Thank you for your comments. The ELP in a thick lens model is defined as the distance between the cornea’s principal image plane and the IOL’s principal object plane. As the full details of the Barrett formula are not publicly available, I cannot make salient comments on the definition and concepts used for this formula and the definition of relevant segments. Regarding the IOL mater 700, I suggest you use the total corneal power for the IOL power calculation if you want to include the measured posterior surface of the cornea in your calculations. Else, you must convert the anterior and posterior astigmatism in vectors or orthogonal functions to perform the computation of resultant astigmatism. This is done automatically in some biometers using ray tracing or a similar approach.

First , I want to thank you for your website ,which solve my question that bother me for years. I am an ophthalmologist working in Universal eye center , Taiwan.

We found that there is limitation in Barrett Universal II since the ELP is an estimated value.
Further more,the Barrett Universal II use Lens Factor to predict the ILP.
However he define the Lens Factor = distance between iris plane and secondary principle plane of IOL. Dose that mean Barrett define the ELP between S1 and Hi’?
According to your website, The ELP t is the distance between principle plane of image of cornea and principle plane of object of IOL(1st principle plane of IOL) .That confused me.

On the other hand,I have problem in entering the posterior cornea R. In our clinic ,we have IOL master 700 and the posterior R can be gained .However the anterior and posterior curvature of cornea are offered in different meridian.Since astigmatism are vectors,I can not just simply add them.How do you solve this problem ? Have you ask Zeiss to offer the cornea posterior radius in different meridians ? If I can get the value from all the meridians ,which meridian do you suggest to choose? (ex:the flat meridian) to calculate the IOL.

Sincerely yours Yuanche ,Tai