From the Greek Optos (what is seen) and -metry (to measure) an optometrist is a doctor, or in some countries a trained specialist, who measures your eye's refractive state, writes your "prescription" and screens for common eye health issues. An ophthalmologist does this as well. Ophthalmologists are medical doctors who require more training and specialize in eye health whereas optometrists are more focused on refractive state.
Even if you are not using your "prescription", you should see an optometrist or ophthalmologist on the recommended schedule to screen for other health issues and objectively measure your progress. High myopes are at higher risk for many eye conditions and should have their eye health screened more often than emmetropes.
Approaching the Opto
Whichever eye care professional you choose, we will just say 'opto' for the sake of this section, many new to the EndMyopia Method wonder how to approach their opto for the first time. For the most part it is probably best not to try to explain a method you are still fairly new to. Additionally, most such professionals are well versed in the main stream approach and are not necessarily supportive of alternative approaches. It is a good idea to get measured objectively for a baseline starting point as you begin the method.
Many optos will measure you for differentials upon request, though not by that name. You will need to ask to be fitted for close work correction, since this is not generally a common practice. It is good preparation to have measured your work space prior to your appointment and then you can include the desired distance in your request. Should you be asked why you desire this second correction you might simply state your concerns that your eyes are being strained by using your distance correction for close work. It might be helpful to reassure your opto that you will only ever use the new correction for close up. Do be sure to decline multifocal lenses, inform them your preference is to alternate between the two corrections. If you find your opto supportive this is a good opportunity to take advantage of that test kit and see if you can shave off some cylinder correction.
Concerning your distance correction; a supportive opto will fit your to 20/25 or even 20/30 as this would still be well within the range of safe and legal even to drive with in most areas. Though you should check your local requirements for visual acuity, so you are informed when you make this request. A less supportive opto generally would, at your request, allow for a just barely 20/20. Anytime you wish to be corrected to less than the maximum correction your eyes will tolerate, you will need to be clear and firm about this desire. It is the default approach for the majority of optos to correct to the maximum (usually 20/15) to ensure your "best vision" under all circumstances. Since this is what most people want, and to prevent customers from returning with concerns about blur in low light, it makes sense that this is their default approach. This is why it is necessary to be clear that you understand you will not always get your "best vision" with the correction you are requesting.
If you are unsuccessful with your opto in getting your correction wrote for any less than the typical maxed out approach, don't fret, you still have your baseline out of it. Decline to fill the "prescription" and take your information with you. Most shops will only release your Pupillary Distance upon request and some shops will not release PD at all, you can measure this yourself, though it is a bit more challenging. Perhaps your next opto will be more supportive.
Where do I go
You can certainly try revisiting your previous opto, you might find they will objectively measure your current vision even while having your records. If you are concerned that they won't, or have already found this to be the case, then a new opto is a good idea. We typically recommend people to see an ophthalmologist as their primary focus is not lens sales.
Rather it is your original opto or a new one it is a fairly common practice for the shop to measure your current lenses. You might avoid some issues with preconceptions if you hand them your differentials or at least a reduction you know is not quite 20/20. Another option is to decline, you can do that as the customer, and ask them to measure you from scratch. Obviously being polite but firm is going to be the best approach.
Getting the best results
Many people want to know what they can do to get the best results from their "official" screening. As you must know by now, vision is highly variable. It is a good idea to not put too much stress on yourself over one snap shot; this is not necessarily a good reflection of your overall vision, you really should just be seeking a baseline. Your own measurements over time will give the most accurate picture of your overall vision, especially if you are making a point to be accurate.
That said, avoiding close up as much as possible the day before and the day of your visit is a good idea. Instead, try getting low stress distance vision time in the place of that close up time while not necessarily doing a lot of focus work and making your eyes tired and/or strained. Drink enough water, get a good night sleep and don't eat a lot of sugar; also good things to help you get better results. If you can manage it, try to schedule in the morning while your eyes are still fresh and well rested.
Points to remember
Remember not to hang too much importance on your screening, a clean bill of health and a baseline is sufficient.
You are not required to purchase correction just because you were measured for it.
You are entitled to your information about your correction, they can not with hold it in order to force a sale.
You do not owe anyone an explanation of your intentions to take your vision into your own hands. It is your right to make inform decisions about your own eyes. Just be sure you know what you are doing when you do so.