Re: [Leica] Re: Distortion

[dannyg1 <dannyg1@IDT.NET>]

Henning,

Thanks for taking the time to write that and futher explain your thoughts. 

While I do believe in you and am sure you are saying with full conviction what you 
believe, I do disagree. Focal length is measured on only the central axis to the film 
plane, because nodal2 to focal point distance is what is being measured. Measuring 
focal length off axis would provide  an 'inaccurate' result, not because of any change 
in the nodal2 to focal plane distance but, solely because the measurement was taken 
from the wrong place. In my view, this again provides a proof that magnification is 
variable across the field and that the variable is directly proportionate to counter the 
effect of diminishing, 3D perspective.

I do, and did, understand that some distortions of rendering are caused by variable 
image magnifications and since this seems to be the central point of your 
conclusions, still do not see critically inarguable evidence that the image is 
unchanged (via edge magnifcation/reduction) by the symmetrical lens. 

It is true that pincushion/barrel distortions have a great effect on apparant 
perspective and your point is well taken that I had not given enough weight to this 
as an obvious difference in rendition between true wides and retrofocus versions. 

I also understand that this difference could very well account for the effect in 
question but it seems to me that a well corrected retrofocus has only slightly more 
verifiable distortion than a symmetrical wide. If telephotos were the object of the 
discussion, there's plenty of these that are distortion corrected in equivalence with 
the best long focus lenses of identical focal length. These should then, image 
identically, but what of the 'telephoto effect'?

I suppose it's time to look for the answers in earnest and I'm going to take some time 
to find the information I need. If you have any interest in what I find, I'll be glad to 
present it to you, as well as any other interested lurkers. 

Thanks again and regards,
Danny Gonzalez