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Author Topic: Aliasing?  (Read 2081 times)

tsjanik

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Aliasing?
« on: August 17, 2013, 04:45:41 PM »

I've attached a cropped image taken with a 645D and 600mm lens.  What looks to be a sharp chair and pleasant OOF flowers in the background, contains bright vertical lines in the OOF area.  Quite pronounced to the lower right of chair.  I've never seen this before.  Any thoughts?

Tom
« Last Edit: August 17, 2013, 04:48:37 PM by tsjanik »
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ErikKaffehr

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Re: Aliasing?
« Reply #1 on: August 17, 2013, 04:49:34 PM »

Hi,

Aliasing occurs in sharply focused areas. I think this may be overcorrected spherical aberration.

Best regards
Erik


I've attached a cropped image taken with a 645D and 600mm lens.  What looks to be a sharp chair and pleasant OOF flowers in the background, contains bright vertical lines in the OOF area.  The 2nd crop shows an area to the lower right of chair.  I've never seen this before.  Any thoughts?

Tom

tsjanik

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Re: Aliasing?
« Reply #2 on: August 17, 2013, 05:15:14 PM »

Thanks for the reply Erik.  I admit to not knowing why overcorrection would cause vertical bands, but it seems that would be a constant feature.  I've never seen this effect with this lens in very similar photos.

By way of example: same day, same lens, same camera sun position
Tom
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ondebanks

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Re: Aliasing?
« Reply #3 on: August 18, 2013, 06:10:55 PM »

I'd suggest that these streaks might caused by refraction through rising thermal air currents. Was it a warm day?

Ray
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BartvanderWolf

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Re: Aliasing?
« Reply #4 on: August 18, 2013, 07:21:03 PM »

I've attached a cropped image taken with a 645D and 600mm lens.  What looks to be a sharp chair and pleasant OOF flowers in the background, contains bright vertical lines in the OOF area.  Quite pronounced to the lower right of chair.  I've never seen this before.  Any thoughts?

Hi Tom,

Well, it's not aliasing, so much is sure. But as to what it is, it's a mystery. It looks a bit like post-processing induced artifacts, but I don't know what would trigger that.

Could you give some shooting details, mainly shutter speed (to perhaps eliminate Ray's thermals suggestion), and post-processing workflow (converter used, specifics about sharpening/clarity).

Cheers,
Bart
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tsjanik

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Re: Aliasing?
« Reply #5 on: August 19, 2013, 09:03:26 AM »

Yes it was warm Ray, but all other shots that day donít exhibit the effect.

I was inaccurate in my title Bart, it should have been Processing Artifact?
It is not a PP artifact; the effect is quite visible in the RAW thumbnail in Bridge; so I was wondering about the in-camera processing. (shots of the chair from f8 through f/9.5 and 1/20 to 1/60 s exhibit the effect)

The effect is only seen in shots in which the chair is present.  I have attached a similar shot taken later in the day where the effect is absent.  The only significant difference I can see is that the back of the chair is no longer in direct sunlight.  So hereís a thought:  The angle of the lens to the openings between the slats in the chair is such that the visible opening is quite narrow; such that itís acting as a diffraction grating and when intense sunlight is striking the back on the chair a diffraction pattern shows up.  However, if thatís the cause, it seems the vertical lines should then also appear on the chair as well.  

I will try to reproduce the effect if the opportunity presents itself.

Thanks all,

Tom
« Last Edit: August 19, 2013, 09:06:07 AM by tsjanik »
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ondebanks

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Re: Aliasing?
« Reply #6 on: August 19, 2013, 09:33:20 AM »

Hmmm. Was there anything at all between the chair and the background flowers? I wonder if there was something that might either create a diffraction pattern or a tenuous, largely vertical oof pattern - a window, a screen, grass stems?

Ray
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bcooter

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Re: Aliasing?
« Reply #7 on: August 19, 2013, 09:41:58 AM »

Hmmm. Was there anything at all between the chair and the background flowers? I wonder if there was something that might either create a diffraction pattern or a tenuous, largely vertical oof pattern - a window, a screen, grass stems?

Ray

It's the ccd gods.

Have that a few times with my phase backs.  One image in the background, way, way in the back ground is a blue light streak like a special effect from the slight headlight of a studio cart.

Of course that image was the select, so we had to rebuild it.

Doesn't happen often, but when it does, it's a mess.

IMO

BC
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ondebanks

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Re: Aliasing?
« Reply #8 on: August 19, 2013, 07:45:42 PM »

In one or two instances, like the right hand side of the 1st attached photo, I've had ripples of light when a strong point light source scattered through my 24/4 ULD fisheye from just outside the frame. I wonder if it is the same phenomenon as in Tom's shot.

Moving the camera to put that light source behind me, the problem disappeared, as in the 2nd photo.

Ray
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tsjanik

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Re: Aliasing?
« Reply #9 on: August 19, 2013, 09:17:50 PM »

Iíve realized a problem with my diffraction idea, the lines should be parallel to the chair slits and they are not. So Iím going with an in-camera processing/CCD phenomenon, i.e. BCís CCD Gods.

Nice sky Ray.
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ondebanks

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Re: Aliasing?
« Reply #10 on: August 20, 2013, 11:35:00 AM »

Nice sky Ray.

Thanks, Tim. This was taken back when I was getting to know my fisheye and how well it would work on my old DB. Pushing low light stuff (ISO 400 @ 30 seconds).

There was a little too much cloud for my liking - I was mainly aiming to capture the stars peeking between the clouds through the deep twilight; those bright dots may look like hot pixels but in fact they're stars in such constellations as Cygnus and Aquila.

As an image, I think it works better in a B&W conversion...with the contrast raised.

[I'll stop hijacking your thread now!]

Cheers,
Ray
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