shows the brightness through red, green, and blue filters all over your sensor, which allows Lightroom, Photoshop, Aperture, or anything else that can read RAW files to calculate color. That process is called demosaicing or debayering, and basically involves using the varying brightness of pixels that each only see one color to create a new, color image, almost as large as the original luminance-only sensor.

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In a sense, because this demosaicing must happen at some point, all jpegs are saved from raw, whether that happens inside the camera or in your computer is up to you.

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This calculation is a large part of what makes the RAW file better for editing. A jpeg file cannot contain the tonal depth of your camera, nor can your screen display it or your printer print it, so at some point, data must be cut. By editing the RAW file, you're choosing how to make that cut: WB corrections change how the debayering calculates colors, and raising the exposure slider actually does bring more information into the image than the editor was previously displaying.

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A jpeg already has color, exposure, contrast, sharpness, and other adjustments applied. New adjustments are calculated from that limited data because the rest was deleted when the image was saved as jpeg. Editors like Lightroom and Photoshop are pretty good at extrapolating from this limited dataset, but it is always better to have as much information as possible.

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