Why do lenses affect AF?

[beno_t_marchal]

Hi

 

I have a question about backfocusing. I noticed a small amount of backfocusing with one of my lens but,

as far as I can tell, it only happens with that lens (for the record, although it matters, this is the 18-70 kit

from the D70 mounted on a D200). I did not noticed a similar problem when mounted on the D70.

 

I searched the photo.net and other sites and I found several references to similar occurences: AF problem

may affect one lens only. I even found that some lenses have a history of causing AF front/backfocusing.

 

My question: how come? I have always assumed that if the AF works properly and is well calibrated, it

should work equally well with every lens.

 

Thanks

 

--ben

[tonybeach]

A visual aid is here http://www.nikon.co.jp/main/eng/portfolio/about/technology/nikon_technology/caf/index.htm and if you imagine the light exiting the lens at an angle you can see how that would change the light arriving at the separator lenses and how that in turn would change how the light exiting those lenses would arrive improperly aligned at the AF sensor which in turn determines where to place the lens to optimize the focus on the image surface. This is why it is important that the lens elements are properly aligned to each other so that the image circle is correctly centered.

[beno_t_marchal]

Anthony

 

Fascinating, thanks for this information.

 

--ben