In order for our glasses to perform at their best, we need to create lenses that can match the distance between your eyes, so the center of each lens aligns with the center of your pupils. For this reason, we need your pupillary distance measurement – also known as the PD measurement which when aligned with the optical centers of prescription lenses allows the lenses to perform to their optimum. Alternatively you can choose to use an average pupil distance, which is provided by our dispensing opticians. You can also use our online measuring guide to get the measurement. Your Optician will be reluctant to give you this measurement in order for you to buy your specs from them and not go to the best online store where you can save up to 80% on your new glasses! This measurement refines the positioning of your lenses within the frame you have chosen. It can be provided in minutes by your optician or doctor, and so we recommend that you ask for it when you have your vision checked. Alternatively, you may ask someone to measure it for you but advise them to be at about an arm's length away from you. You can also measure by yourself by facing a mirror. For single vision lenses, you may choose to use an average PD of 63mm, but for Bifocal or Progressive lenses, we still strongly recommend to get this information from your optician. If you have a high Prescription please contact us for advice before ordering.