Does “the same” lens, with different autofocus systems, have the same optical quality?

Question

_{I'll give examples with Nikon only, but the question is pretty general.}

So, is this a fact or not?

Examples:

• Nikkor 50/1.8G AF-S, Nikkor 50/1.8 AF-D and Nikkor 50/1.8 AF
• 70-300 AF and 70-300 AF-S
• Nikkor 80-200 AF and Nikkor 80-200 AF-D
• etc.

So, may I rely on the fact, that these lens will have the same optical quality (will produce the same image quality), or there's no such thing and this is completely wrong?

_{NOTE: I know the differences between AF/AF-S/AF-D, I know what G is and what VR is, please ignore these.}

Answer 1

Simply, those are not the same lenses, so no, they do not perform the same. Dig for optical formulas and you'll find that they are often changed between lenses. Example: the 50 1.8G is 7 elements in 6 groups. 50 1.8 AF-D is 6 elements in 5 groups. That's not to say some formulas won't change, but more often than not they will be different.

I think the next natural question is "why are they different?" improving the current design, new processes, new coatings, faster AF, etc.

Answer 2

Autofocus system as such has nothing to do with image quality, but at the same nothing prevents manufacturer from making changes to lens design (for various reasons) So, no, you should not rely on AF and AF-S lenses to be the same optically.