Did you use detail sharpening?
Try setting it to 0 and try an export.
On my version the effect of detail sharpening is not shown right in the develop module unless viewed at 1:1 or larger.
This is how it looked like for me, less contrast and detail.
One problem is that the image shown in IrfanView is resized/zoomed to 563x845 and IrfanView probably has different resizing algorithms, which can result in different appearances of sharpness or lack of sharpness.
What happens if, as a test, you import this exported photo back into LR, does the sharpness appear the same as the original, or not?
I sharpened the image with "adjustment brush / sharpness".
At the "export / Output Sparpening" the sharpening is turned off.
I used an old M42 44M 58/2 Helios lens for this photo with an adapter on a Canon 600D.
I opened the jpeg with IrfanView, XnView and ACDSee. All the same.
The exported jpeg looks like the photo in LR library mode.
In LR develop mode the photo is much sharper.
The reimported (from jpeg) photo in LR looks like the previously exported jpeg.
The first source was canon's RAW format.
Oh, and thank you for your answers/ideas!
Sharpening applied in develop module is only showed correctly at 1:1 or larger, and should only be applied at 1:1 or larger. I think you will se that there is no difference switching between library and develop at 1:1.
Your jpg suggests to me that you have applied lots of detail sharpening, as that will have a similar effect on my pictures.
Im hoping Lightroom 6 will have the same resizing algorithms in develop module as in library module..
Yes, in 1:1 the photo is the same in library and develop mode too.
The bad thing is that the photo in develop mode looks sharper than the exported one.
Maybe the algorithm in develop mode is more aggressive.
The intresting thing is that I only saw this with the photos took with M42 lens.
Thank you for your answers!