to measure a defocus curve
It is important to take into account that changing optotype size is not the same that correcting vergence by proximal distance. This is a concept that generates confusion as we have seen in many of the courses we have conducted about Defocus Curves. The question we are asked is:
If my optotype is already calibrated for my cabinet distance, why do I have to put in a lens when doing the Defocus Curve?
Considering that the far point of the emmetropic patient is at infinity when we approach the test at 6 m, we are inducing a myopia of -0.1667 D. In other words, a patient refracted at 6 m and from whose refraction we obtain 0 D, may not really be emmetropic but myopic of -0.1667 D (1/-6). This is generally not clinically relevant because we measure the refraction in steps of 0.25 D or even the depth of focus might compensate for this small error.
What happens if I do the refraction and the defocus curve at 6 meters?
If we refract the patient at 6 meters and make the curve at 6 meters with that refraction and without correcting the (-0.17 D) with (+0.17 D) in theory our desired defocus curve in steps of 0.5 D from +1.00 D to -4.00 D would not describe the following dioptric values or distances:
It should be considered that the proximal distance generates a myopia of -0.17 D and that the exact defocuses we are measuring or distances would be:
What is our recommendation?
Why make approximations when we can optimize our clinical procedure? Our recommendation is to refract at 4 m (for cabinets >= 4 m) or 2 m (for cabinets < 4 m). Emphasizing the following considerations: