# Red/blue edges around my product photos...



## Hovik

Hi guys, these aren't really visible unless zoomed in, but it annoys me when I'm spending hours editing those photos. Any idea why this happens?

I use about 4 light sources to take those photos, camera is T2i with 24-70 2.8L

Thanks!


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## cgipson1

CA! What make is the lens?


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## Hovik

Canon.


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## mjhoward

Those are called Chromatic Abberations and are flaw found in every lens.  They happen when different wavelengths of light don't focus at the same point.  Some lenses exhibit this worse than others.


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## cgipson1

Can't help you there... But the CA can be cleaned up in PS or LR....  Pixel peeping can drive you nuts!   

did find this:  

Canon EF 24-70 F/2.8L fringing

Canon 24-70 f/2.8 L fringing/CA: Canon SLR Lens Talk Forum: Digital Photography Review


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## Hovik

Something to live with then, I suppose. 

Appreciate the quick responses!


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## Garbz

CA is also focal length and aperture dependent. Try shooting at a different aperture if the situation allows.


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## o hey tyler

I'd be interested to know what aperture you used on that photo.


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## TCampbell

At first I couldn't figure out why your CA was uniform -- then I noticed the 200% zoom level and realized we're just seeing one section of the image.

CA radiates from the center...  the farther from center, the worse it gets.  If you look through a common magnifying glass (I pick on this example because it's (a) a single lens element with no corrective elements involved and (b) it shows a worse-case scenario of the CA problem) at a sheet of newspaper, you'll notice that in the very center of the magnifying glass, the image looks sharp.  But near the edges of the magnifying glass you'll see this same red/blue fringing around the letters.  Whether the red is on the "inside" edges and blue is on the "outside" edges depends on whether you're looking at black letters on white paper or white letters on black paper.

The edges of the lens element behave like a prism (imaging slicing the lens in half and look at the cross section of the glass).  That prism shape splits the light into it's constituent wavelengths just like a prism would.  Since red is at one end of the visible spectrum and blue (well ... technically "violet" but eyes can more easily see blue light than violet light, so you generally "notice" the blue fringing more than the violet.)

The problem happens on ALL refractive lenses.  The bigger the lens diameter, the worse it gets.  This is why HUGE prime & zoom lenses (e.g. 300mm f/2.8 primes ... 600mm f/4 primes, etc.) cost ludicrous amounts of money.  Manufacturers try to combat the problem by using low-dispersion elements (usually made out of semi exotic materials rather than common optical glass -- for example, Canon uses fluorite which is a synthetic crystal that they "grow" in kilns.  To "grow" them requires a carefully controlled amount of heat to make sure that while a crystal "grows", they want it to grow slowly.  If a crystal grows too quickly it'll have flaws in it.  They also add extra elements in the form of achromatic doublets.

But all of this extra expense and optical engineering simply "reduces" the effect of CA -- it doesn't really eliminate it.  Larger telescopes use parabolic mirrors to focus light rather than lenses because CA occurs when light passes through the glass and since a mirror simply reflects the light (without letting the light pass through) it eliminates the CA problem.  There is such a thing as a catadioptric camera lens (these use mirrors to focus light instead of lenses) and it has a central obstruction just like a catadioptric telescope.  They do a good enough job on the focused subject, but the "bokeh" created by such lenses is doughnut shaped (has a black spot in the middle of the circle -- because the lens has a central obstruction.)

Fortunately, since CA is virtually non-existent at the central of an image (because light passing through the central axis of the lens doesn't really go through any curved glass... the elements are all just about "flat" at that point in the lens) and gets progressively worse as you get to the edge of the image.  THAT means that if you split the image into it's red, green, and blue channels, you can "shift" the red & blue but leave the green alone (imaging if you were take a red-channel version of an image and scale it 1% bigger, leave the green-channel version of the image alone, and take the blue-channel image and scale it 1% smaller... and then re-combine the three images back into one.  What you'd end up with is an image where the red, green, and blue channels actually line up.  That's basically what they do to try to combat the effects of CA by using software.

Just make sure you have software that can correct for CA such as Photoshop, Lightroom, or Aperture, and learn to use the CA correction tool.


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## Espike

Mr. Campbell, you sir, know your sh*t! 

I will be using your post in justifying my future L lens purchase to the wife when she finds out.


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