1Fit on Screen

Displays the entire image as large as possible. This function can be found under View -> Fit on Screen

2Fit layer(s) on screen

Displays the selected layer(s) as large as possible. This function can be found under View -> Fit Layer(s) on Screen

3Actual size

Displays the entire document in its original size, e.g. if it is to be printed later. This function can be found under View -> Actual Size

4Custom Zoom Levels

Displays the document at the zoom level you have set. You can set your individual zoom levels under #5.

Alt/Option key

If you hold down the Alt/Option key, you can switch back to 100% at any time using buttons #1 to #4. This allows you to switch back and forth between the selected setting and 100% simply by pressing and releasing the Alt/Option key.

5Settings

Goes to the settings where you can set the values for buttons #4. If you leave a field blank or enter 0, this button will be hidden.

6-/+ Buttons

Reduce/increase the current value by 1%.

Alt/Option Key

Holding down the Alt/Option key reduces/increases the current value by 5%.

Ctrl/Command Key

Holding down the Ctrl/Command key reduces/increases the current value by 10%.

Shift Key

If you hold down the Shift key, the current value is halved/doubled.

TIPRight-Click

Right-clicking on the buttons shows you all available functions, so you don't have to remember the keyboard shortcuts.

TIPMenu

In the top right corner of the panel, there is also an export and import function, allowing you to copy your settings to another version of Photoshop or create a settings file for different projects.