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Old Jun 7, 2008, 8:50 AM   #1
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Hi folks,

I posted in the "digiscoping" section, and I'm not getting an aswer.
I'm using a 60X 60mm lens in conjunction with my digital camera, and I'm looking for a filter that will help reduce chromatic aberration. I just need an inexpensive 67mm threaded addition, I believe, but I don't know what to call it.

Thank you.
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Old Jun 7, 2008, 1:19 PM   #2
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If anything, a filter would probably make it worse. ;-)

I did some experimentation some years back using a variety of different filters (including "Hot Mirror" filters designed to block IR) with an Epson PhotoPC 3000z (a 3 Megapixel digital camera).

Some people had a theory at the time that purple fringing was being caused by light outside of the visible spectrum versus the optical quality of the lens (and because of microlens design on a modern camera's sensor, light refraction can occur, leading to purple fringing).

But, I found zero evidence that a filter could reduce purple fringing with a modern diigtal camera (most newer models already have filters over the sensors that block most IR and UV), when taking photos of the same subjects in controlled conditions with and without the filters (the fringing was identical, with and without the filters attached).

Basically, you need higher quality optics versus a filter. You may want to try using a smaller aperture (represented by a higher f/stop number) and see if that makes any difference (keeping in mind that you'll need slower shutter speeds for proper exposure if you use that technique).


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Old Jun 9, 2008, 1:47 AM   #3
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What software do you use? There are lots of programs that address chromatic aberration. Look for a tutorial about this with your current editor.

If what you have won't do this, shop some of the fuller-featured, moderately priced editors like Photoshop Elements or Paint Shop Pro and see if they can lessen CA.

Gimp claims to be effective against CA -- and it's free!

Specialty corrective software like DxO can handle a wide range of image defects beyond CA.

Grant
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Old Jun 10, 2008, 8:07 AM   #4
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I use Paint Shop Pro. The program has a "purple fringe fix" that gets rid of fringing about 50% of the time. On some photos it does a great job on others not so good or not at all. It's a hit and miss situation.
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Old Jun 12, 2008, 7:34 AM   #5
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Thanks, guys.

JimC, thanks for letting me know it's a true limitation of hardware. At least I won't go chasing my tail in an expensive manner by buying more equipment that won't work!

I have all three processing softwares mentioned. PFing was never a concern for me before, so I never sought a remedy in the softwares. I will try letting in less light and utilize the software.

Perhaps shooting in RAW will help me? (Or make it more complicated...)
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Old Jun 12, 2008, 11:09 AM   #6
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If what you're talking about is 'purple fringing', I've had good luck with UV filters. Something like a Tiffen 1A,2A, or a Warm UV filter migth do just what you need.
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Old Jun 12, 2008, 1:02 PM   #7
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I don't buy it with a modern digital camera. It's probably only a coincidence if you are seeing less fringing in your images (i.e., the conditions are not identical to when you had fringing)..

I've seen this subject discussed before, and I've done quite a bit of testing myself. With an older model that's UV and IR sensitive, I've seen some evidence that hot mirror filters can help in extreme conditions. For example, some of the early 2MP and lower resolution sensors didn't have filters over them that block most IR and UV light). Ditto for some of the early sensors used in some dSLR models (some actually had removable versus permanently attached filters for the sensors). But, not for newer cameras.

Take some images of the same subjects, in the same conditions, with the same settings (i.e., controlled conditions), with and without a filter and see what you get.

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Old Jun 12, 2008, 1:24 PM   #8
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If your photo editing software can use Photoshop-type filters (.8bf files), don't miss the free Photoshop filter that removes purple fringeing, PFree from Thomas Fiddaman, http://www.sd3.info/pf828/PFree/PFree0-1.html

(Note that this is a software filter, not a physical filter that goes on your camera lens.)

It works without greatly affecting overall color balance. For those with limited image processing skills (for example, me), the default settings work for most problem images.

If you think you want to play around with the sliders, you might want to jot down the default settings when it first begins, as there is no 'default' button to re-set them.

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