Check if Lightroom is up to date.
It should be 6.3 version.
When you hold down the ALT key on the slider it shows how much of a mask you are applying to your image (it goes black and white and by moving the slider you can see the amount of effect).
This works for me in Lightroom 5.7 and Lightroom 6.3. You have to hold the ALT key down and then hold the mouse button down on the mask slider.
Yes it is LR 6.3
Yes I do "hold the ALT key down and then hold the mouse button down on the mask slider" - it does not work on my Windows 7 PC.
(On my other computer- an old Windows XP with LR3 , it does work this way and it always has worked)
In fact it has nothing to do with my Nikon D800 as old images on the same PC taken with my old Nikon D90 also do not work - so it is a LR problem.
Maybe a 64 bit issue?
Hi I cannot find the file at this location ..
C:\Users\[user name]\AppData\Roaming\Adobe\Lightroom\Preferences\Lightroom 5 Preferences.agprefs
How far down the path do you get? The AppData folder is hidden so if you're wanting to click on it in Explorer then you'll need to unhide hidden and system files and folders in the Explorer options, or you can just click down to your username folder then type \AppData in the Explorer adderss bar after it and hit Enter, then continue clicking down to the Prefs area, or you can type %APPDATA% as the only item in the Explorer address area and hit Enter and you'll be down into the Roaming folder.
Does the Alt key work with any of the other Sharpen sliders? It should work with all of them. If it doesn't work with any of them, I'd guess your Alt key is inoperative and you can try the other one on the opposite side of the space bar.
Does the image not even turn gray or does it just not change when you hold the Alt key and move the slider? The effect can be subtle.
As far as which preferences files to work with: instead of deleting, I would just rename the file from .agprefs to .xgprefs or something similar. And rename all of them, in case creating a new one looks at the previous version for it's starting point. If renaming the preferences turns out NOT to help then you can delete the newly-created preferences file and rename the old one(s) back. After renaming all of them, I'd start testing with LR 6 because that's the most important one to get working.
You'd want to do the renaming when LR is closed.
You can leave the LR 6 Startup Prefs file alone.
Deleting the preference file is the Lightroom 5 and earlier method. Use the Lightroom 6 method.
Hold down [Alt]+[Shift] and launch Lightroom
Overwrite the preferences when prompted by the dialog.
No need to go hunting for files and it bypasses El Capitan's nasty habit of safeguarding preference files.
My point was to not delete/reset the preferences and only rename them so they could be put back and not lose the customizations done over time, since this problem has existed for many years it's likely not a preference-file corruption issue.
Obviously the OP has Windows not El Capitan but that was just general information about the new method.
Obviously (that is why I used Alt instead of Opt in my instructions).
Regardless of the OS, the preset reset method has changed. It is much easier now and platform agnostic.
Additionally, for those El Capitan viewers of the thread, there is an added benefit of bypassing the built in recover that will stymie and older method of preference reset. .
Yeah, there should have been a start-up-reset in LR for years, already.
Thanks to you guys for your advice - it is now solved!
Based on your recommendation "Does the Alt key work with any of the other Sharpen sliders? It should work with all of them. If it doesn't work with any of them, I'd guess your Alt key is inoperative and you can try the other one on the opposite side of the space bar." I tried the Alt key (marked "Alt Gr") and it works. I have never used this key before for anything before. Duh.
I cant believe it was so easy to fix and I have missed it for like a year when I moved to this PC and also LR 4 - never used this forum before. So thanks again for your persistence.