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Author Topic: raw adjustments - loss of quality?  (Read 3223 times)

spphoto

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raw adjustments - loss of quality?
« on: December 08, 2005, 01:30:53 pm »

hi,

If i would need to compensate exposure a lot ( 2 or 3 stops) in a raw format, would i lose any quality?

thanx,

sp
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DarkPenguin

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raw adjustments - loss of quality?
« Reply #1 on: December 08, 2005, 02:08:30 pm »

Compared to what?  A properly exposed image?  If so, then yes.
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Tim Gray

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raw adjustments - loss of quality?
« Reply #2 on: December 08, 2005, 03:02:30 pm »

well, a lot less loss in quality than if you did that kind of adjustment on a jpg or 8 bit tif.
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Das Bosun

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raw adjustments - loss of quality?
« Reply #3 on: December 08, 2005, 08:03:18 pm »

Quote
If i would need to compensate exposure a lot ( 2 or 3 stops) in a raw format, would i lose any quality?

thanx,

sp
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In a raw converter images taken beyond about a plus 1.8 EV exposure increase start to look softer (even if they were captured in focus).  The image saturation also starts to fade a little beyond plus 1.8 EV.

This is because with increases around 2 stops you're essentially making dark shadows (Zone I) into shadows with detail (Zone III) - shadow detail (Zone III) becomes mid tones (Zone V) and mid tones (Zone V) become highlights with detail (Zone VII).  If you then accord this with the break down of how the 12bit data in a raw file is distributed, the softening effect on an image starts to make sense.

See Michael's Expose Right article for a break down of tonal information (under the heading Home on The Dynamic Range):

[a href=\"http://luminous-landscape.com/tutorials/expose-right.shtml]http://luminous-landscape.com/tutorials/expose-right.shtml[/url]

I've also noticed that substantial white balance OR tint value shifts can make individual channel noise more visible.  A good example of this is tungsten/incandescent shooting with the camera set to auto white balance.  

It seems to me no camera manufacturer has got auto white balance in tungsten/incandescent light correct yet.  Auto white balancing relies on a mild shift of its Kelvin values (the sweet spot seems to be between 4000K and about 7000K), but inturn the camera's auto white balance seems to love compensating for the lack of kelvin value movement with drastic shifts in the white balance tint values.  

If auto white balancing, under tungsten/incandescent lighting, delivers an incorrect result it's necessary to make fairly substantial shifts, to the post production white balance, in a raw converter if the image is to be neutralized.  

Fairly drastic white balance and exposure shifts made in a raw converter therefore also reveal more individual channel noise.
 
My experience suggests 'correct exposure' and preset white balances (OR custom) are the simplest solution to highest quality raw shooting results.  Conservatively, one has about 0.5EV of lost highlight detail that can be saved and 1.5EV of shadow detail that can be lifted out of the mud.

Das Bosun
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