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Compression Problem ?


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#1 snoack

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Posted 30 April 2002 - 07:14 AM

The following picture shows a part of an unmodified photo of a clownfish with a strange effect: The white stripe seems to blur into the background and the brown and yellow areas of the fish.
I've taken several pictures of clownfish so far and never seen something like this before.
The compression was set to HQ (as usual).

Could this be a compression (jpg) problem ?

??? - Sabine

image

#2 SharpDiver

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Posted 30 April 2002 - 07:41 AM

I think the problem is that the shot is somewhat over-exposed. The "purple-fringing" problems often seen with digital cameras almost always appear in areas of over-exposure -- often unavoidable with backlit subjects. Here I think if you had used a faster shutter speed, and a smaller aperture you might have had better results.

#3 james

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Posted 30 April 2002 - 07:46 AM

This is an excellent example of "Chromatic Aberration" or "Purple Fringing" which is commonly seen with consumer digicams.

Areas of high contrast - especially black building edges against a bright sky for example - will see this fringing effect. The effect is totally an artifact from the CCD and is not brought about by software or JPEG compression.

It's definitely a "bummer" as it occurs often underwater - especially on "electric colored" fish such as chromis, anthias, etc.

HTH
James
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#4 MikeO

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Posted 30 April 2002 - 07:49 AM

Actually, it is due to both the CCD and the optics . . .

http://www.dpreview....rrations_01.htm



[Edited on 4-30-2002 by MikeO]

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#5 SharpDiver

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Posted 30 April 2002 - 08:11 AM

Hey, James, let's not give up quite so quickly.

Chromatic aberration -- particularly underwater -- is not unavoidable, except in one situation.

If you are trying to include a sun in your image, CA is going to be very hard to control.

But, brightly colored or not, some experimentation with lighting, speed and aperture should reduce or eliminate CA in a strictly reflected scene (no light source shining directly into the lens).

What is required is an understanding of when CA can rear its ugly head. High contrast areas and bright colors will require extra bracketing and a tendency toward under-exposure instead of over-exposure.

My point is, don't throw up your hands - work through the problem.

#6 MikeO

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Posted 30 April 2002 - 09:10 AM

Here's an example of a shot with the sun in the background in which the CA was managed pretty well. You can see a little bit of a line around the shark, but all in all, it's not distracting . . .


Posted Image

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#7 james

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Posted 30 April 2002 - 10:30 AM

For anyone who takes aquarium digital photos:

Try taking a picture of a Pacific Blue Tang (Paracanthurus hepatus) under artificial lighting. You will be knocked flat to see that the fish that to you looked blue turns out a neon purple in your shot. It's very frustrating.

Here's an example with a Blue Chromis (a fish most of you have seen before):

Posted Image

I'm telling you, that fish is actually blue! I know this is sort of off topic, but just wanted to vent my frustration a bit. I wonder if setting the white balance to "flourescent" would help?

Cheers
James
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#8 herbko

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Posted 30 April 2002 - 03:38 PM

James,

Have you tried using your flash?

Put your lens as close as you can to the glass.
Use the highest shutter speed of your camera, and
adjust the exposure with your aperture.

Herb
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#9 rstark

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Posted 03 May 2002 - 02:19 AM

About the aquarium shot. As I remember, most good aquariums have either metal halide or actinic lighting. The color temp on these are going to be unusual so you will have to do a manual white balance.

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#10 james

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Posted 03 May 2002 - 05:13 AM

That's exactly right. Flourescent (VHO) lighting emits light in a few narrow spikes.

Most MH are broadband emmitters but they also have spikes as well.


Cheers
James
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#11 Andi Voeltz

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Posted 03 May 2002 - 11:16 AM

Hello everyone,

very interresting posts! I have learned a lot about "chromatic aberration" now. Even if I was not able to experience it with my Nikon Coolpix 990 yet. Most problems I encountered so far could be succesfully solved by underexposing or playing around with the white balance. Anyway I'd like to have your opinion on this new "hyped" SIGMA CCD and how digital underwater photographers could benefit from it.

http://www.sigma-pho...news_sd9_fs.htm
find a housing for your digicam! oOo. http://www.digideep.com .oOo.
market overview of the essential equipment for digital uw photography

#12 snoack

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Posted 05 May 2002 - 06:00 AM

Aha,

that explained a lot. As C3030Z owner (which was mentioned to be especially sensible to this problem), i'll have to be more careful next time.

Thanx for your help - Sabine