If they have an optometrist on site they may suggest you use them for final prescription, but in either case you are not committed to buy glasses from them. In my case, years ago, an optometrist walked out on me after I could not see difference that he thought I should see and as said there are very few such trained persons in Thailand as it is a new profession here. A normal ophthalmologist visit in my experience includes 20 foot test, computer test and pressure test so basic information is available to doctor from these - if he feels closer testing is required he is capable of doing himself.
As for "western" eye exams - some of the worst vision prescriptions I have ever had were from US optometrists - they could not even get eyeglass size right - and vision was totally bad. Being western is not a guarantee of being good.