their mean age was 21.20 years ( standard deviation [ sd ] = 1.46 ; range 1824 years of age ) .
the study followed the tenets of the declaration of helsinki and informed consent was obtained from all subjects after an explanation of the nature and possible consequences of the study .
all subjects had best - corrected visual acuity of at least 6/6 in each eye at both 6 m and 40 cm .
subjects were excluded from the study if they demonstrated any of the following : ( i ) strabismus at 6 m or 40 cm ( as assessed by the cover test ) ; ( ii ) pu accommodative amplitude below the normal range as quantified by hofstetter 's equations ; ( iii ) lag of accommodation ( assessed using monocular estimate method retinoscopy at 40 cm ) outside the range of + 0.25 to + 0.75 d ; ( iv ) monocular accommodative facility < 10 cycles / min using 2.00 d flippers ; ( v ) mean spherical equivalent refractive error outside 3.00 d or astigmatism > 1.75 d ; ( vi ) any history of ocular trauma , ocular disease , refractive surgery , or aphakia .
the order of testing , i.e. whether the 33 or 40 cm distance was tested first , was alternated across subjects .
all testing was performed on the subject 's dominant eye ( as determined by the hole in the card test ) while the nondominant eye was occluded . for testing at 40 cm , subjects viewed the middle letter within a row of  20/30  letters ( corresponding to a visual acuity of 40/60 or a decimal equivalent of 0.67 ) . using lenses mounted in a trial frame , ml power was added in 0.25 d steps on top of the subjective refractive correction until the letter first became  slightly blurry  and could not be cleared by the subject .
subjects were allowed up to 510 s for each lens presentation to clear the letters . at the first noticeable blur point ,
the subjects were asked to try with maximal accommodation expended and clear the print to sure reaching to the end point .
the near acuity chart was positioned on a reading stand perpendicular to the line of sight of the subject with a luminance of approximately 50 cd / m .
the same procedure was used for testing at 33 cm , except a row of  20/25  letters was used ( corresponding to a visual acuity of 33/50 or a decimal equivalent of 0.67 ) .
thus , the target selected subtended an angle of 7.5 min of arc at both 33 and 40 cm , respectively .
the aa was calculated as the amount of ml power added before the subject reported the first slight sustained blur , plus the dioptric value of the test distance ( 2.50 d or 3.00 d ) . to determine the repeatability of the procedure
, these measurements were repeated for both target distances after a time interval of at least 24 h but not more than 48 h after the first measurement . for pu amplitudes ,
the same  20/30  row of letters was placed at a distance of about 75 cm and was moved slowly toward subject .
subjects were requested to keep the target as clear as possible to get to the point of first sustained blur .
the distance between target and the spectacle plane was measured with a millimeter ruler and converted to diopters .
altman method was used to assess the agreement between the viewing distances taking into account of the replicates .
the limits of agreement were determined as the mean difference 1.96 times the sd of the differences ( sd ) , calculated by means of a mixed model with the interactions between subjects and distance and between subjects and replicates as random factors , owing to the fact that the replicates are nonexchangeable within each method ; in the above mixed model , have been included subject and distance as factor with distance as a fixed factor .
furthermore , the 95% confidence intervals of the upper and lower limits of agreement were calculated as the limits of agreement  1.96  the standard error . for this purpose
, the standard error of these limits was calculated from the formula 3sd / n , where n is the sample size and sd is the standard deviation of the differences .
the presence of a proportional error has been assessed by testing the pearson 's correlation coefficient between the differences on the first replicates of the two distances and their mean ; in addition , the same analysis has been carried out on the last replicates and on the difference between the means of the replicates and their mean .
intraclass correlation coefficients together with their 95% confidence intervals ( cis ) have been calculated for the two measurements ( first and second ) at each distance to evaluate the repeatability for each method that , in addition , has been quantified by the usual formula of 2 2sd where sd is the sum of the variance of the replicates at each distance and of the interaction subject by replicates .
