Quick Look and thumbnails: how a Mac previews what is inside a file

Q: Does clearing the Quick Look cache delete anything important?

No. qlmanage -r cache removes cached thumbnails only. Your files are untouched, and thumbnails are redrawn when needed.

26 September 2026 · 6 min read

A Mac shows what is inside a file without opening it in several places: Quick Look (select a file and press Space), icon thumbnails in Finder, the preview pane, gallery view and the Spotlight window. For Quick Look and thumbnail privacy on a Mac, that means anyone at your keyboard can read page one of a document just by selecting it. To stop Finder drawing file contents as icons, choose View, Show View Options (Command-J) and untick “Show icon preview”; to clear the thumbnails macOS has already cached, run qlmanage -r cache in Terminal.

None of this is a flaw. Previews are one of the best things about the Mac. They are also copies of your content, drawn and sometimes stored, and it helps to know where.

Every place a Mac shows what is inside a file

Quick Look. Space or Command-Y on a selected file opens a full-size preview: every page of a PDF, a playable video, a whole spreadsheet. The arrow keys step to the next file without closing it. It works in Finder, in Open and Save dialogs, on Mail attachments and in Time Machine. Quick Look from the keyboard covers every shortcut.
Icon previews. In Finder and on the Desktop, a photo’s icon is the photo and a PDF’s icon is its first page. At large icon sizes the text is readable.
The preview pane. View, Show Preview (Shift-Command-P) adds a column on the right of a Finder window with a large preview and the file’s details.
Gallery view. Command-4 shows one big preview of the selected file above a strip of thumbnails. It is a preview by design.
The Spotlight window. Highlight a search result and a panel beside the list shows its contents.
Get Info. The Info window (Command-I) has a Preview section at the bottom.
Dock stacks. A Downloads stack in the Dock shows each recent download as a thumbnail when you click it.

The last two catch people out, because they show contents in places nobody thinks of as viewers.

How previews are made

Quick Look previews and Finder thumbnails come from the same system service. macOS includes preview generators for the common formats (PDF, images, video, audio, plain text, Apple’s iWork files and more), and apps can add their own through Quick Look extensions. To see which extensions are installed, and switch any off, open System Settings, General, Login Items & Extensions and look for Quick Look. On macOS 13 Ventura the list is under Privacy & Security, Extensions.

You can watch the service work from Terminal. These commands only read the file:

qlmanage -p ~/Documents/report.pdf
qlmanage -t ~/Documents/report.pdf

The first shows the Quick Look preview; the second shows the thumbnail Finder would draw.

Some files also carry a preview inside themselves. Many photos include a small embedded thumbnail in their metadata, and a PDF can hold page thumbnails that were saved at an earlier stage and never updated. That matters when a document has been edited or redacted: What a PDF still hides covers old thumbnails along with layers and attachments.

Turn off thumbnails in Finder

Icon previews are set per folder and per view, and you can make the change the default.

Open a Finder window in the view you use most (icons, list or columns).
Choose View, Show View Options, or press Command-J.
Untick Show icon preview. In column view you will also see Show preview column, which controls the large preview on the right; untick it as well if you want that gone.
Click Use as Defaults at the bottom of the panel, where it is offered, so folders that follow the default change too. Folders you have customized before keep their own settings, so check the ones that matter.

Then the rest:

Desktop: click an empty spot on the Desktop, press Command-J, and untick Show icon preview there.
Preview pane: View, Hide Preview, or Shift-Command-P again.
Gallery view: the large preview is the whole point of that view, so switch sensitive folders to list view with Command-2.

With icon previews off, a PDF looks like a generic PDF icon rather than the first page of your bank statement. That is what protects you during a screen share, a presentation or a glance over your shoulder. It does not stop Quick Look: there is no setting to turn Quick Look off, and anyone who can select a file can press Space. The protection for that is locking the screen whenever you step away.

Quick Look thumbnail privacy: the cache on your Mac

Finder does not redraw every thumbnail each time you open a folder. The system keeps a cache so folders appear quickly. Apple does not document it and its location has changed over the years. On macOS 15 Sequoia it sits in a per-user cache folder outside your home folder, under the path that getconf DARWIN_USER_CACHE_DIR prints, inside com.apple.quicklook.ThumbnailsAgent in a folder named com.apple.QuickLook.thumbnailcache. On the Mac this guide was written on, simply listing that folder from Terminal returned “Operation not permitted”, so macOS does guard it against casual reading.

Why it is worth knowing about: a thumbnail is a small picture of your file. In 2018 security researchers showed that this cache kept thumbnails of files stored on encrypted drives, and that those thumbnails stayed readable after the drive was ejected. How current versions handle every case is hard to confirm from outside, so the safe assumption is that anything Finder has drawn a preview of may be kept for a while.

To clear it:

qlmanage -r cache

qlmanage’s own help describes this as “Reset thumbnail disk cache”. It does not touch your files. Finder redraws thumbnails as you browse, so switch icon previews off for sensitive folders first, then clear the cache.

What a locked or hidden item shows

Different kinds of “protected” look very different to Quick Look.

Ticked Locked in Get Info. Previews exactly as before. That checkbox only prevents changes; see Locked in Finder is not password protected.
Hidden files. Nothing shows until someone presses Shift-Command-period in Finder. Then they preview like any other file.
A PDF with an open password. Its pages are encrypted, so they cannot be drawn without the password. The thumbnail can only be a generic PDF icon, and Quick Look cannot show a page either.
An encrypted zip. The contents are protected, but the file names inside stay readable to anything that lists the archive.
An encrypted disk image. While it is ejected you see a disk image icon and nothing of what is inside. Once you open it, it becomes an ordinary disk: every file previews and gets thumbnails like anywhere else.
A Hushbox locked item. Hushbox locks a file or folder where it sits, and while it is locked what is inside stays out of Quick Look previews and thumbnails, as well as Spotlight and Finder Recents. The item keeps its name in Finder (a file shows without its extension, so Taxes 2025.pdf shows as Taxes 2025), because Hushbox hides contents, not names. An unlocked folder opens in its own Finder window, so the view settings above apply there as well.

Questions

Does Quick Look send my files anywhere? Apple’s built-in previews are drawn on the Mac. Third-party Quick Look extensions are code from the apps you installed, so review the Quick Look list in Login Items & Extensions if you have added any.

How do I stop Finder showing what is inside files as icons? Press Command-J in a Finder window, untick Show icon preview and click Use as Defaults. Do the same on the Desktop, and hide the preview pane with Shift-Command-P.

Does clearing the Quick Look cache delete anything important? No. qlmanage -r cache removes cached thumbnails only. Your files are untouched, and thumbnails are redrawn when needed.

Can I turn Quick Look off completely? There is no switch for it in System Settings. Keep sensitive files locked instead, and lock the screen when you leave the Mac, since Quick Look can only preview what someone can select.