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02-23-2012, 09:46 PM   #46
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QuoteOriginally posted by wlachan Quote
The way I see it, lens CA and sensor blooming are 2 different issues. PF is a generic term which can be associated to either one, but somehow being firmly associated to sensor blooming by some. I have no idea why.
I always prefer to use precise language when discussing technical matters.

What I can't understand is why some people want to call all purple or bluish aberration PF? If a lens has CA, it is easily detected on the optical bench or by using tools like Imatest. PF (sensor bloom) is not easily tested for except by actual photographs of stereotypic subjects (cellophane, chrome, and such). PF can ruin an otherwise good photo. CA can keep all photos taken with a particular lens from being as good as they might be otherwise.

PF (sensor bloom) is distracting and a pain (the bane of digital wedding photographers), but CA is more than just false color. CA degrades the image as a whole.


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02-23-2012, 09:47 PM   #47
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QuoteOriginally posted by Lowell Goudge Quote
As one of those who would prefer purple fringing to be only addressing sensor bloom I will reiterate my earlier statements. Fringing of any color due to CA is and should always be called CA and not purple fringing. Call it what it is don't confuse the issue by using imprecise or vague terms when a correct accurate and precise term exisis. If you want a good example of how messed up things get, just look at the "crop factor" and all the confusion it has caused. By dumbing down the terms used we throw people into confusion when there is no need
I am by no mean have the last say on any definition but rather interested in the truth of any discussion. If confusion is the issue, then SB (sensor blooming) will be the correct and precise representation of sensor blooming. Purple fringing itself is vague to begin with. To this day, there is no concrete definition that PF must mean sensor blooming. Until then, PF is a free representation of any fringe in purple colour. But I guess this will be my last reply to this futile discussion.
02-23-2012, 09:48 PM   #48
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QuoteOriginally posted by Winder Quote
If we are going to be exact then we should call it axial chromatic aberration.
I have seen both axial (or longitudinal) and lateral CA called purple fringe. To me as long as we call it CA and add an adjective when or if needed we are accurate enough
02-23-2012, 09:57 PM   #49
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QuoteOriginally posted by Lowell Goudge Quote
Call it what it is don't confuse the issue by using imprecise or vague terms when a correct accurate and precise term exisis.
Unfortunately, the term "purple fringing" is by its nature generic and imprecise. It describes the symptoms, not the cause. That means it covers any purple fringes, regardless of what effect produces them. Trying to make the term apply to only "a special type of purple fringing" goes a bit against its own nature.

02-23-2012, 11:26 PM   #50
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QuoteOriginally posted by Lowell Goudge Quote
What I see is CA not purple fringing, so let's define the two, because they are different.

CA is the behavior of the lens to focus different colors at different spots, there are two principle types, longitudinal CA where one in turnout of focus regions, you generally get green fringing behind the plane of focus, and purple in front of the plane of focus. The other is Lateral CA where as you move out from the center you get purple fringing on one side of a subject and Green on the other.

True purple fringing is a bright purple halo arund all sides of a subject that has a very high contrast to the background. It is a property of the photo sensors

One of the best ways to avoid purple fringing is to use a flash in back light situations, the other is to shoot film for those shots, since film does not display purple fringing.

There is no way to avoid CA in a lens, if it has it it will be there, but one way to reduce the impact of CA is to avoid high contrast out of focus regions.
Capture One has separate tools for dealing with CA and PF. They work reasonably well, but there are times when no amount of post-processing can completely remove CA or PF.

Rob
02-24-2012, 09:09 AM   #51
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QuoteOriginally posted by robgo2 Quote
Capture One has separate tools for dealing with CA and PF. They work reasonably well, but there are times when no amount of post-processing can completely remove CA or PF.

Rob
I use PSP X4 which also has deprecate CA and PF tools. The PF tool is specifically tuned for PF from sensor bloom, theCA tool is a fringe removal tool where you pick the fringing you want to remove and the pixel radius. It works well for out of focus CA but I also have a plug in that rescaled the color layers for lateral CA in the focus plane since each color has slightly different magnification for lateral CA. This gets back edge sharpness

I ran a thread in the processing forum where I posted correction by fringe removal and rescaling and made a raw file a available to members to use their programs to evaluate the different tools
02-25-2012, 11:49 AM   #52
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QuoteOriginally posted by everydaylife Quote
When I asked Pentax why the 77 Ltd exhibited purple fringing a Pentax technician explained that it was mainly because the FA lenses are optimised for film therefore do not focus the light at exactly the right focal plane because the sensor is much thinner than 35mm film.

I am sorry I cannot remember exactly how he explained it so this explanation may be a bit thin so I hope someone else may be able to explain it in a more technical way.
You're correct, the grain of film has some thickness to it so the senstive layer is thicker and it's even stack as far as i know, with digital you have one nice orderly laid out pattern and the sensative part is simply the surface.

