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CS2 lens correction affects sharpness - why ?

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  #11  
Old 07-30-2006, 03:05 AM
Nacoya Nacoya is offline
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Posts: 89
hi again,

i made a small (well 8.5meg) psd with the original and 3 rotations for ref.

measure method, lens correction and rotation prior to photoshop.

if you wish to dl it is ~ here ~

having done a quick google it seems i have been fumbling around oblivious to this for years . It seems that it's the interpolation used for small arbitrary rotations that cause the apparent loss of sharpness.

added : ~ here ~ is some good bumf
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  #12  
Old 07-30-2006, 03:48 AM
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Photo678 Photo678 is offline
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Quote:
Originally Posted by Nacoya
It seems that it's the interpolation used for small arbitrary rotations that cause the apparent loss of sharpness.
this should have been obvious......photoshop only does the mathematics ,,,,,,,,,,garbage in, garbage out
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  #13  
Old 07-30-2006, 09:21 AM
Nacoya Nacoya is offline
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Quote:
Originally Posted by Photo678
garbage in, garbage out
implying what exactly.

Quote:
Originally Posted by Photo678
this should have been obvious......photoshop only does the mathematics ,,,,,,,,,,
and the mathematics are bound by the fact that sharpness is a factor of edge contrast and the limitation to that is a pixel is 1x1 and on rotation of less than 90deg or indeed inbetween the 4 poles, the original registered pixel may have to overlap it's neighbours = no defined edge as per original = no contrast = no sharpness.

but thanks for your reply.
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  #14  
Old 07-31-2006, 06:18 AM
Dm_Cher Dm_Cher is offline
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Sharpening lost

Quote:
Originally Posted by Nacoya
Hi,

Just noticed something i had never picked up on before.

I have an image i took last week

~ here ~

i then used lens correction in photoshop. Straight line to level the image in line with the boat. The result as can be seen is a complete loss of sharpness.

~ straightened ~

nothing else done to the image.

can anyone explain why ?

thanks,

dave

"Lens correction" is a geometric distortion of your image. So, when you shot using "bad" or specialized lens (for example,"fish-eye lens"), straight line will looks as curve line on shot. "Lens correction" deforms image back in order to straight line on scene will straight line on image. When transformation is executed where is sub-pixel precision calculations and interpolation is needed.
Any interpolation is a low-frequency filter (like lens MTF defocus) and any sharp details are lost.
You can take sharp image and rotate it several times, and your image will smooth without sharp details.
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