There is no way for Lightroom (or any software) to know what the quality level of the original photo was.

By setting the quality manually, I can't be sure that I don't loose information/ quality in my picture. Alternatively, setting the quality too high means I am increasing the file size without gaining anything in terms of quality...

Incorrect. There is always a loss in quality when you save a JPG. Doesn't matter what number you choose, or what the quality level was before you did the edit.

If you set the quality "too high", you are always gaining things in terms of quality, when compared to a lower setting.

Your comments seem to indicate you think that quality level is an absolute number ... that quality of 80 is always the same. It is not; it really represents the amount of compression of the JPG photo, and an image saved at quality 80 once has higher quality than if you save it a second time also at 80.

bakgwei wrote:

 

Interesting. I was under the impression, that quality is limited. Example: The images come out of my camera at the maximum quality that the camera can offer, with a size of 4MB for instance.

No. The camera takes the raw sensor data and renders it down to a JPEG or other pixel format using an image conversion engine. Compression in JPEG is lossy, so any amount of compression in-camera during this process throws away information which is lost forever.

But yu now say that it is indeed possible to gain in quality.

I most definitely did not say that. I said: "If you set the quality ‘too high’, you are always gaining things in  terms of quality, when compared to a lower setting." The key phrase is "when compared to a lower setting". It's relative.

In other words, if you save at 100, you have a higher quality result than if you save the same image at 70. In either case, the image is never as high quality as before you did the save.

From what I have understood so far I have to set a quality setting of below 100% in LR at export to get back to my original quality and file size...

I think this is incorrect. If you take a JPG and save it as a JPG, it is ALWAYS lower quality than before the save, even if you choose 100%. The differences might not be noticeable to most human eyes, but there is a loss of information, even at 100%.

You are confusing image quality and file size. Within a single save, these two items are related, higher quality means larger file size and vice versa. Across multiple saves, or comparing original JPG straight out of camera to the first save, these two items are NOT related.

THe process of editing a file and then saving it (or in Lightroom terms, exporting it) causes the following things to happen: the image is de-compressed, or expanded, to its full pixel size. JPG Compression then shrinks the file down to a new size (which may be larger than the original, because it was expanded just a second ago).

This is true that every time you save Jpeg images, they are being compressed and loose on a quality that you cannot get back. Even if you save Jpeg with the best possible quality, it gets compressed and some pixels are lost forever.

Please read the link to Jeffrey Friedl's explanation

I haven'ty completely understood all the bits of it, but I figure it boils down to 2 options:

 

1) I can either accept a lower quality (if I choose a quality of, say, 80% during export).

2) I have to accept way higher file sizes if I want to keep the original quality (exporting at 100%).

Both are wrong. You haven't seemed to grasp the simple fact that EVERY time you save a photo as a JPG, you are losing quality. There is no way to keep the original quality when you do a save, even at 100%.

Everyone talks about the loss of quality every time a JPEG image is saved.  But my experience has been that the loss is so negligible that it would, in almost every case, be difficult/impossible to quantify the loss.  Especially if you import the image from the camera into Lightroom, make the necessary adjustments, and then export your final image.

clvrmnky wrote:

Just try a few rounds and see what happens.  A few times is not really noticeable (depending on the information the image holds) but do it even a few more times (3-5 or even more) and an experienced eye can see it.  JPEG artefacts are like digital noise anywhere else: once you perceive it, it is hard not to perceive it.

 

There are a few examples on the web of someone saving to JPEG over and over.

Of course that is true!  But as I indicated, download, edit, and export is going to render a new JPEG that it will be difficult to quantify any loss.  If you want to keep loading and saving an image to prove a point, anyone can do that.  But why would they want to?  We all know that there is a loss of quality when saving JPEG images.  So all you have to do is minimize the number of times you have to save a JPEG copy.  If you follow a normal Lightroom workflow, you only have to export once from the original to get what you need.

All I wanted was to save my images at the same quality level and roughly the same size during export as they used to be before I imported them into LR. I guess this proves much harder than I thought, and there is no definite answer.

I don't think anyone actually answered your question directly! When your camera creates a JPEG file from the RAW image it uses a quality level that is selected in the 'Quality' menu as High, Low, or Medium, or something similar. You are using the highest quality setting and want to use the same compression level and resolution for exported images. Suggested procedure:

1) Find a typical image that you have imported into LR and make sure all Develop settings are at default zero (0).

2) Export this image using JPEG, sRGB settings, 'Limit File Size to' unselected, 'Resize to Fit' unselected, 'Sharpen for' unselected.

3) Using the above settings set the 'Quality' setting and 'Export.'

