A frequent question asked is why the Multifocal Lens Analyzer is designed to measure Defocus Curves at 2m or 4m and not at 6m which is considered the standard distance for measuring refraction. To explain the reason it is important to have the following concepts:
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:
[1.00 0.50 0 -0.50 -1.00 -1.50 -2.00 -2.50 -3.00 -3.50 -4.00]
[Inf Inf Inf -2 -1 -0,67 -0,50 -0,40 -0,33 -0,29 -0,25]
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:
[0,83 0,33 -0,17 -0,67 -1,17 -1,67 -2,17 -2,67 -3,17 -3,67 -4,17]
[Inf Inf -6 -1,50 -0,86 -0,60 -0,46 -0,38 -0,32 -0,27 -0,24]
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: