[dms]

A. If the sensor resolution is so much greater than the lens, then the lens resolution is the determinig factor, and magnifying the lens image does nothing, and when the respective resolutions are not very dissimilar they both contribute--as the square root of the sum of the squares of the lens and film resolution.

I did a really quick search and found (as I expected) the following in [http://www.dpreview.com/forums/post/24792592] **

1/Rm^2 = 1/RL^2 + 1/Rs^2 , where Rm is the combined resolution, and RL and Rs are lens and sensor resolutions.

Thus the two resolutions both contribute, and only when the lens resolution is really low, does the TC do no good. If the RL is 1/200 (1/lpmm) and the sensor is 1/2000, the lens is the only important resolution, and using a 2x TC reduces the resolution by 2, and thus there is no benefit using the TC. But in reality the sensor and lens resolution are likely not so different.

B. Another mitigating effect is with TC you are magnifying the center of the lens which likely has better resolution.
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** The address I give above seems to not work? (Thus) If you do a google search of the string "sum squares for lens and sensor resolution" it will be the first hit.

And here is discussion in PentaxForums: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/110766-lens-resolution.html

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Last edited by dms; 03-17-2016 at 07:15 PM.

[jpzk]

A. If the sensor resolution is so much greater than the lens, then the lens resolution is the determinig factor, and magnifying the lens image does nothing, and when the respective resolutions are not very dissimilar they both contribute--as the square root of the sum of the squares of the lens and film resolution.

I did a really quick search and found (as I expected) the following in [http://www.dpreview.com/forums/post/24792592]

1/Rm^2 = 1/RL^2 + 1/Rs^2 , where Rm is the combined resolution, and RL and Rs are lens and sensor resolutions.

Thus the two resolutions both contribute, and only when the lens resolution is really low, does the TC do no good. If the RL is 1/200 (1/lpmm) and the sensor is 1/2000, the lens is the only important resolution, and using a 2x TC reduces the resolution by 2, and thus there is no benefit using the TC. But in reality the sensor and lens resolution are likely not so different.

B. Another mitigating effect is with TC you are magnifying the center of the lens which likely has better resolution.

Way too complicated for my taste !

I think it boils down to the "apparent" result and, most of the time (in my case anyway: K5/K3 + DA*300/4 + HD 1.4X TC)) there is always some sort of IQ degradation when using a TC on APS-C cameras.
As for using that on a FF camera (with the same set up) .... no idea but I would only guess the results would be similar.

[dms]

Yes, it is mathematical, but how else to quantify the loss of IQ with TC.

Granted this is a mathematical model, and does not consider other possible artifacts due to the TC, nevertheless it is likely the technical basis, and counter point to what apparently was stated by Tony Northrup.

And as the FF (presumably) would have a lower sensor density, the lens effect on the combined resolution would be less (compared to APS-C)--which is correct--but likely overstates the case, as shown in my prior post.