Image Assets For iOS– Similar to Android, iOS too has a way of dealing with different pixel densities of iPhones, iPads and other Apple devices.
They have categorized the displays as Retina 2x and 3x. Here is a table of different apple devices and the densities they fall in.
Device Density Resolution
iPhone 5/5S Retina 2x 640 x 1136 px
iPhone 6/6S Retina 2x 750 x 1334 px
iPhone 6 Plus/6S Plus Retina 3x 1242 x 2208 px (1080 x 1920 on device.)
Here is how to go about optimizing your designs and export them for specific densities.
To export your design elements at different densities just press Ctrl + Alt + I in Photoshop and make sure the constrain ration link icon is pressed then enter the desired value in one of the fields to resize your document to another size.
Right click on the layers or groups and select Export as (Photoshop CC) to cut them out for sending them to developers.
NOTE: if you do not have Export as function in your layer panel then you probably have an outdated version of Photoshop, Install Creative Cloud Desktop App and update/Install the latest version of Photoshop to get the feature.