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How do I fix that?


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#1 aczyzyk

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Posted 08 January 2006 - 02:02 PM

I had my first try at D70 with 10.5 lens this weekend at a pool.
Despite of warnings I read here I still managed to photograph my own strobe :(

Anyway, it seems to be a really cool lens, but I got this ugly violet cast on window frames in the top left corner of the frame.

How do I get rid of that (the cast not the lens)?

A crop of that photo:

2006_01_Warsztaty___dzien___2___19_2.jpg


Andrzej
Andrzej Czyzyk

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D200, Sea&Sea DX-D200, Nikkor 10.5, Nikkor 105 VR, Tokina 12-24, 2xDS-125

#2 mattdiver

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Posted 08 January 2006 - 09:46 PM

What you see is called chromatic aberration. I feel lazy today, so you'll find an explanation of this phenomenon here :( :
http://www.dpreview....atic aberration

As for how to correct it, it depends at which stage you want to do it.

To minimise this effect, you'd have to reduce contrast. In this case, that means exposing for the windows or area around them, and using a higher strobe power to illuminate your subjects underwater. A better way around it is to use a split ND filter, but this will restrict your composition options, and has its own difficulties to implement. Either options will get rid of the blown highlights too.

If you want to minimise it in post-production and you've taken the shot in RAW format, you can use APS or other converters' built-in functions to reduce or eliminate chromatic aberration. If you shot in JPEG, you can try to convert the picture to Image>Mode>Lab Mode, and then tweak the 'a' curve (Ctrl+M) to eliminate the purple altogether. Note that this last option will also affect the purple on the rest of the picture, so you'd have to select the area affected, e.g. using the Lasso tool.

Cheers,
Mat

#3 aczyzyk

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Posted 09 January 2006 - 05:56 AM

Thanks.

I meant in post processing. I know I could have exposed that better but its too late to fix it now. It's a RAW.

I was not sure if it was cromatic abberation because I imagined abberation was something smaller. This one is really nasty. Anyway, I tried looking for chromatic abberation correction in Nikon Capture last night, but could not find it. Maybe I was too tired.
Andrzej Czyzyk

www.flickr.com/photos/czyzyk/
D200, Sea&Sea DX-D200, Nikkor 10.5, Nikkor 105 VR, Tokina 12-24, 2xDS-125

#4 mattdiver

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Posted 09 January 2006 - 06:59 AM

Not sure about Nikon Capture, tried it once only, and didn't like it...

In Photoshop RAW converter, click on the "lens" tab on the right, and move the top slider. That should fix it...

CA.jpg

Mat

#5 aczyzyk

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Posted 09 January 2006 - 12:13 PM

I tried PS RAW converter and managed to reduce that a bit. I could not get rid of it completely, but it looks better now.
Thanks

Andrzej
Andrzej Czyzyk

www.flickr.com/photos/czyzyk/
D200, Sea&Sea DX-D200, Nikkor 10.5, Nikkor 105 VR, Tokina 12-24, 2xDS-125

#6 Photobeat

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Posted 09 January 2006 - 05:04 PM

You could use the square tool to quickly surround the purple areas separately (and a little of the window) or even select the entire upper section. Next go to the Edit and click on hue/saturation. Select the colors (whatever affects purple) Play with the sliders to get what you want. You could also do this with the channel mixer.
Aquatica Housing - D100 - 10.5dx - 17-55DX - Nikonos 105 strobes - TLC arms

#7 mattdiver

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Posted 09 January 2006 - 05:52 PM

Another quick fix is to select the whole window area using the lasso tool, then open the Hue/Saturation diolog (Ctrl+U), choose the Magenta range of colours, and set saturation to -100%, as shown below...

CA_Correction.jpg

#8 aczyzyk

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Posted 16 January 2006 - 06:37 AM

I did as you said and it helped.
Thanks.

Andrzej
Andrzej Czyzyk

www.flickr.com/photos/czyzyk/
D200, Sea&Sea DX-D200, Nikkor 10.5, Nikkor 105 VR, Tokina 12-24, 2xDS-125