the mean spherical equivalent refractive error for the dominant eye was 1.56 d ( sd = 1.12 ; range = plano to 3.00 ) . the mean ( sd ) for the pu technique was 10.70 d ( 1.01 ) in all subjects and separately in females and males were 10.80 d ( 1.04 ) and 10.56 d ( 0.96 ) , respectively ( p = 0.21 ) .
mean values of aa measured using the ml technique , the limits of agreement , and the confidence limits at the two working distances are presented in table 1 .
mean values of the accommodative amplitude using minus lens method ( d ) , the limits of agreement and confidence limits at the two working distances tested ( n=240 ) the agreement band is 2.72 ( from 2.72 to  0.19 ) that is about the 29% of the mean of the two replicates at the two considered distances and consequently , it has to be considered too wide for claiming between the agreement of the measurement at 33 and 40 cm .
in addition , the presence of a proportional measurement error turned out to be not statistically significant ( p = 0.5093 first measurement , p = 0.2994 , second measurement , and p = 0.9735 , mean of the measurements ) .
the aa measured with the target at 33 cm was significantly ( p < 0.001 ) higher than that measured with the target at 40 cm , by on average 1.17 d [ fig . 1 ] .
altman plot showing the difference between the measurements of amplitude of accommodation at the two test distances as a function of the mean finding ( n = 240 , two measurements for each subjects ) .
the mean difference is shown by the solid horizontal line , while the 95% limits of agreement are indicated by the dashed horizontal lines the mean and sd of accommodative amplitude measured by the pu method were 10.70  1.01 d ( 95% ci : 10.5110.88 ) .
the mean difference of the pu amplitude with the ml amplitude measured at 33 cm and 40 cm was 0.67  1.55 d ( 95% ci : 0.390.95 ) and 1.84  1.51 d ( 95% ci : 1.502.11 ) , respectively . table 2 shows mean , sd , mean difference , and the repeatability of the repeated measurements of accommodative amplitudes at the two distances .
repeatability between the repeated measurements of the amplitude of accommodation with the minus lens method at 40 and 33 cm ( n=120 ) the mean differences between the two trials were different for the two viewing distances ( 0.24 d for distance 33 cm and 0.16 d for 40 cm ) and the coefficient of repeatability ( cor ) was higher for the 33 cm ( 1.24 d ) when compared with the 40 cm test distance ( 0.99 d ) .
a significant correlation was observed between the first and second readings at both distances 33 cm ( r = 0.77 ; p < 0.001 ) and 40 cm ( r = 0.86 ; p < 0.001 ) as expected .
the results of the present study demonstrated a significantly higher mean ml aa when tested at 33 cm , compared with the value obtained at 40 cm ( mean difference = 1.17 d ) .
it seems likely that a large proportion of this difference is due to a greater proximally induced accommodative response at the closer viewing distance ( note : the term proximally induced accommodation is chosen since this is likely to include both proximal accommodation and proximal vergence driving convergence accommodation ) . in addition , the repeatability of the measurements was superior at the farther distance , with a cor of  1.24 d and  0.99 d being recorded at 33 and 40 cm , respectively .
the mean aa results found in this study were similar to previous studies using the ml procedure at 40 cm and at 33 cm , while the cors determined in this study at either distance were generally smaller compared to previous reports .
if the goal of the aa procedure is to achieve maximum accommodation , then using a closer viewing distance is preferable . given that greater proximally induced accommodation will become manifest at even shorter distances , one might speculate that even closer viewing distances are preferable .
when working at more proximal distances , the target will subtend a larger angle at the eye , and given the reduced blur sensitivity with larger targets , an increased aa may simply reflect the failure of the subject to perceive target blur resulting in a delayed endpoint rather than increased accommodation . an additional factor to consider
it is well established that the pupil diameter decreases concurrent with an increase in accommodation .
however , phillips et al . showed minimal change in pupil size when accommodation was stimulated with blur - driven accommodation alone .
they suggested that cues such as target size and/or proximity may be a more significant stimulus .
accordingly , one might speculate that testing at a closer viewing distance may result in a smaller pupil diameter ( and therefore larger depth of focus ) when compared with a longer distance .
