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07-02-2010, 05:53 AM   #1
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Pentax K-x single channel clipping

Hello everybody,
I'm kinda new to the DSLR-world, bought my first DSLR (K-x) 2 weeks ago, and I like it a lot. Very usable image-quality up to ISO 6400, what more could I wish for!

But! I experienced something strange: whenever I shoot something lit with a bright blue LED, I get strange results:

I noticed very similar problems in the "K-x in use"-thread, more specifically this post: https://www.pentaxforums.com/forums/1094163-post86.html

Is this the result of a defective camera, or expected behaviour?

cu!

07-02-2010, 06:06 AM   #2
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The [very detailed] discussions in the thread of UnknownVT's you link to on the subject of how camera sensors render LED light in the blue/violet spectrum would seem to suggest that it is an issue with all digital camera sensors. So it's not a K-x specific issue. It's akin to the camera not being able to capture infra-red or ultra-violet wavelengths properly.

UnknownVT's thread also provides a master-class on how to post-process your images in order to remove this sort of LED light weirdness.
07-02-2010, 07:34 AM   #3
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Yes, I read most of it, and I agree that it's to be expected that shooting something that's lit only by LED-lights will give unnatural effects. But in this case there's also a 'normal' light, shining more or less on the front of the face.

I would expect that where there are highlights caused by the LED-light in a region that's also lit by a normal light, that one would get an additive blend of both the effect of the LED-light and the normal light, which in this case would be more or less (blueish) white but certainly not overly saturated blue.

I tried post-processing it, also some test-shots, both in JPEG and in RAW, but I could only partly solve the problem, by lowering the contrast in-camera.

Also, I've never noticed this effect with my previous cameras, a Panasonic Lumix TZ3 and a Panasonic Lumix FZ38.
07-02-2010, 04:42 PM   #4
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I should also have referenced this multi-page forum thread started by UnknownVT on the specific subject of LED lighting, where he does work through solving some of the problems of LED lighting:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/93809-modern-led-s...-problems.html

07-06-2010, 03:31 AM   #5
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ok, read that thread. Quite interesting thread actually!

I think I'll just try to avoid LED-lighting for the time being

Thanks for your replies!
07-08-2010, 09:27 AM   #6
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QuoteOriginally posted by gaeriel Quote
Is this the result of a defective camera, or expected behaviour?
Looking at your example that's quite a good exposure considering the circumstances - the blue is clipped -
I think the camera tried its best to expose for the overall scene and clipped the blue channel.

On its default Bright Color setting the Pentax K-x tends to give slightly over-saturated colors (not as heavily as previous Pentax dSLR like the K100D which I also own). This doesn't help in an exposure with mixed or extreme colored lighting - certain colors will tend to clip - blues actually is one of the weak spots with a tendency to clip - although the natural color setting may help - I still much prefer the bright Color setting on the K-x as it generally gives punchier results (whereas on the K100D it was too much and I have Natural Color as my setting)

Once there is clipping in the photo - there is very little one can do PP in JPG or RAW - although RAW may allow a little more flexibility - but as shown in my thread Kx in Use although RAW can sometimes do better - it is by no means an instant answer or solution to all one's problems (some need a psychiatrist! )

in that thread Modern LED Stage Lighting & photography problems

I showed these - pretty severe clipping examples in blue Post #16 -
This lovely artist had the misfortune to have have her entire set lit with blue (LEDs) only:

This photo technically is kind of OK - just not very flattering to her.

However with the much maligned technique of chimping -
I saw this problem almost straight away, and was able to mitigate it somewhat:

I think what has happened on this flash assisted (second) shot is that I used slow-sync fill flash, and the camera attempted to expose for the scene as-is - and notice the background and surroundings were dark so probably over-exposed the center main subject - so the blue definitely will clip (since it already has a tendency to over-saturate) the the exposure due to the slow-sync fill-in flash is as set (-2/3 flash compensation) mitigated the blue wash on the face - hence the more presentable face exposure.......

- not great, a little psychedelic and Andy Warhol-ish, but better, at least to my eyes.

Sometimes it's impossible to avoid entirely -
but when seen - a huge advantage of chimping -
it's probably better to try to take "advantage" of that characteristic -
and utilize it to to take unique photos.

PS - many thanks for the kind comments from rawr
07-08-2010, 10:20 AM   #7
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QuoteOriginally posted by UnknownVT Quote
I showed these - pretty severe clipping examples in blue Post #16 -
This lovely artist had the misfortune to have have her entire set lit with blue (LEDs) only:

This photo technically is kind of OK - just not very flattering to her.

However with the much maligned technique of chimping -
I saw this problem almost straight away, and was able to mitigate it somewhat:
Yeesh - somewhat mitigated indeed in some ways, but even stranger in others! I know at first I was doubting that LED's are any worse than other lights, but I've pretty much been convinced by your examples and a couple more experiences of my own.
07-10-2010, 02:18 AM   #8
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LEDs are full of blue (possibly UV as well) light, which is why they are so bad as lighting sources - our eyes are less sensitive to blue than to yellow, so LEDs have to burn very brightly to put out enough visible light (containing yellow) and then overdo the blue. LEDs need to be filtered with yellow filters to cancel out the blue in order to approximate daylight.

Try putting some yellow cellophane over the front of an LED flashlight and see how much more 'visible' the light becomes.

I suggest either using a yellow filter over the lens when next shooting in LED lit scenes or changing the WB to a custom setting that emphasizes the yellow spectrum. In mixed lighting scenes though, one is a bit screwed as a camera cant correctly balance for both blue and yellow light at the same time. I'd rather tone down the blue with a yellow balance and the PP selected colours to shift the overall tonal balance towards acceptability.

09-20-2010, 02:27 AM   #9
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possible solution to blue clipping

Moin,

I recently discovered that there is a strong correlation of "Custom Image" setting and the bad rendering of saturated blue: Only Bright/Vibrant settings seem to result in clipped blue colours, all others seem to work just fine.

Stefan
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