As a community eye care specialist, I sometimes examine people who are too unwell to come to my practice. I examine them in their own homes with portable equipment. Some people have never had prescription glasses or have lost them so I have no prescription to use as a starting point. Of course, I can use my retinoscope to get a starting point for subjective refraction but sometimes the light in the room is unsuitable for retinoscopy; often too bright from sunlight and not thick enough curtains. Do you have any advice as to how I can start subjective refraction without having a sense of the previous prescription?
I also carry out some examinations in people’s houses, don’t have information about previous lens powers and the light conditions that prevent retinoscopy.
I set up my trial frame and cover over one eye. I measure distance vision in each eye separately. With the left eye covered, I’ll place a +0.50 DS over the left eye and ask if the letters look the same or clearer. If the answer is yes I’ll increase the lens power by +0.25 DS. I keep increasing by +0.25 DS until the person tells me that the increase makes the letters less clear.
If the first +0.50 DS makes the letter less clear I will then use a -0.50 DS lens and ask if the letters are clearer. If the +0.50 DS made the letters less clear then in my experience a -050 DS lens makes them clearer.
Once I have found the optimum plus or minus spherical power I place a -0.50 DC (cylinder) lens at 90 degrees and ask if this makes the letters clearer. If the response is yes I will use my cross-cyl to refine the power and axis. If the answer is no I will move the -0.50 DC lens to 180 degrees and ask if the letter looks clearer. If the response is yes I will use my cross-cyl to refine the power and axis. If the answer is no I remove the -0.50 DC lens and prescribe a spherical correction.
Of course, the level of visual acuity I achieve in each eye with just spherical lenses will tell me if there is any astigmatism and if so roughly how much; as long as the eye is free from disease.
This way I can quickly determine the optimum lenses without having a conventional starting point.