Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-03-2009, 04:11 PM   #1
Veteran Member
glasbak's Avatar

Join Date: Apr 2009
Location: Netherlands
Posts: 343
Purple Fringing on Film

Some people think purple fringing is a problem what only happens on digital, and must be related to the CCD/CMOS sensor and not to the lens.
I think it is only chromatic aberration made visible by the different sensitivity of the CCD/CMOS sensor, compared to film.
On film it takes some more overexposure to see the same effect.

Here an example of my early astrophoto work, its comet Austin (1990?) with a pentax 85mm wide open on ektachrome 400 slide film.



And a center crop:




And you will see, blue fringing, even at the center of the image.
I learned then two things :
- never shoot wide open with fast lenses, always stop down one or two clicks, unless you want colorful blobs i.s.o. stars.
- you can't record red nebula's with ektachrome 400 (its red sensitivity sucks, so no purple but blue fringes)


As a consequence, I do not have a lot of wide open shots which show extreme CA samples.
But this Pentax 135mm shot on kodak gold 100 shows what we used to call chromatic aberration, but now its better known as 'purple fringing'.

Comet Neat (may 2004):



And a center crop:



In the universe, purple stars are extremely rare, but not on this image.


On this dpreview page they have an example (leaves) of purple fringing, compare that with the samples on this page.
The aberations seem fairly identical, no microlenses required for this.

10-03-2009, 06:00 PM   #2
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,312
there are two aspects to PF, one is sensor related and the other is good old CA that has been around for ever.

CA was ovisible on my 300F4 when I shot film so why wouldn't it be there on digital?
10-04-2009, 03:55 AM   #3
Site Supporter
knyghtfall's Avatar

Join Date: Jun 2007
Location: Singapore
Photos: Gallery
Posts: 348
hmm... is it purple fringing, or some other aspect of light bending when it hits the atmosphere, etc etc resulting in the colour shift? Any pentaxian physicists out there who can shed some light on this issue?
10-04-2009, 05:37 AM   #4
Veteran Member
ytterbium's Avatar

Join Date: Jan 2008
Posts: 1,076
PF is a result of CA. But digital sensors see more colors and render them differently than film, and reproduce them differently.

Its like colour and BW film if i exaggerate. In what colour a B&W film would show you CA's if the lens had them? I believe it would show as slight softnes around contrasty edges, or wouldn't even show at all if the BW film was sensitive mostly to green colors.

So you still get PF FROM CA on film where the lens has less correction for blue colour. If the digital sensor sees even further than blue and deep violets where the lens correction is even worse and probably decreases by power of two or three (depending on the correction "strength" in visible range) you need only small ability to see some more of deep violets to get much more PF.
And its not even certain that the fringe is actually purple or blue, because you can capture IR diode of remote control in blue colour. It is only registered by blue photosites.

Film still sees some of the light not corrected by lens, so with longer exposure and dark background, you can accumulate that light long enough for it to become visible.

Btw just discussed in here: https://www.pentaxforums.com/forums/pentax-dslr-discussion/75319-k7-compared-...0-ca-pf.html#2

Here i made an example of specific colour extraction in GIMP (that's close to desaturation of cyan/magenta in photoshop, but with little original colour recreation):

With that i tried to illustrate, that if sensor was blind to some colours or registered them in a different way (like CMY layer in film), can make the camera blind to uncorrected light or not render it so obviously. In reality such processing done in software can end up really bad, because this unneeded colour information has already been mixed in some useful colour channel:

P.S. I adjusted Magenta-Yellow layer transparencies "blind" and separately for G+(M-Y) and B-(M-Y), and only saw the result after recomposing grayscale channels so i could have missed the colour balance a bit. I did this with a JPEG file as well, where the colours have already been shifted by sRGB, Bright color mode and jpeg transforms, so this is not an actual violet extraction. It should be done RAW.

If you've red on colour theory, you can register the same colour scene in very different ways. Most visible/colour images in google earth for example are actually captured as multi "colour" component infra red images. Images seen are calculated afterwards by knowing which infra red "colours" sometimes happen to match with which visible colours.


Last edited by ytterbium; 10-04-2009 at 06:45 AM.
10-04-2009, 12:25 PM   #5
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,167
I am the guy who has never seen PF on film. The star pictures are interesting, except that I don't know why you would consider it to be PF as opposed to plain old CA. What I would be interested in seeing is the typical purple fringing of specular highlights on water or crumpled cellophane that are generally used to demonstrate the phenomenon.

