First of all, Dagors and Artars are very different lenses of very different construction. Experience with one tells you exactly nothing about the other.
Most symmetrical lenses exhibit minimal focus shift. When used converted, the spherical aberration will increase the "focus shift". But even the Symmar isn't too bad! In nearly every case the "focus shift" is completely covered by the increase in DoF when stopping down. Only when planning extreme enlargements and super-critical sharpness should it be necessary to even think of it - and in those cases, you shouldn't be using half a lens in any case...
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