Purpose Far objects appear blurred when viewed through a screen mesh (Mandelbaum effect) presumably because accommodation is biased towards the near screen. Owens reported this bias is maximum when the screen is near the dark focus point. We studied dynamic accommodation while a far target is viewed and an intervening screen is added.

Methods The Mandelbaum effect was demonstrated to five untrained emmetropic subjects (age = 22 ± 2). Then, eye focus was continuously measured (40Hz) with an infrared optometer as subjects monocularly viewed a 20/70 Snellen E on a eye chart located 26 feet away. Each 30-second trial had four consecutive viewing conditions: (1) look at E directly (baseline), (2) look at E through screen, (3) look at screen, and (4) look at E through screen again. In separate trials, the screen was at the distance of maximum blur (124 cm ± 13), or 40 cm. Dark focus was also measured.

Results (1) Eye focus did not change from baseline with screen addition. (2) Eye focus was near veridical when attention was directed to the screen. (3) Eye focus returned to baseline when attention was redirected to the far E. All subjects reported blur of the far E while viewing through the screen. The screen distance of maximum blur was not predicted by dark focus.

Conclusion This objective experiment, along with some subjective observations, indicate the Mandelbaum effect is not likely due to involuntary mis-accommodation.

Owens, D.A. (1979). J. Opt. Soc. Am., 69 , 646-652