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Photography Question 

UV filter?

© Cristina
In most of my pictures, I get a purple edge around trees, snow, etc. Cand this problem be solved with a UV filter? Or it's not from the UV rays?

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7/4/2008 6:43:34 PM

Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
  Not with a uv filter. That's chromatic aberration. It's the result of light of different wave lengths and diffraction as light passes through the glass of the lens.
Better quality optics have less of it. Doubt if any can get rid of all, but good lenses can reduce it to be unnoticeable.

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7/4/2008 7:46:53 PM

David Van Camp
BetterPhoto Member Since: 1/27/2008
  Some software programs can help with CA... to some extent. For example, PaintShopPro (I've used JASC versions 8 & 9, typically avail on ebay for ~$25US) has a CA filter that is better than nothing... does a good job sometimes, but depends on how severe the problem is.

I cannot comment on other products. Perhaps others can.


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7/4/2008 8:43:45 PM

Bob Cammarata
BetterPhoto Member Since: 7/17/2003
  Purple fringing will be most prevalent around out of focus edges toward the outside of the frame.
This common aberration will occur with cheap lenses (as Greg described) or with mis-matched lens/teleconverter combinations.
Being aware of it will help to avoid it.
Keep your main subject in tack-sharp close to the center of the frame.
(...or just buy a better lens.)

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7/5/2008 2:10:23 AM

Cristina    I don't think I can change the lens to my camera... ( canon S5 IS ). Where I bought the UV filter from, it says that it helps a little removing the chromatic aberration. It's this true?

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7/5/2008 5:35:16 AM

Gregory LaGrange
BetterPhoto Member Since: 11/11/2003
  A uv filter will reduce a blue cast to a photo like you get shooting in the shade, or on an overcast day.
You may see more of a reduction if you change the time of day you shoot. Like later in the day instead of noon. Or even change shooting angles

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7/5/2008 3:59:16 PM

Alan N. Marcus   Hi Cristina,

I own a Canon S5 IS and I love it. I donít know of another in this price range that is as feature packed.

As to chromatic aberration: All camera lenses suffer from this affliction. It is a phenomenon that affects all lenses (except mirror lenses). The job of the lens is to cause light to bend inward. This causes an image to form behind the lens. The further behind, the more magnified the image. The S5 features a zoom lens, user settable, 6mm ~ 72mm. That quite a lot, it covers a 12x change in magnification. Most often this expanse is explained by association with the equivalent optics as mounted on a standard 35mm film camera. We can do this using a multiplying factor. In the case of the S5 the imaging chip is 6 times smaller than a standard 35mm image area. So doing the math the equivalent is 36mm ~ 432. Let me tell you, thatís a lot of magnification change. 36mm is wide-angle, 432 is a respectable telephoto.

Additionally, this camera as 4x more magnification in the form of an electronic zoom. That multiplies the magnification to a whapping 48x. Thatís a lot! When zoomed all the way out thatís equivalent to a super telephoto focal length 1728mm (by 35mm standards). Another way to look at it, using 50mm as normal, that magnification is equivalent to a telescope with a power of 34 Ĺ x (sports photographers and naturalist go nuts for a reach like this.

When a camera lens or a telescope is operating at these high magnifications, chromatic aberrations become pronounced. Optical science has never been able to overcome this. You see, each color comes to a focus behind the lens at a distance based upon its wave length. Red comes to a focus first, followed by all the other colors, blue comes to a focus last. Since the blue image forms further away from the lens, it is the biggest image (more magnification). The purple fringe you see is the larger blue image superimposed on top of the other colored images, each a different size, red is the smallest.

Now let me tell you that the S5 is remarkably well color corrected for the price. I have noted that images examined via the electronic viewfinder show accentuate purple fringe. This it is not as perceptible a defect when viewed on the computer screen or on a print. Additionally the purple fringe is only noticeable at high magnifications.

The countermeasure is digital editing software that features purple fringe removal.

As to a UV filter. They are over rated; they only work at high altitudes, providing slight haze penetration. A UV doesnít work effectively at sea level; it is not a warming filter like a light salmon colored ďskylightĒ. You donít need a warming filter with digital as enhancements can be preformed after the fact, in you computer, with a reasonable editing program. In passing, the most valuable filter to put in your gadget bag is a polarizing screen.

Alan Marcus (truly marginal technical gobbledygook)

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7/6/2008 5:35:19 AM

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