You'll have to show us screen captures of the photo in Lightroom and in Photoshop, and I believe we'd need to see the same area of the photo shown a 1:1 zoom for both Lightroom and Photoshop.
You also haven’t said what your quality setting is when saving from PS. This can affect sharpness if you compress the JPG significantly.
Ok, I made two screenshots, but you can't see the difference. So it's useless that I post them here. I passed a JPG to PS as copy and saved it as a JPG with the quality set to max 12.
As I said, it only seems to be a view bug in LR, because if I compare this two files with an other programm, ie PS or Irfan View, they look the same.
It also only seems to happen in the "fit to content" view in LR. If I zoom in to 1:1 and compare the affected parts of the image, I can't see a difference.
In the 4:1 view I can see that some pixel changes when I switch between the two photos.
I can give you the original JPG and you can try it yourself
You have to look at the branches on the left.
When you view an image at less than 100%, calculations are done to figure out which pixels you will see and which you won't. It would be reasonable for Lightroom and Photoshop to share this code, but I would not be surprised if they do not share.
Since the window size of Lightroom and Photoshop do not match in size, "fit content to window" will give two different zoom levels in the two different applications. When viewed at the same numbered zoom level in each app, I would expect you not to be able to tell the difference, mostly due to human eyes not being that good.
(The JPG on filedropper is damaged.)
It's actually not reasonable that LR and PS share the same display code, because LR is starting with original image pixels and applying all the edits that have been made, and then displaying the result, all within a few seconds, so the speed of the resampling algorithm has to be extremely fast and therefore somewhat inaccurate to keep the display delay to a minimum. PS on the other hand has the end result of the last edit already cached in memory so merely needs to send this to the video card for display. The allows PS to do a better job at resampling.
That's all theoretical stuff, please try it with the attached image from above.
@ManiacJoe Why damaged? What's the problem? I downloaded it on a different PC and had no problem to view it with IrfanView.
I used of course the same zoom level in LR and switched between the two files and you can see clearly a difference.
If Lightroom is not showing the reality of a picture, it's useless. Then because of that I might be attempted to sharpen it. And saving in PS with max quality shouldn't affect sharpness, right?
First of all I should say that I was able to download the file, but initially I got an error that said it was not found. It was probably just an incompatibility of the dropfiles site and my default browser download plug-in and it finally worked just using the standard browser download function. Looking at the contents of the photo, itself, there are no LR adjustments that have been saved to the JPG, only a few metadata changes such as adding the photo location as being in Hawaii.
Anyway, there are several issues that are being mixed together:
1) When a JPG is opened and resaved it will be slightly different due to JPG compression being a lossy format and that the color workspace in PS is slightly different than in LR so when PS converts the colors from its 16-bit workspace to 8-bits the output pixels won't be exactly the same as the input pixels even if there were no substantive changes in PS, just an open and resave.
In this 4:1 screenshot the PS photo does have more variation in colors between adjacent rows of pixels than the original in LR:
2) This variation in colors can lead to an apparent difference in sharpness in LR when a resampled-smaller zoom level is used--in this case Fill with a small LR window size:
3a) The PS resampling for display and the LR resampling for display use different algorithms due to LR having to be as quick as possible, so what you see at those zoom levels doesn't accurately reflect the actual image sharpness, only resampling algorithm accuracy and differences.
3b) In LR there is a warning badge in the Detail panel when using zoom less than 100%. Unfortunately the warning disappears when I press a key to do a screen capture of it:
So to summarize, pixels don't remain unchanged across an Open/Save cycle when you're using the lossy JPG format -- a reason to use TIFs, and less than 100%/1:1 zoom levels don't show sharpening accurately in any program and differences between resampling algorithms in different programs result in differences in the inaccuracy of the resampled result. In other words, with less than 100%/1:1 zoom PS's inaccuracy is different than LR's inaccuracy so neither should be used to judge sharpness, and you should use 100%/1:1 zoom or higher.
Ok, I made two screenshots, but you can't see the difference.
Always compare using the Develop Module at 1:1 not Library or other modules. Have the image you are comparing in Photoshop to be at 100% zoom ratio. That's the only way to properly view the image data accurately in LR.
Thank you ssprengel for the detailed posting!