I was asked a question in an interview .”If I have n number of lenses in our optical system and our system is showing spherical aberration due to the last surface then how can I remove spherical aberration in my system“
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Is the interview over with? 
Is the interview over with?
Yes Sir, It’s over now. 
Now I want to be good in Optical system designing.
I think that’s a very worthy ambition. I have found it to be a never ending quest, and a lot of fun.
Since the question states that the spherical aberration is generated by the last surface, let’s stick to that. Spherical aberration is caused by the rays at the edge of the lens being brought to focus at a shorter distance than those near the center. This is a property of the spherical lens profile. One solution is to make the lens an asphere. This works very well. However, aspherical surfaces can be more difficult to manufacture that spherical surfaces, and so cost more. (For molded optics, that is not necessarily true.) Another possible solution is to split the last element into two or more elements. This can also reduce spherical aberration.
It’s worth mention that these changes can affect other aberrations, and reoptimizing the entire lens is a good idea.
Welcome to the Zemax community,
I think that’s a very worthy ambition. I have found it to be a never ending quest, and a lot of fun.
Since the question states that the spherical aberration is generated by the last surface, let’s stick to that. Spherical aberration is caused by the rays at the edge of the lens being brought to focus at a shorter distance than those near the center. This is a property of the spherical lens profile. One solution is to make the lens an asphere. This works very well. However, aspherical surfaces can be more difficult to manufacture that spherical surfaces, and so cost more. (For molded optics, that is not necessarily true.) Another possible solution is to split the last element into two or more elements. This can also reduce spherical aberration.
It’s worth mention that these changes can affect other aberrations, and reoptimizing the entire lens is a good idea.
Thank you sir for such a nice explanation 
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