I always believe the fringing problem is mainly from the lens (as they
spread much wider than a few pixels). I still not understand why it is
caused by the image sensor. I have tested some zuikos, they all have
fringing problem on the blown areas wide open but eliminated when stopped
down (of course the exposure time is adjusted with lens stopped down).
C.H.Ling
----- Original Message -----
From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
To: "Olympus Camera Discussion" <olympus@xxxxxxxxxxxxxxxxx>
Sent: Sunday, February 23, 2014 10:39 PM
Subject: Re: [OM] IMGS: Rug Weaver
> If the fringing is reduced by stopping down then the 180/2 suffers from
> axial chromatic aberration. Check this Wiki description of CA types.
> Note especially the last paragraph which discusses the difficulty of
> correcting axial CA relative to transverse CA.
>
> Chuck Norcutt
> --------------------------------------------------------------------------
> There are two types of chromatic aberration: axial (longitudinal), and
> transverse (lateral). Axial aberration occurs when different wavelengths
> of light are focused at different distances from the lens, i.e.,
> different points on the optical axis (focus shift). Transverse
> aberration occurs when different wavelengths are focused at different
> positions in the focal plane (because the magnification and/or
> distortion of the lens also varies with wavelength; indicated in graphs
> as (change in) focus length). The acronym LCA is used, but ambiguous,
> and may refer to either longitudinal or lateral CA; for clarity, this
> article uses "axial" (shift in the direction of the optical axis) and
> "transverse" (shift perpendicular to the optical axis, in the plane of
> the sensor or film).[4]
>
> These two types have different characteristics, and may occur together.
> Axial CA occurs throughout the image and is specified by optical
> engineers, optometrists, and vision scientists in the unit of focus
> known widely as diopters,[5] and is reduced by stopping down. (This
> increases depth of field, so though the different wavelengths focus at
> different distances, they are still in acceptable focus.) Transverse CA
> does not occur in the center, and increases towards the edge, but is not
> affected by stopping down.
>
> In digital sensors, axial CA results in the red and blue planes being
> defocused (assuming that the green plane is in focus), which is
> relatively difficult to remedy in post-processing, while transverse CA
> results in the red, green, and blue planes being at different
> magnifications (magnification changing along radii, as in geometric
> distortion), and can be corrected by radially scaling the planes
> appropriately so they line up.
> ------------------------------------------------------------------------
>
> On 2/23/2014 5:24 AM, C.H.Ling wrote:
>> I have not idea how to judge where the purple fringe came from but I'm
>> sure
>> OM 180/2.8 has purple fringe with both film and digital. At the mean time
>> the pruple fringe can be reduced by stop down the lens even on a digital
>> sensor.
>>
>> C.H.Ling
>>
>> ----- Original Message -----
>> From: "Chuck Norcutt" <chucknorcutt@xxxxxxxxxxxxxxxx>
>>
>>
>>> Regardless of what you think the lenses are guilty of, what you showed
>>> on that image is not chromatic aberration. It's not the lenses but
>>> could be the sensor combined with blown image areas. I don't think
>>> you're using Canon lenses on the Leica are you? Perhaps you're more
>>> likely to overexpose the Canon? Rather than complain about the lenses
>>> or software you need to critically examine some of them as I did that
>>> one to get to the real cause. It could still be hardware but it's not
>>> likely the lenses.
>>>
>>> Chuck Norcutt
>>>
--
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