This study has the following limitations. First, although
p value, which is the magnification factor of the imaging system, was a constant for the OCT device, it might vary among the different OCT devices. However, the
p value of 3.382 was calculated with Stratus OCT or Cirrus HD OCT, and was quoted in this study based on previous reports,
4–7,35 because the
p value of 3D-OCT 2000 used in current study is also unknown to researchers. Therefore, the cpRNFL thickness corrected with Kang's method might be slightly different from that corrected with Littmann's method. However, it would have had an insignificant effect on cpRNFL thickness because the differences in
p of the magnification factor of the imaging system would be quite negligible. Although the telecentric design with 3D-OCT 2000 has not been made public, we can assume it is because the default axial length, corneal radius, and refraction are determined as 24.39 mm, 7.7 mm, and 0 diopter, respectively, based on the Gullstrand schematic eye in 3D-OCT 2000. Littmann's method corrected for the ocular magnification by using Equation 1:
t = p * q * s. In 3D OCT-2000, the magnification factor in an eye with an axial length of 24.38 mm is 1 (i.e.,
t = s). Substituting
t = s into the formula, we arrive at
p = 1/q and
p can be calculated as 1/(0.01306 [24.38 − 1.82]) = 3.394. Additionally, because the corneal radius is considered in 3D-OCT 2000, both
p values would be slightly different. However, a previous study reported that the effect of the corneal radius on magnification factor is very small.
6 On the other hand, with Stratus OCT, the default axial length is 24.46 and refraction is 0 diopter.
35 The Cirrus HD OCT system has the same magnification factor as that of the Stratus OCT system.
7 Therefore,
p can be calculated as 1/(0.01306 [24.48 − 1.82]) = 3.382 in Cirrus HD OCT. It can be speculated that the value of
p obtained from Cirrus HD OCT and 3D OCT-2000 are nearly equal. Second, this study included only young healthy participants, and further studies in patients with glaucoma may be necessary.