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:

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.

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:

• Defocus Lenses (D)

[1.00    0.50         0   -0.50   -1.00   -1.50   -2.00   -2.50   -3.00   -3.50   -4.00]

• DiCorresponding distances (m)

[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: 

• Defocus Lenses (D)

[0,83   0,33     -0,17   -0,67   -1,17   -1,67   -2,17   -2,67   -3,17   -3,67   -4,17]

• Corresponding distances (m)

[1,20   3,00     -6        -1,50   -0,86   -0,60    -0,46   -0,38   -0,32   -0,27   -0,24]

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:

• The refraction obtained will not be subjective at infinity but at 4 m or 2 m.
• To calculate the refraction with spectacles of the patient (Rx at infinity) we have to add -0.50 D of Sphere (if we have refracted at 2 m) or -0.25 D (if we have refracted at 4 m).
• If we start from the refraction at infinity we will have to use the vergence correction lens of +0.25 D (MLA at 4 m) or +0.50 D (MLA at 2 m).
• The MLA automatically integrates the values of these lenses with the Rx and the defocus lens to avoid lens build-up. You can learn more about this process in our article Preparing the lenses to measure a Defocus Curve
• If you use the MLA you can rely on the VisionC test during the refraction process. This App calibrates the ETDRS optotype chart for any desired distance from 4 m to 20 cm in order to perform the refraction at 2 m or 4 m or to measure the high and low contrast Visual Acuity at multiple distances. 

• No hay que confundir el calibrado de tamaño del optotipo (ajusta su tamaño para la distancia a la que se presenta con el fin de que la notación de agudeza visual sea la correcta) con la compensación de la vergencia proximal.
• Al hacer una curva de desenfoque a 6 m, sin corrección de vergencia por distancia proximal de -0.17 D con una lente de +0.17 D, los pasos de desenfoque medidos en teoría no se corresponden con los que frecuentemente se utilizan en las gráficas de curvas de desenfoque.
• Al realizar una curva de desenfoque se debe refraccionar a 2m o 4m, la refracción en infinito se calculará añadiendo (-0.50 D) o (-0.25 D), respectivamente. Si partimos de la refracción en infinito y ponemos el MLA a cualquiera de estas dos distancias debemos añadir un (+0.50 D) o (+0.25 D) a la refracción en infinito para corregir la vergencia por distancia proximal.