I am actually pretty open minded and am willing to admit that I am often wrong. I am also willing to admit that I have never done a controlled test for film vs. digital regarding PF. It seems that there are a limited number of possibilities:
  • PF is a digital issue (lens-sensor interaction)
  • PF occurs on film and I have just never seen it
  • PF occurs on film, but the film is not able to record it
  • PF does not exist per se, but is just plain old CA amplified by the sensor
If I get a chance, I will try a controlled setup where I will try to get some good PF on digital and swap out the K10D for one of my film camera with the same lens. If it is a lens issue, the same aberrations should be visible for both shots. This assumes, of course, that the film is capable of detecting any PF that might be present.

Steve
10-04-2009, 02:37 PM   #6
Veteran Member
glasbak's Avatar

Join Date: Apr 2009
Location: Netherlands
Posts: 343
Original Poster
QuoteOriginally posted by stevebrot Quote
The star pictures are interesting, except that I don't know why you would consider it to be PF as opposed to plain old CA.
Well, why would PF be something different than plain old CA, but in the way it manifests itself on the silicon sensor.

QuoteOriginally posted by stevebrot Quote
  • PF is a digital issue (lens-sensor interaction)
  • PF occurs on film and I have just never seen it
  • PF occurs on film, but the film is not able to record it
  • PF does not exist per se, but is just plain old CA amplified by the sensor
option 4 describes best how I see it.
You need a serious overexpose to see it on film, thats why it shows on star images.

If someone can tell me how to get PF regardles of lens mounted, as a pure silicon sensor (+microlenses?) effect on my K10D, please tell me so I can try to reproduce it.
But until now, all PF I have seen on my K10D is plain old CA.

George
10-04-2009, 03:17 PM   #7
Veteran Member
krypticide's Avatar

Join Date: Dec 2007
Photos: Gallery
Posts: 1,079
All the PF problems I see on digital have been sharp. I think that's what bothers me the most. It just screams "digital" to me. At least the PF on film is grainy.
10-04-2009, 04:37 PM   #8
New Member




Join Date: Oct 2009
Location: Ashland, OR
Photos: Gallery
Posts: 7
Hi,
Purple fringing absolutely does occur on film and has always been an issue. It tends to get worse with the focal length, being more noticeable in long lenses, and APO lenses were used to defeat it. I believe reasons it can be more evident on crop sensor DSLRs is that 1. the photos need more enlargement to get to a given size and 2. we are now able to zoom in on the screen and detect them. In the past, unless you really enlarged the photo from film, you may have not even noticed the phenomenon.
Scott

10-05-2009, 08:24 AM   #9
Site Supporter
gofour3's Avatar

Join Date: Oct 2008
Location: Vancouver, Canada
Posts: 5,119
QuoteOriginally posted by stevebrot Quote
I am the guy who has never seen PF on film. The star pictures are interesting, except that I don't know why you would consider it to be PF as opposed to plain old CA. What I would be interested in seeing is the typical purple fringing of specular highlights on water or crumpled cellophane that are generally used to demonstrate the phenomenon.

I am actually pretty open minded and am willing to admit that I am often wrong. I am also willing to admit that I have never done a controlled test for film vs. digital regarding PF. It seems that there are a limited number of possibilities:
  • PF is a digital issue (lens-sensor interaction)
  • PF occurs on film and I have just never seen it
  • PF occurs on film, but the film is not able to record it
  • PF does not exist per se, but is just plain old CA amplified by the sensor
If I get a chance, I will try a controlled setup where I will try to get some good PF on digital and swap out the K10D for one of my film camera with the same lens. If it is a lens issue, the same aberrations should be visible for both shots. This assumes, of course, that the film is capable of detecting any PF that might be present.

Steve
I’ve been shooting slide film since the early 70’s and have never noticed purple fringing either. That’s with enlargements up to 24 x 36 and via a slide projector.

This might be due to the lenses I mostly used (K28/2.5, K55/1.8 & K135/3.5) and filters.
I read that you can help reduce purple fringing on film by using a good UV filter???

Phil.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
center, crop, ektachrome, film, image, k-mount, page, pentax, pentax lens, samples, sensor, slr lens, stars
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Purple Fringing stevewig Digital Processing, Software, and Printing 13 09-08-2010 01:01 PM
Purple fringing on DA* 16-50 spinmar Pentax SLR Lens Discussion 14 02-03-2009 07:03 AM
Purple Fringing paolojackson Digital Processing, Software, and Printing 2 10-06-2008 05:25 PM
Oh no! Purple fringing! NaClH2O Post Your Photos! 6 03-27-2008 05:28 AM
Purple Fringing Stratman Pentax SLR Lens Discussion 7 04-29-2007 01:07 PM



All times are GMT -7. The time now is 10:21 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top