such an increase in depth of focus would delay the perception of blur , thereby resulting in a higher measurement of aa .
it should also be noted that all subjective methods for assessing the aa overestimate the maximum accommodation as a result of the depth of focus of the eye , when compared with objective techniques that measure the actual change in response , such as the use of an autorefractor or dynamic retinoscopy . in confirming this results , anderson and stuebing investigated the differences between objective measures of aa as compared to subjective aa measures ( pu or mls ) and mentioned that the objective measured amplitude is substantially less than the subjective pu technique .
wold et al . suggested that while subjective measurements of the aa are not adequate for assessing the change in the optical power of the eye , they may provide a useful assessment of near reading ability .
for example , a common clinical procedure to determine a near addition lens is to calculate the difference between the required stimulus ( in diopters ) and 50% of the subjective amplitude .
it is also worth considering why 40 cm ( 16 inches ) has been widely chosen as the standard near test distance and whether this is still appropriate given modern near - vision demands . while this value is often quoted as a typical near viewing distance for adults , wittenberg and grolman examined the accommodative demand for a range of 23 occupations and reported mean values ranging from 1.50 to 4.63 d , with an overall range between 1.38 and 6.50 d ( equivalent to viewing distances between 15 and 72 cm ) .
further , an examination of children between 6.4 and 10.75 years of age found mean habitual reading and writing distances of 27.2 and 27.7 cm , respectively .
in addition , the use of modern technology such as smartphones has also changed viewing conditions .
a recent study showed that the mean viewing distance when reading an internet web page on a smartphone was 32.2 cm , with a range from 19.0 to 60.0 cm .
accordingly , the use of 40 cm as a near test distance for all subjects may no longer be appropriate that is in contrast with the current study which indicate better repeatability of the measurements at 40 cm .
these results indicate that although testing the ml aa at 33 cm produces a higher response , greater test repeatability was observed with testing at 40 cm . however , it is unclear whether the larger amplitude reflects a larger increase in accommodation or a decrease in the ability to perceive the first slight sustained blur .
further studies to evaluate the optimal distance for performing this standard clinical procedure would be valuable .
hence , it will be a good idea to include various distances , and this way to evaluate the effect of different parameters , such as blur sensitivity and proximal accommodation , to determine that which factor ( s ) has ( have ) the most effect on the measurement of the aa with this method .
 purpose : the purpose of this study was to compare the mean findings and the repeatability of the minus lens ( ml ) amplitude of accommodation ( aa ) at 33 cm and 40 cm.materials and methods : aa was measured from the dominant eye of 120 fully corrected subjects using the ml procedure when viewing the target at both 33 and 40 cm . each measurement was repeated between 24 and 48 hours after the first trial.results:mean aa when tested at 33 cm and 40 cm was 10.20 diopter ( d ) ( standard deviation [ sd ] = 1.24 ) and 8.85 d ( sd = 1.23 ) , respectively ( p < 0.001 ) . 
 the limits of agreement of the measured amplitude calculated with taking into account of the replicates at 33 and 40 cm were  0.19 ( 95% confidence interval [ ci ] : 0.34 to 0.04 ) and 2.53 ( 95% ci : 2.38 to 2.68 ) , respectively . 
 the repeatability of testing at the two distances 33 and 40 cm was  1.24 and  0.99 , respectively . 
 in addition , the retest reliability of measured amplitude using the intraclass correlation coefficient was 0.87 ( 95% ci : 0.7890.920 ) at 33 cm and 0.91 ( 95% ci : 0.8720.945 ) at 40 cm.conclusion:there is no agreement in the obtained amplitude at the two measurement distances . testing the ml aa at 40 cm may be superior given that a lower repeatability coefficient was observed . 
 however , it is unclear whether the larger amplitude measured at 33 cm reflects a larger increase in accommodation ( greater proximity effect ) or a decrease in the ability to perceive the first slight sustained blur . 
Materials And Methods
Results
Discussion
Conclusion
Financial support and sponsorship
Conflicts of interest