Originally Posted by
arri
You need this correction formular when you use a single meniscus element or with a two elements lens when it were made of two identic glass types, like the Bistigmat.
This optical systems are not colour corrected and this is the reason for the focus shift.
An achromatic system, like the Rodenstock Imagon has enough colour correction to avoid a focus shift.
The single cells of the set lenses donīt need correction because this once are colour corrected as well, like the cells of a Dagor, Protar, Eurynar, etc.
Here are two glass types in use, crown and flint, and with it a colour correction were possible.
What you can get with this lens cells is a focus shift by the closed aperture but you see it when you check the sharpness again when the lens is stoped down.
This focus shift has another reason and has nothing to do with a bad colour correction.
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