The optician will take a photo of your retina.
To do this they will need to clearly see into as much of the back of your eye as possible.
To enable them, to do this they will give you eye drops which will expand your pupils. It may take up to 20 minutes for the pupils to get large enough.
The eye drops can sting a bit so just be ready. Try to keep your eyes open as best you can to avoid having to having to have additional attempts.
The fluid needs to cover the centre of your eye to make your pupils larger.
When your pupils are sufficiently expanded the optician will sit you down in front of a machine which will take the photograph of the retina in each of your eyes.
Your eyes must be fully open for a good photograph to be achieved. Be aware that it may take a few attempts to get the photograph just right.
As well as taking a retinal photo, you will also have your sight measured.