REMOVING COLOUR ABERRATIONS/COLOUR FRINGING/ATMOSPHERIC DISPERSION FROM AN IMAGE IS VERY SIMPLE IN PHOTOSHOP. THIS PROCESS USING THE LAB COLOUR MODE AND BLUR TOOL CAN ALSO BE USED WITH SOME SUCCESS IN DAYTIME CAMERA SHOTS AS WELL, IF IT IS QUITE BAD. THE ONE DOWNSIDE IS THAT IT REDUCES THE SATURATION A LITTLE, BUT THAT IS EASILY SOLVED INCREASING SATURATION WHEN FINISHED.

THIS IMAGE OF SATURN SUFFERS BADLY FROM ATMOSPHERIC DISPERSION (THE SPREADING OF LIGHT INTO ITS COMPONENT COLOURS BY THE ATMOSPHERE), FROM SATURN NOT BEING PLACED HIGH IN THE SKY WHEN THIS IMAGE WAS TAKEN AND IS A PRIME EXAMPLE TO USE. NOTICE THAT THE RED, GREEN AND BLUE CHANNELS DON'T ALIGN AND AREN'T ALL FOCUSSED TO THE SAME POINT. THE IMAGE IS A STACK OF 900 IMAGES IN REGISTAX WITH SLIGHT WAVELET SHARPENING, NO OTHER PROCESSING HAS BEEN DONE. ***

STEP 1

OPEN YOUR IMAGE IN PHOTOSHOP.

ONCE OPENED GO TO IMAGE > MODE AND YOU WILL SEE A LIST OF POSSIBLE MODE CHANGES THAT CAN BE SELECTED. FOR THIS OPERATION WE WANT TO CHOOSE 'LAB COLOUR'. CHANGING TO LAB COLOUR IN PHOTOSHOP IS A NON-DESTRUCTIVE MODE CHANGE SO THERE IS NO NEED TO WORRY ABOUT LOSS OF IMAGE QUALITY.

STEP 2

ONCE CONVERTED TO LAB COLOUR YOU WILL NOTICE ON THE CHANNELS PALETTE THAT WE NOW HAVE THREE DIFFERENT CHANNELS THAN WHAT WE PREVIOUSLY HAD. BEFORE WE HAD RED, GREEN AND BLUE AND NOW WE HAVE LIGHTNESS, A AND B. TO CARRY OUT THIS OPERATION WE SHALL BE USING CHANNELS A AND B, THESE CHANNELS HOLD THE COLOUR INFORMATION IN THE IMAGE.

SELECT 'CHANNEL A' FROM FROM THE CHANNEL PALETTE, YOU WILL NOTICE THIS IS A VERY DIFFERENT IMAGE THAN WHAT YOU OPENED UP. SELECT FILTER > BLUR > GAUSSIAN BLUR FROM THE MENU AT THE TOP. NOW YOU HAVE THE GAUSSIAN BLUR TOOL ENABLED MOVE THE SLIDER TO THE RIGHT TO ADD MORE BLUR. WHAT IS IN EFFECT HAPPENING IS THAT YOU ARE SOFTENING THE COLOUR INFORMATION IN THE SELECTED CHANNEL. A GOOD PLACE TO START IS TO REMOVE THE SHARP EDGES IN THE CHANNEL AND CLICK OK AND SEE WHAT YOUR IMAGE LOOKS LIKE WITH ALL THE CHANNELS SELECTED, SO CLICK OK AND SELECT LAB AGAIN. YOU WILL SEE THE RED FRINGING ON YOUR IMAGE HAS DECREASED SLIGHTLY.

NOW DO THE SAME PROCEDURE WITH CHANNEL B AND REMOVE THE SHARP EDGES IN THAT COLOUR CHANNEL, CLICK OK THEN REVIEW YOUR IMAGE BY SELECTING LAB AGAIN WITH BOTH CHANNELS NOW SLIGHTLY BLURRED. YOU WILL SEE THIS HAS SIGNIFICANTLY REMOVED THE FRINGING FROM THE EDGES IN THE IMAGE.

EVERY IMAGE IS GOING TO BE DIFFERENT IN THE AMOUNT OF COLOUR FRINGING SO JUST EXPERIMENT HERE BY ADDING MORE BLUR OR IF YOU FEEL YOU ADDED TOO MUCH TO START WITH JUST GO BACK USING HISTORY AND RETRY AGAIN. REMEMBER NOT TO OVERLY BLUR ANY OF THE CHANNELS AS BY DOING THIS YOU ARE REMOVING THE COLOUR INFORMATION FROM THE IMAGE.

