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Chromatic aberrations terminology
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PostPosted: Thu Dec 08, 2011 8:25 am    Post subject: Reply with quote
poilu wrote:
AhamB wrote:
poilu wrote:
a sample of sensor bloom
it appear only where the sensor is overcharged (overexposed),
it is called sensor bloom because it doesn't appear on negative who cannot get overcharged


Were those taken with a camera with CCD sensor? CMOS sensors don't bloom.

Explanation of CCD blooming: http://dpanswers.com/content/tech_defects.php#bloom

on a cmos
hey, your link tell the exact opposite of my explanation Laughing


I strongly disagree that CMOS cannot bloom, there has been a very recent issue with a CMOS based camera:

http://www.dpreview.com/news/2011/12/07/fujifilmplansfirmware


PostPosted: Thu Dec 08, 2011 5:13 pm    Post subject: Reply with quote
taunusreiter wrote:
color fringing can be caused by coma or by longitudinal chromatic aberrations (German: Farblängsfehler).

The first should be quite easy to correct for an optical designer. The latter isn't that easy. First, you have to correct the errors with objects in focus, but that don't mean that the objects out of focus are corrected as well. With a zoom lens this is quite impossible.

If you don't have any color fringing it's probably an apochromatic corrected lens.


With LoCA, there should be no fringing with the objects in focus, so there should be nothing to correct (unless you meant objects that you wanted in focus, but missed focus on). Personally, I don't mind this type of CA - I think that in some images it adds "character". PF I don't like, but I'm starting to think that in many cases, it may be exaggerated by WB issues. Lateral CA is bad for wide angles, but I don't use them often, so it's PF issues I usually try to avoid.

One other thing to note is that none of these CA are really relevant for B&W photography. Which is why I find it "interesting" when people provide B&W sample images in support of their arguments that a lens works great.


PostPosted: Thu Dec 08, 2011 5:40 pm    Post subject: Reply with quote
Not quite right. It may result in less sharp edges, even in B&W. But of course it's more visible with color film.


PostPosted: Fri Dec 09, 2011 10:52 am    Post subject: Reply with quote
Ymmot wrote:

I strongly disagree that CMOS cannot bloom, there has been a very recent issue with a CMOS based camera:

http://www.dpreview.com/news/2011/12/07/fujifilmplansfirmware


I think that's a bad example, as Fujifilm didn't use a normal CMOS sensor in their X10, and for all we know the extreme blooming seen in that sensor (only on ISO below 800) may be a firmware problem.


PostPosted: Fri Dec 09, 2011 10:56 am    Post subject: Reply with quote
taunusreiter wrote:
color fringing can be caused by coma or by longitudinal chromatic aberrations (German: Farblängsfehler).

The first should be quite easy to correct for an optical designer. The latter isn't that easy. First, you have to correct the errors with objects in focus, but that don't mean that the objects out of focus are corrected as well. With a zoom lens this is quite impossible.

I have heard the opposite. Having more lens elements means you have more options to address LoCA. The Canon 70-200/2.8L II has 23 elements, for example, and it has very good control of LoCA.

@Laurentiu: I have read that spherochromatism is the combined effect of LoCA and spherical abberation. In other words: wavelength dependent SA.


PostPosted: Fri Dec 09, 2011 7:46 pm    Post subject: Reply with quote
AhamB wrote:

@Laurentiu: I have read that spherochromatism is the combined effect of LoCA and spherical abberation. In other words: wavelength dependent SA.


Thanks - I never heard the term before. While looking for more info on it, I found this interesting older thread:

http://photo.net/canon-eos-digital-camera-forum/00Ko8F

There is some good info from Bob Atkins on page 2.


PostPosted: Sat Dec 10, 2011 3:48 pm    Post subject: Reply with quote
bogolisk wrote:
+1

I'm so confused about that whole colourful mess: LoCA, PP, etc.

I hope some one (like Orio, Arkuu, Chris, etc.) would give a nice explanation.


Its actually quite simple..Lateral CA (LaCA) is caused when a lens is insufficiently corrected to focus the three primary colours simultaneously to the exact same point across the plane of the film or sensor, ie: Some colours can splay or spread away from the ideal focal point on the desired plane of focus, resulting in visble CA, usually at the edges of frame.
It is usually caused by poorly corrected spherical abberation.
Longitudal CA (LoCA) is when a lens is insufficently corrected to focus the three primary colours simultaneously onto the exact plane of the film or sensor, ie some colours can actually be focussed infront of, or behind, the desired plane of focus instead. LoCA can be seen anywhere in an image, not just at the edges of frame, often making it harder to remove than LaCA.
Just because a lens suffers from LaCA does'nt necessarily mean it will also suffer from LoCA, and visa versa...It depends entirely on the optical properties of the lens design, rather than sample variation caused by poor QC of a batch of seemingly identical lenses.
The better quality the lens, the less CA it will exibit of either kind.
Generally speaking, lenses with Aspherical elements usually have the lowest levels of LaCA and LoCA, but if not properly designed even the latter can suffer from either form of CA, so its not a 100% guarentee.


