instruction: hello doctor please help me with my assignment I am an optometry student my question is what will happen to the foi when correction lenses are placed in the case of mixed astigmatism that has one focal point in the front which is nearsighted and one behind the retina which is farsighted will they merge as one when you give spectacle correction or the focal points will remain at their own places even though there is correction already please reply
output: hello welcome to icliniq com the condition of one focus in front of the retina and another behind arises in mixed astigmatism as you have already mentioned to correct this we prescribe toxic lenses to the patient which as you must already know have spherical and cylindrical components with opposite signs as we are treating mixed astigmatism the process of how a clear image is formed in this case will be illustrated using a strums conoid this image is easy to find online or can be seen in a lot of basic ophthalmology textbooks here the different meridians come to focus on the retina such that they produce a circle of least confusion this makes the image of the object form with any distortion on the retina anywhere in front of or behind the circle of least confusion the image will be defocused I hope this helps