STEP 3

IF THE SATURATION IN THE IMAGE HAS DECREASED A LITTLE BY DOING THIS THEN NOW IS THE TIME TO ADD A BIT OF SATURATION TO THE IMAGE, REMEMBERING NOT TO OVER DO IT.

STEP 4

ONCE YOU ARE HAPPY WITH THE OUTCOME JUST CONVERT YOUR IMAGE BACK TO RGB USING THE IMAGE > MODE ROUTE AND SAVE YOUR IMAGE AS A TIFF ONCE AGAIN.

BEFORE                                     AFTER

COMPARING THE IMAGES YOU CAN SEE THERE IS A GREAT REDUCTION IN THE REMOVAL OF THE COLOUR FRINGING AROUND THIS SATURN IMAGE, ALTHOUGH THERE IS A LITTLE SHOWING THROUGH IT LOOKS A LOT BETTER NOW.

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THE IMAGE I USED ABOVE AS AN EXAMPLE WAS AN EXTREME VERSION OF THE ABOVE MENTIONED PROBLEM. IF YOU STACK YOUR PLANET AVI'S IN REGISTAX THEN YOU WILL SEE AN OPTION CALLED RGB ALIGN, WHICH ALIGNS THE 3 COLOUR COMPONENTS SO THEY FIT A LITTLE BETTER. WITH RGB ALIGN YOU CAN MANUALLY MOVE THE COLOURS YOURSELF OR LET REGISTAX DO IT AUTOMATICALLY. (YOU CAN ALSO MOVE THE RGB CHANNELS SEPARATELY IN PHOTOSHOP USING THE MOVE TOOL FROM THE TOOL PALETTE, SELECTING ONE COLOUR COMPONENT AT A TIME AND SHIFTING IT SLIGHTLY)

THE THING WITH DOING THIS IN REGISTAX OR ANY OTHER PROGRAM FOR THAT MATTER IS THAT THE 3 COLOUR COMPONENTS ARE NOT THE SAME SHAPE AS EACH OTHER, THE COLOURS ARE FOCUSSING AT SLIGHTLY DIFFERENT POINTS SO THE RED AND BLUE ARE STRETCHED OUT A LITTLE, HENCE THE COLOURS WILL NEVER BE ALIGNED PROPERLY.  AN EXAMPLE OF THIS IS BELOW.

 BEFORE ALIGNMENT                            AFTER ALIGNMENT                              AFTER LAB COLOUR OPERATION

NOTE THAT THE COLOURS HAVE BEEN MOVED IN THE MIDDLE IMAGE YET THERE IS STILL A SLIGHT BIT OF FRINGING EVIDENT. THE LAST IMAGE SHOWS THE IMPROVEMENT ON USING THE LAB COLOUR OPERATION TO REMOVE IT.

ALIGNING THE COLOURS FIRST I THINK IS A BETTER WAY OF DOING THIS AND AFTER DOING THE LAB OPERATION IT NEARLY ELIMINATES ANY COLOUR FRINGING THAT MAY SPOIL THE IMAGE.

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AS FOR TERRESTRIAL PHOTOGRAPHS, BELOW IS AN EXAMPLE OF THE ABOVE PROCESS OF BLURRING THE A AND B CHANNELS AND SHARPENING THE IMAGE, THEN ADDING SOME COLOUR SATURATION. THE PHOTO BELOW IS A 100% CROP FROM A PHOTO I TOOK WITH AN 8MM FISHEYE LENS WHICH SUFFERS A LITTLE FROM CHROMATIC ABERRATION. THIS LENS WOULD GIVE A 180 DEGREE FIELD OF VIEW ON 35MM AND FULL FRAME SENSORS, BUT WITH CROPPED SENSORS LIKE THE 10D, SOME OF THE FIELD IS CROPPED DUE TO THE SENSOR SIZE DIFFERENCE.

AFTER THE CORRECTION PROCEDURE, THE RESULTING IMAGE LOOKS LIKE THIS, WHICH IS A GREAT IMPROVEMENT ON THE ORIGINAL !

© All photographs in the galleries copyright of Gordon Mackay

Please ask permission if you would like to use any of the photographs on this page, thanks.

email gordon@mackayphotography.co.uk