Your surgically induced astigmatism (SIA) from your standard phaco incision is probably far less predictable than you think. There are so many factors that are involved such as corneal diameter, corneal elasticity, corneal pachymetry, incision architecture, and more. If you keep the phaco incision on the steep axis it will reduce the magnitude of the astigmatism and not affect the axis. If you make the phaco incision on the flat axis, it will increase the magnitude but again not affect the axis. However, if you make your phaco incision off-axis then it will affect both the magnitude and direction of the astigmatism and may change the axis. This video gives helpful pearls to minimize this effect and make the post-op results more predictable in cases where your incision will be off-axis. So much to learn!