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Comparing six lenses at 135mm
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PostPosted: Wed Jan 21, 2009 9:09 pm    Post subject: Comparing six lenses at 135mm Reply with quote
Here are 6 lenses tested at 135mm, one zoom in KAF2 mount, one prime in K, and 4 different Takumars in M42. All pics were taken on tripod, RAW > jpeg conversion only, crop100%, the lighting (sun) was almost constant during the test, a little bit more shining for the forst two lenses, I volontary corrected nothing.

It's here (2000x2000, 1800ko) : http://pix.nofrag.com/d/f/c/78d884bf27c0f5782f75c165b964f.html

Wink


PostPosted: Wed Jan 21, 2009 9:26 pm    Post subject: Reply with quote
Thanks for the great comparison work! Nice to see the differences.

While the old pre-set Takumar (I have that one also, also in Asahiflex mount and in chrome) seems a little less sharp it still strikes me that this old lens has almost no red fringing around highlights in the lens of your subject, while the newer ones do. Do you have an explanation?

I saw the same in older, non-MC lenses. For instance the Jupiter-11 and Tair-11-2 are old and not multicoated, but there's also no fringing to be seen. Strange, but I do not have much optical knowledge. What do the experts on this forum say?


PostPosted: Wed Jan 21, 2009 9:31 pm    Post subject: Reply with quote
Many forms of purple fringe are caused by axial (longitudinal) chromatic aberration.


PostPosted: Wed Jan 21, 2009 9:38 pm    Post subject: Reply with quote
no-X wrote:
Many forms of purple fringe are caused by axial (longitudinal) chromatic aberration.


OK, but what does that mean in this case? Are you implying that the later lenses are less corrected for aberrations?


PostPosted: Wed Jan 21, 2009 10:09 pm    Post subject: Reply with quote
The most common form of CA is transverse CA = contrasty lines near image borders are surrounded by colored lines (red/cyan). This form of CA can be easily corrected in post-processing. This form of CA is visible mostly in in-focus areas. Changing aperture doesn't fix it. It doesn't influence bokeh.

But axial CA is almost invisible in IF areas, but it displays in (even slightly) out of DOF areas. Because this form of CA doesn't affect in-focus objects, it doesn't affect lens resolution - and the resolution was (and is) the main priority for many manufacturers. Why to care for axial CA, when ISO 12233 chart, kodak tables etc. must be in focus while picturing them, so this defect won't show?

Priority is resolution and correction of transverse CA - these can be easily measured, they are visible on testing charts and most of reviews judge lenses according to these aspects. Axial CA can't be easily measured, it affects bokeh (bokeh can't be measured too), so it is no wonder, that correction of axial CA is not the main priority for manufacturers.

I'm not expert, but I think that axial CA is a fallout of other corrections, especially of those, which increase sharpness. Majority of old (40+) lenses don't show this issue.


PostPosted: Wed Jan 21, 2009 11:42 pm    Post subject: Reply with quote
Hello,

Thanks for the great explanation ...

Best regards


PostPosted: Thu Jan 22, 2009 8:00 am    Post subject: Reply with quote
no-X wrote:
Many forms of purple fringe are caused by axial (longitudinal) chromatic aberration.


I believe that purple fringing is mostly caused by electrons bleeding between individual sensors of a digital camera's light-sensitive matrix. I've never seen purple fringing on film. This way, purple fringing is not CA. In this theory, older lenses don't have purple fringing because of their lower contrast and acutance.


PostPosted: Thu Jan 22, 2009 8:58 am    Post subject: Reply with quote
In this case, there would be no difference in purple fringe intensity between different lenses and existance of purple fringe wouldn't be dependant on DOF. I wouldn't be dependant on aperture settings, too. But in reality, it depends on particular lens, aperture value and DOF. Digital sensors can amplify these defect (because they can be more sensitive to specific wave-length), but they aren't the cause.

Sensor bloom and purple fringe are two different phenomenons. It would be quite difficult to explain it in English for me, so here is scientific explanation...

Quote:
Purple fringing is often blamed on sensor bloom, which is odd as blooming is a quite different phenomenon [14, 15]. In fact, there are as many arguments against sensor bloom as there are in favor of chromatic aberration to account for purple fringing. The examples shown on the present page are all demonstrably due to the lens and not to the sensor. Sensor bloom has no known color preference, and if it had, it would not change its colors upon defocusing a lens (as in Fig. 3) or upon swapping lenses (as in Fig. 4). The list of arguments is long [16].


more info and examples:

http://www.vanwalree.com/optics/chromatic.html


PostPosted: Thu Jan 22, 2009 9:28 am    Post subject: Reply with quote
On a side note: at home I have cable TV with a HD Receiver and a Samsung HD Plasma screen. Some of the channels I like are National Geographic HD, Discovery HD and History Channel HD. I cannot remember things like CA from the past, but in several documentaries (apparently shot with digital HD cameras) I see green and purple fringing all over the place, most notably in wide angle shots. Is this a sign of not-so-good lenses being used?


PostPosted: Fri Jan 23, 2009 4:09 pm    Post subject: Reply with quote
Spotmatic wrote:
no-X wrote:
Many forms of purple fringe are caused by axial (longitudinal) chromatic aberration.


OK, but what does that mean in this case? Are you implying that the later lenses are less corrected for aberrations?


I think it means that the later lenses are computer-optimised - but for other things, like in-focus sharpness and removal of transverse CA. That means that longitudinal CA is either not optimised for, or is actually made worse by optimising for other things.

Transverse CA is can be removed (or at least reduced) during RAW processing. Longitudinal CA cannot, it seems.
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