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A question about "off-axis" lens performance
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PostPosted: Fri Apr 15, 2011 5:24 am    Post subject: A question about "off-axis" lens performance Reply with quote
I'm not sure I understand what "off-axis" lens performance means.
I'm guessing it means image quality degrades towards the corners?

If that is true, then wouldn't that also mean that a lens designed for 35mm film that suffered in the "off-axis" department would perform BETTER if used on a APS-C sensor size (or a 4/3 or micro 4/3)? Because essentially you are cropping out the part of the frame where the image degrades the most.

If I'm not understanding off-axis properly, then the above paragraph is probably all garbage.

Was thinking about it when I read that the Tamron 03A was superceded by the Tamron 103A with better off-axis performance. I was inventorying my lenses and found a Tamron 03A that I have never tried to use yet. I think I'll have to put it on my Pentax K-x and have a go.


PostPosted: Fri Apr 15, 2011 10:06 am    Post subject: Re: A question about "off-axis" lens performance Reply with quote
cheekygeek wrote:

If that is true, then wouldn't that also mean that a lens designed for 35mm film that suffered in the "off-axis" department would perform BETTER if used on a APS-C sensor size (or a 4/3 or micro 4/3)? Because essentially you are cropping out the part of the frame where the image degrades the most.


Yes and no.
Yes you are using the "sweet" spot.
But no, its not better. As every lens has "resolution", its measured in lines per millimeter. So if you put a lens that has lets say 50 lpmm on a 10 mpx fullframe and then on a APS-C 10 mpx camera, then you can imagine what happens. The lens will not be able to "resolve" that amount of "lines" as the pixel on a 10mpx APS-C is smaller than on a 10mpx full frame.
You will use the mid part of the lens, but the resolution on that mid part will stay the same and as the pixel amount stays the same, then you will need more lpmm on a lens to resolve the smaller pixels.

Its pretty hard to explain it.
But try to play it through your head.


PostPosted: Fri Apr 15, 2011 11:07 am    Post subject: Reply with quote
With a µFT camera you are cropping even more, so you don't see the abberations outside the centre, but the image is 2x magnified so you basically need two times as high a resolution to show the same sharpness as on a FF camera (presuming you have the same pixel density on both). Spherical and chromatic abberations that are present in the centre are also magnified, which is usually not mentioned by people talking about the "sweet spot".
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