PostPosted: Sat Dec 10, 2011 4:14 pm    Post subject: Reply with quote
DSG wrote:
Generally speaking, lenses with Aspherical elements usually have the lowest levels of LaCA and LoCA, but if not properly designed even the latter can suffer from either form of CA, so its not a 100% guarentee.

Where did you get the idea that aspherical elements are used to correct CA? As far as I know they are used primarily to correct spherical aberrations (SA). The primary means of correcting CA is to use types of glass with different refractive indices, especially partial anomalous dispersion (Fluorite for instance). Most of the basic explanations of CA show a single lens element with the usual CA, and then an achromat doublet. The different glass types help bring different wavelengths into common focus. See: http://en.wikipedia.org/wiki/Achromatic_lens

Perhaps you misunderstand SA -- it's a monochromatic abberation and it's caused by the inherent inability of a spherical lens of non-zero thickness to bring all rays to a common focus. SA is not caused by dispersion (wavelength dependent refractive index, which causes CA) but by lens geometry.

Besides, there are plenty of zoom lenses out there with aspherics and the main thing these elements do is to reduce the number of elements necessary to achieve the desired level of correction (without aspherics, additional elements are necessary to correct SA). It's absolutely no guarantee that the CA of these lenses is lower than other lenses without aspherics.


PostPosted: Sat Dec 10, 2011 5:44 pm    Post subject: Reply with quote
AhamB wrote:

DSG wrote:
Generally speaking, lenses with Aspherical elements usually have the lowest levels of LaCA and LoCA, but if not properly designed even the latter can suffer from either form of CA, so its not a 100% guarentee.

Where did you get the idea that aspherical elements are used to correct CA? As far as I know they are used primarily to correct spherical aberrations (SA). .


Because Lateral CA is caused by poorly corrected spherical abberation, of course. Wink


AhamB wrote:

The primary means of correcting CA is to use types of glass with different refractive indices, especially partial anomalous dispersion (Fluorite for instance). Most of the basic explanations of CA show a single lens element with the usual CA, and then an achromat doublet. The different glass types help bring different wavelengths into common focus. See: http://en.wikipedia.org/wiki/Achromatic_lens
Perhaps you misunderstand SA -- it's a monochromatic abberation and it's caused by the inherent inability of a spherical lens of non-zero thickness to bring all rays to a common focus. SA is not caused by dispersion (wavelength dependent refractive index, which causes CA) but by lens geometry.


I never mentioned dispersion...

AhamB wrote:

Besides, there are plenty of zoom lenses out there with aspherics and the main thing these elements do is to reduce the number of elements necessary to achieve the desired level of correction (without aspherics, additional elements are necessary to correct SA). It's absolutely no guarantee that the CA of these lenses is lower than other lenses without aspherics.


Which is exactly what I already stated.


PostPosted: Sat Dec 10, 2011 9:25 pm    Post subject: Reply with quote
Can't tell if trolling or...

How do you make the connection between LaCA (a chromatic aberration, caused by dispersion) and SA (a monochromatic aberration)? Do you have a reference?

SA should be practically zero on a wideangle lens, especially when stopped down to f/11 or so, but LaCA will stay practically the same at all apertures. Where does SA come into the picture to influence LaCA at all?

Can you mention examples of lenses that have good CA correction due to aspherics?


PostPosted: Sat Dec 10, 2011 10:23 pm    Post subject: Reply with quote
AhamB wrote:
Can you mention examples of lenses that have good CA correction due to aspherics?

Here's one that hasn't - Carl Zeiss 24mm f/1.8.

Quote : Aspherical lenses located on either side of the aperture effectively compensate for distortion, while an ED element suppresses chromatic aberration.


PostPosted: Sat Dec 10, 2011 10:38 pm    Post subject: Reply with quote
DSG wrote:
Because Lateral CA is caused by poorly corrected spherical abberation, of course. Wink

Not everyone agrees. Toothwalker tells us that chromatic aberration is caused by dispersion whereas sperical aberration is a monochromatic effect. But I guess that AhamB has already told us that.


PostPosted: Sun Dec 11, 2011 1:47 am    Post subject: Reply with quote
AhamB wrote:
Can't tell if trolling or...

How do you make the connection between LaCA (a chromatic aberration, caused by dispersion) and SA (a monochromatic aberration)? Do you have a reference?

SA should be practically zero on a wideangle lens, especially when stopped down to f/11 or so, but LaCA will stay practically the same at all apertures. Where does SA come into the picture to influence LaCA at all?

Can you mention examples of lenses that have good CA correction due to aspherics?


Yes, the Canon FD 85mm f1.2 L.
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