4) Repeat as necessary by fine-tuning the 'Quality' setting until the "exported" file size is the same, or close to the "original" camera file size. That may be a number such as 74, or whatever!

5) You have now found the closest quality setting of LR's JPEG compression engine that matches your camera's! You can use this number with confidence, for any JPEG picture shot using that specific in camera 'Quality' setting (i.e. High) with that camera.

Please remember that LR is a non-destructive editor and therefore never over-writes your original image files (JPEG or RAW). Your original files will NOT degrade due to any work done in LR. This is only an issue for exported JPEG copies that you continue to “resave” after opening in other applications. If you never have to “resave them, they will never degrade! If you need to do extensive work on a specific image outside of LR, use the “loss-less” TIFF file format in LR’s Export options. Yes the file size will be bigger, but you can select ‘ZIP’ compression to reduce the file size somewhat.

A JPEG compression setting of 100 will not necessarily produce a "sharper" picture than a 70 setting. The biggest issue with JPEG compression is introduction of artifacts, which is well described and illustrated at the link PhotoGap provided. Also, keep in mind that full-size exports images are rarely viewed at 100%. For smaller enlargements up to 8” x 12” and when using resized exports for web posting, JPEG artifacts will be much less visible.

IM Jolly wrote:

 

An interesting solution, but it still doesn't resolve the issue described in 'post 27' where several images from a batch that were converted from RAW to JPEG, resulted in a larger file size than the original RAW file. 

RAW files can also be compressed - usually losslessly (i.e. no data is lost when a RAW file is decompressed). Although, some cameras have an option for lossy RAW compression also.

In very simple terms, an uncompressed image will be the dimensions your camera produces * 3 (plus other metadata and so on, and for simplicity let's not get into 8/12/16 bit).

So if your camera produces images sized 4000 * 3000 pixels, an uncompressed image will be around 36,000,000 bytes, about 34MB.

Lightroom will uncompress (JPEG or RAW) any image in order to work on it. So it's starting out with that 34MB of data.

Again, in simple terms, you could think of the JPEG quality slider as "the amount of data I'm prepared to lose in compression", where 100% is none and 0% is all of it.

An interesting solution, but it still doesn't resolve the issue described in 'post 27' where several images from a batch that were converted from RAW to JPEG, resulted in a larger file size than the original RAW file.

I think I have a better explanation as to why some of your LR JPEG export files are larger than the the original RAW file format. I just took an image shot at ISO 6400 with my Canon 5D MKII and increased LR Sharpening to 100 Amount, 1.0 Radius,100 Detail and no Noise Reudction (0) settings. I exported to JPEG, sRGB, full-size with 100 Quality setting and no other setting options. The JPEG file size is 37.1 MB and the orginal .CR2 RAw file is 31.13 MB. I reset LR Sharpening to a more normal 25 Amount, 1.0 Radius, 25 Detail, and 25 Luminace and Color Noise Reduction. The JPEG file size is only 14.7 MB.

The more processing you apply to a RAW image in LR, the more likely the JPEG or compressed TIFF file size will inflate. REASON: You are increasing the picture information. If you shoot a Gray Card or other subject with very little detail the JPEG file size will be very small at ISO 100, but much larger at ISO 6400 due to noise introduction. It will get bigger "additively" as you increase Exposure, Sharpening, Color, Contrast and just about any adjustment in LR that brings out more detail in the picture.

You're welcome and glad I could help provide a better explanation. "Noise" is normally not a big issue with today's DSLRs, except at very high ISO and when using very high Sharpening Amount settings in LR. There are numerous posts in this forum where exported JPEGs were showing color shift in certain areas and less sharpness. One poster finally revealed he was using a Sharpness setting of 150, which was causing this issue. He was trying to export "resized" JPEGs for website posting and thought that the 150 setting was providing better sharpening of the smaller size JPEG images. 'Output Sharpening' in the Export settings is what should be used for setting sharpness of reduced size images.

Best advice is to only use the full amount of any LR setting for "special effects," since it has been well documented that undesirable artifacts are introduced. Fill greater than 50 can cause a "halo" outline around images against a bright background. Sharpness greater than 100 can cause color shift, reduced sharpness, and yes inflated JPEG file size. Adobe increased the original LR maximum Sharpening Amount from '100' to '150' due to interaction between the 'Amount' and 'masking' controls. This allows using a large amount of Masking and still achieve desired sharpness. Adobe never intended a 150 Amount setting should be used without Masking for normal image processing.

I hope this helps explain the "strange" differences some LR users are seeing in their exported JPEG images.