Trial lens kit
A trial lens kit is a set of lenses of various powers and usually a frame shaped like a pair of glasses but with mounting points for the lenses. The number of lenses and powers will vary by what kit you buy. If you are a very high myope, try to find a kit that includes powers close to your Full Prescription.
Legal: Check your local laws before performing any trial lens tests on someone other than yourself. This may require a licence in your jurisdiction.
Medical: If you have any vision problems not compensated for by refraction, go see your optometrist.
Minimalist Trial Lens Kit
A small kit that will allow you to test small changes to your existing prescription would be:
Trial frames come in many forms, they may have lots of adjustments or come in fixed PD widths. Using a frame with the correct PD is important so the wearer is looking through the optical centers of the lenses. Adjust the frame for the best comfort and so the wearer is looking through the optical center of the lens. A trial frame that holds four lenses and has axis adjustment is best.
Placing lenses in the trial frame
Always place the strongest lens in the stack closest to the eye to reduce the effects of vertex distance.
Flip lenses are designed to be hand held in front of the trial lens or customary lenses and test plus vs minus at a set power. Can be used to determine if the customary lenses need tweaking up or down. They may be monocular or binocular They are also used in vision training.
How to test myopia with a trial lens kit
You will use the sphere lenses for this test. This is a very extensive topic, here are some lectures for optometrist students:
A shorter overview of the process:
How to test Astigmatism
Testing for astigmatism is tricky, and you should master testing for myopia first. Errors such as being off by 90 degrees are common.
The JCC Lens has two cylinders at opposite angles and powers. By holding the lens in different ways over a trial cylinder lens, you can determine if the angle or power of the cylinder lens should be changed. (This is the part of the refraction test where the doctor is happy if both options are equally blurry.)
A stenopeaic slit is just an opaque disk with a slit in it. By putting the slit at different angles and taking standard myopic refraction, one can calculate the angle and power cylinder needed.
JCC vs Stenopeaic
The use of a Stenopeaic slit vs a JCC lens seems to be cultural. Most US doctors will use JCC lens, while middle eastern doctors prefer the stenopeaic slit. Both tests work and it's more about how your doctor was trained which he/she will use. For DIY types, the stenopeaic slit is easy to make at home and is compatible with the simpler cm measurement when a full trial lens kit is not available.
The pinhole accessory in many trial lens sets may help distinguish between refractive issues and medical issues. If the vision does not improve or gets worse with use of the pinhole lens, this is an issue that should be addressed by an optometrist.