Thorough, traditional, professional approach
I have had an issue three times since 2005 of getting prescriptions where the prescription for the left eye is ever so slightly 'off', certainly in environments with fluorescent lights, cloudy days or at dusk. Not serious, but persistently niggling. This happened with a large chain optician's prescription one week before this appointment. I was hoping for some modification to the left eye prescription. To my chagrin, the left eye prescription remained exactly the same, but the right one had been modified. Rather sheepishly, I approached Intellisight in Redcar for a second opinion. The approach was rather more measured and thorough than that from the chain store. Intellisight do not use the automated lens-chaining apparatus which, to my mind, can sacrifice thoroughness and precision for speed (depending on the individual user). At Intellisight, I also had more precise attention given to underlying medical conditions (e.g., a betablocker blood pressure prescription rather than a 'standard' one).
To my relief, I was offered the chance to compare looking at the eye chart using the prescribed lenses and also with my old lenses. I have never asked to do this as I would feel rather sheepish - as if I would be 'second-guessing' or distrusting the optician's judgement. I was able to see a minor, but clearly observable, difference - certainly on the left eye. I left the optician feeling relieved. The charge for a private prescription, at the time of writing, was £30 compared with £25 for the chain store. Well worth the difference for a thorough and reassuring prescription. Highly recommended.