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I make many product photos for the company I work for. For this I use Canon 400D with two external flash devices Walimex CY-600K.

I've noticed an interesting effect on many of the photos I take with white background. On the resulting image there is a red/blue area visible between object and the background. The effect is strongest for black objects and also seems too decrease when I make a brighter photo. It is mostly visible only if zoomed in really close and is not a real problem for me as the resolution is quite high and the resulting images are only 500px wide.

I am just curious as to what is causing it exactly.

For the following example photo I used manual mode with these settings: 1/200 F10 ISO200:

photo.

You can see blue area left from the object and red area right from it. What is causing this?

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Possible Chromatic Aberration? –  fluf Jan 19 '12 at 12:03

2 Answers 2

up vote 4 down vote accepted

This is called Chromatic Aberration (CA). In photography its is also known as Purple Fringing. It occurs because lenses have a different refractive index for different wavelengths of light. The refractive index decreases with increasing wavelength. Its most visible when you shoot a dark object against a bright background.

To overcome this problem totally, you'll need to use a better optics/lens. Some lenses have very good control over CA, others do not. You did not mention what lens you are using on your 400D, but I assume its the 18-55mm kit that comes along. If you're not ready to purchase a new lens, I'd suggest shooting in RAW format, and use the Canon's Digital Photo Professional software which have the ability to fix CA issues mostly if not completely in post processing.

As long as you are not doing professional product photography or making large prints, I wouldn't want to spend thousands of dollar for a better lens as you've mentioned you re-size the image to a smaller size.

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Since it's not purple, I don't think it can be described as "purple fringing", can it? –  mattdm Jan 19 '12 at 12:11
    
Ah I see! Thank you, I am reading about it now. And yeah, the lens is the bundled 18-55mm. –  rbx Jan 19 '12 at 12:13
    
@mattdm I think they both originate from the same optical limitation on cheaper lenses. –  fahad.hasan Jan 19 '12 at 12:17
1  
I can confirm that Canon's DPP software is great for removing such fringing for Canon lenses. It makes the cheaper consumer lenses very usable when shooting in RAW –  Dreamager Jan 20 '12 at 12:26

In addition to the first answer, it's also a byproduct of digital sensor technologies. The sensors for red, green, and blue light are NEXT to each other on flat surface, therefore they each 'see' the light being focussed onto them from an ever so slightly different angle, but when this is recombined into a photo, they don't quite line up properly and you get this effect. Some considerable efforts have been made by the manufacturers over the years to reduce this. I believe one of the big selling points of the Leica M9 was their arrangement of photosites to try to minimise this.

In older film technology, where the colour-sensitive layers of emulsion where overlaid on TOP of each other, this never happened because the red/green/blue sensitive layers saw the exact same image. That is why you will not see this with film, only digital...

(That's my understanding, anyway) :)

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Interesting! But if this was the prime cause, we would then see this effect on every photo, not just under described conditions? –  rbx Jan 19 '12 at 12:37
    
Just to point out: The photodiodes on the M9 are symmetrical, the microlenses in front of them are not. Also, because of the camera design, the M9 has to be able to capture angled light rays and not only parallel light. And it is a CCD sensor over the more common CMOS. –  DetlevCM Jan 19 '12 at 12:44
    
@rbx You notice it on any areas of hard contrast, deliniating lines between bright and dark. On parts of a photo where there is more a graduation from one colour to the next it is not noticable. @ DetlevCM - thanks for the clarification, though you understand my point that they have tried to account for it in some way :) –  Mike Jan 19 '12 at 16:52
    
You're describing colour aliasing which happens with regular patterns when the detail is finer than the spacing between pixels. The image in question is suffering from (lateral) chromatic aberration, which is causes by the lens and shows up on film as well as digital. –  Matt Grum Jan 19 '12 at 19:32

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