Lenses - CA's?

Archived from groups: rec.photo.digital (More info?)

Conrad wrote:

>
> Hi,
>
> On lens comparison site the letters CA's are used. What are CA's?

Chromatic aberrations... Which are colors that shouldn't be there..

Light refracts when it travels through glass. Different wavelengths
refract at different angles. This can cause colored fringing
to appear in photos.. Especially in bright areas. With digital
cameras, purple seems to be a common CA.

Lenses can be constructed in such a way that they can avoid the CA
but it adds to the cost.

Archived from groups: rec.photo.digital (More info?)

Conrad commented courteously...

> On lens comparison site the letters CA's are used.
> What are CA's?

Conrad, a bunch of other folks have already told you about
chromatic aberation, the meaning of CA. I'm adding my 2
cents American to plug Corel's Paint Shop Pro 9 which has
an excellent CA correction tool, as well as a superb
filter called DCNR (Digital Camera Noise Reduction).

If you aren't a PSP 9 user, and have either a CA or noise
issue, I'd encourage you to download the trial version and
see if it helps.

On the other hand, if your question was like some of mine
- simply curiosity - please forgive my intrusion.

--
ATM, aka Jerry

Archived from groups: rec.photo.digital (More info?)

With digital
cameras, purple seems to be a common CA.

Good point but why does it show up more on Digital Cameras.



Good point but why does it show up more on Digital
"Jim Townsend" <not@real.address> wrote in message
news:113pb6g88n45f58@news.supernews.com...
> Conrad wrote:
>
>>
>> Hi,
>>
>> On lens comparison site the letters CA's are used. What are CA's?
>
> Chromatic aberrations... Which are colors that shouldn't be there..
>
> Light refracts when it travels through glass. Different wavelengths
> refract at different angles. This can cause colored fringing
> to appear in photos.. Especially in bright areas. With digital
> cameras, purple seems to be a common CA.
>
> Lenses can be constructed in such a way that they can avoid the CA
> but it adds to the cost.
>
>
>
>
>
>

Archived from groups: rec.photo.digital (More info?)

In article <Conrad.1m5w6y@no-mx.Newsgroup.Gateway>, Conrad
<Conrad.1m5w6y@no-mx.Newsgroup.Gateway> writes
>
>Hi,
>
>On lens comparison site the letters CA's are used. What are CA's?
>
>Thanks,
>
Chromatic aberration.

There are two types of CA caused by lenses, longitudinal and lateral.
There is also a problem caused by the interaction of light with digital
sensors, which is not strictly a lens aberration.

Longitudinal is where light of different colours comes to a focus at
different distances behind the lens; this is caused because short
wavelength (blue) light is refracted (bent) more by the glass lens
elements than longer wavelength (red) light. It is particularly common
in long lenses, but can largely be cured by using fluorite elements, or
ultra-low dispersion glass. This is fortunate, as it is impossible to
correct by post-processing. It results in a loss of sharpness of the
image and lowered contrast.

Lateral chromatic aberration is where light of different wavelengths
forms images of different sizes - it is sometimes called "chromatic
difference of magnification". It results in colour fringing in the
image. Lateral CA is one of the hardest lens aberrations to correct by
lens design, but fortunately it is one of the easiest to correct by
post-processing: as the very digitisation of the image involves
separation into red, green and blue channels, most of the effect can be
removed by careful tweaking of the relative sizes. There are software
packages to do this. I remember being particularly impressed by a piece
on this on Bryan Caldwell's web site (sorry, you'll have to Google him).

The third problem - not, strictly speaking, CA at all - is caused mainly
when the light from the periphery of the lens strikes a digital sensor
at a very fine angle. It is largely an issue of the design of the sensor
and the suitability of the lens for this use. Incidentally, this also
explains why colour fringing is a particular problem with digital
cameras, especially compacts which have less-than-ideal designs.

David
--
David Littlewood

Archived from groups: rec.photo.digital (More info?)

"Charle" <C@comcast.net> wrote in message
news cydnegpbLeBKaHfRVn-jA@comcast.com...
> With digital
> cameras, purple seems to be a common CA.
>
> Good point but why does it show up more on Digital Cameras.

Probably because people never looked at their film images that closely.
(Seriously, film does have problems (including purple fringes!) with the
extreme contrasts that cause purple fringing in digital.)

Also, though, the digital cameras do have far more extreme lenses than the
film P&S cameras did, so you shouldn't be surprised by problems. The great
film P&S cameras often didn't even have zooms. (GR21, Rollei35, etc.)

David J. Littleboy
Tokyo, Japan

Archived from groups: rec.photo.digital (More info?)

David J. Littleboy wrote:

>
> "Charle" <C@comcast.net> wrote in message
> news cydnegpbLeBKaHfRVn-jA@comcast.com...
>> With digital
>> cameras, purple seems to be a common CA.
>>
>> Good point but why does it show up more on Digital Cameras.
>
> Probably because people never looked at their film images that closely.
> (Seriously, film does have problems (including purple fringes!) with the
> extreme contrasts that cause purple fringing in digital.)

Yes... When you view a 6MP image on a home monitor at 100% zoom, it's
the equivalent of viewing a film image printed at 36 inches wide.
That's a pretty big enlargement  

FWIW, You're right. The lens does have a *lot* to do with it.. I have
two telephoto zoom lenses.. My Canon EF 100-300 produces noticable purple
fringing when I use it on my 10D shooting objects with bright highlights.

My EF 100-400 produces NONE.


> Also, though, the digital cameras do have far more extreme lenses than the
> film P&S cameras did, so you shouldn't be surprised by problems. The great
> film P&S cameras often didn't even have zooms. (GR21, Rollei35, etc.)
>
> David J. Littleboy
> Tokyo, Japan

Archived from groups: rec.photo.digital (More info?)

"Jim Townsend" <not@real.address> wrote:
> David J. Littleboy wrote:
>
> FWIW, You're right. The lens does have a *lot* to do with it.. I have
> two telephoto zoom lenses.. My Canon EF 100-300 produces noticable purple
> fringing when I use it on my 10D shooting objects with bright highlights.
>
> My EF 100-400 produces NONE.

You must be quite wrong here: Stacey's told us over and over again how
terrible a lens that is. Not.

David J. Littleboy
davidjl@gol.com
Tokyo, Japan

Archived from groups: rec.photo.digital (More info?)

David J. Littleboy wrote:

>
> "Jim Townsend" <not@real.address> wrote:
>> David J. Littleboy wrote:
>>
>> FWIW, You're right. The lens does have a *lot* to do with it.. I have
>> two telephoto zoom lenses.. My Canon EF 100-300 produces noticable purple
>> fringing when I use it on my 10D shooting objects with bright highlights.
>>
>> My EF 100-400 produces NONE.
>
> You must be quite wrong here: Stacey's told us over and over again how
> terrible a lens that is. Not.

:-)