With the Windows 10 May 2019 update, Microsoft made a simple but highly noticeable change to the login screen. It now features the "Acrylic" translucent blur effect from Fluent Design, which makes the background image almost unrecognisable in an attempt to focus you on the login form.
If you'd rather keep your background photo visible, without the extra blur, there are a couple of ways to disable the new effect. They'll both restore your login screen to the same style used by the Windows 10 October 2018 update and prior.
The quickest way to achieve this is to disable transparency altogether. Launch the Settings app and click the "Personalisation" category, followed by the "Colours" page from the left menu. Turn the "Transparency effects" toggle button to the off position.
This method disables transparency completely, which might not be what you want. If you'd like to retain Fluent Design transparency but remove the login screen blur, you'll need to manually edit a registry key. Our usual warning applies – take care when editing the registry, since mistakes could lead to an inoperable system.
Search for and launch the registry editor from the Start menu. Paste the following key into the address bar at the top of the screen and press enter:
From the left sidebar, right-click the "System" key in the expanded node. From the context menu, choose New > DWORD (32-bit value). Per HowToGeek, you'll need to name this value "DisableAcrylicBackgroundOnLogon". Next, double-click the value to open the value editor and change the data field to "1".
You're now done. Try pressing Win+L to lock your PC and display the login screen. You should find the Acrylic effect is gone, so you're back to having Fluent Design with a standard login screen – how it was before the May 2019 update.