But how much this will effect everything, i don't know...
02-25-2012, 11:55 AM   #53
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QuoteOriginally posted by stevebrot Quote
As for why it is more prevalent with some lenses...the last time I looked, it was thought that angle of incidence to the sensor might be the issue along with the size of the rear element. My big question is why, if it a lens thing, is there no mention of PF in the photo magazines, books, and reviews prior to the digital era? I have been active in photography since the late 1960's and never heard of PF until I was shopping for my Canon G2.
The tamron 70-300 is known is a PF monster but the design of the lens itself is not so different compared to other longer lenses, and with these long focal lengths the angle at which the light hits the sensor is also pretty straight.
I at least can not blame the sensor in anyway why the Tamron is preformaing so poor here compared to other lenses around this range so it must be a lens design.

I would love to see the Tamron on a film camera just to see if it PF on it as well.

02-25-2012, 12:06 PM   #54
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QuoteOriginally posted by Anvh Quote
You're correct, the grain of film has some thickness to it so the senstive layer is thicker and it's even stack as far as i know, with digital you have one nice orderly laid out pattern and the sensative part is simply the surface.

But how much this will effect everything, i don't know...
The forum is acting so weird since i can't change my comment.

But wanted to say that the thickness might indeed effect on the longitudinal chromatic aberration and maybe that will cause the PF.
If you change the focus the colour should change as well as far as i know, so you can try that out with the Tamron.
02-25-2012, 01:14 PM   #55
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QuoteOriginally posted by Anvh Quote
The forum is acting so weird since i can't change my comment.

But wanted to say that the thickness might indeed effect on the longitudinal chromatic aberration and maybe that will cause the PF.
If you change the focus the colour should change as well as far as i know, so you can try that out with the Tamron.
This idea has been discussed at length on various forums and is a good possibility. Also discussed is the nature of the micro-lenses that are part of each sensor location. The uniformity, alignment, and reflectivity (internal and external) of these extremely tiny lenses may influence the propagation of stray light at certain wavelengths to adjoining sites. That is why so much attention is given to light striking the sensor at oblique angles.


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02-25-2012, 02:10 PM   #56
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QuoteOriginally posted by stevebrot Quote
That is why so much attention is given to light striking the sensor at oblique angles.
But if we take for example the Tamron 70-300 again, at 300mm the light already hits the sensor very straight so surely you can't not just blame the sensor for this no matter how you look at it.
02-25-2012, 03:05 PM   #57
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QuoteOriginally posted by stevebrot Quote
This idea has been discussed at length on various forums and is a good possibility. Also discussed is the nature of the micro-lenses that are part of each sensor location. The uniformity, alignment, and reflectivity (internal and external) of these extremely tiny lenses may influence the propagation of stray light at certain wavelengths to adjoining sites. That is why so much attention is given to light striking the sensor at oblique angles.


Steve
Actually this is the explanation for vignetting not sensor bloom. But that is addressing non retrofocus wide angles
02-25-2012, 04:09 PM   #58
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QuoteOriginally posted by Lowell Goudge Quote
Actually this is the explanation for vignetting not sensor bloom. But that is addressing non retrofocus wide angles
Applies to both, Lowell. Visualize a low-angle ray striking the surface of a microlens and imagine the unavoidable refraction/reflection to places where it is not supposed to be.


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02-25-2012, 04:25 PM   #59
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QuoteOriginally posted by Anvh Quote
But if we take for example the Tamron 70-300 again, at 300mm the light already hits the sensor very straight...
One would think, however the angle to the sensor is not related to focal length per se, but to the size of the sensor and distance to the rear element. Light from a long lens is incident at the same angle as light from a short lens. That is what the lens is for. A wide-angle fills the same frame as a tele.

That being said, there seem to be more PF problems with lenses originally designed for 35mm film. Stray light at lower angles? Larger rear element diameter? You got me. It has been a long time since I took physics. I do know that a few of the lenses in my collection have more of a tendency to PF(sensor bloom) than the others; the FA 77/1.8 limited (not real bad actually), the Tamron 70-150/3.5, and the Zenitar fisheye. The others are PF-free for the most part.


Steve

Last edited by stevebrot; 02-25-2012 at 04:55 PM.
02-26-2012, 08:51 PM   #60
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QuoteOriginally posted by Anvh Quote
The tamron 70-300 is known is a PF monster but the design of the lens itself is not so different compared to other longer lenses, and with these long focal lengths the angle at which the light hits the sensor is also pretty straight.
I at least can not blame the sensor in anyway why the Tamron is preformaing so poor here compared to other lenses around this range so it must be a lens design.

I would love to see the Tamron on a film camera just to see if it PF on it as well.
That Tamron is strange. On one day, I was getting PF issues in almost any shot. Since then, I hardly had any PF issues with it. I am still not sure what makes it produce PF. I like the lens - for its price, it is very capable.
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