Determination of Image Size and the Range of Clear and Singl
The purpose of this study was to determine whether changing fixation target size in 3D viewing significantly affects the vergence ranges and, if so, then to determine whether the target size effect is driven by fusional vergence gain changes, threshold of blur changes, or both.

21 visually normal adults aged 18 to 28 years viewed 3D images at 40 cm in an electronic stereoscopic. The fixation target, a Maltese cross, moved in depth at 2/s by way of changing crossed or uncrossed disparity until blur and diplopia ensued. Researchers used four target sizes: (1) small (width × height, 0.21° × 0.63°), (2) medium (1.43° × 4.3°), (3) large (3.6° × 10.8°), and (4) 3D (size changing congruently with disparity). The effect of target size on responses was tested by mixed ANOVAs.

--Mean convergence blurs and breaks increased with target size by 40% and 71%, respectively, and in divergence by 33% and 30%, respectively.

--The increases in break magnitude with target size implicate fusional vergence gain change in the size effect.

--Increasing target size raised the threshold of blur from 1.06 to 1.82 D in convergence and from 0.97 to 1.48 D in divergence.

Finally, in 3D seeing, increasing the size of the fixation target raises the fusional vergence gain and blur thresholds, which extends the limits of clear and single binocular vision. As a result, the clarity of a 3D image is determined by both its disparity and the scale of the observed image.