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01-03-2008, 08:58 PM   #1
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Why CA in most New Pentax lenses?

Somebody please educate me. With improvement in knowledge and technology, one should be able to overcome atleast few of the problems.

Most of the Pentax lenses like 16-45, 16-50 (supposedly professional grade), Limited Series seem to suffer from CA while cheap Pentax 18-55 or a Sigma 17-70 seem to be relatively free.

Please don't tell me that it can be easily corrected in PP and is not an issue at all. A correction is always a correction.

01-03-2008, 09:31 PM   #2
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The DA 16-45mm has some CA at the wide end, all the other lenses are perfectly fine. CA is very controlled in those lenses.
01-03-2008, 09:58 PM   #3
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Yea, I think I've actually only seen significant CA in a few of my shots ever... If memory serves, they were shot with the kit lens 18-55. I don't have the 16-45, so I can't comment on that, but I'd have to disagree with your generalization based on personal experience, I got quite a few DA's and no problems really.
01-04-2008, 04:16 AM   #4
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From the lenses I had tried, Fa 135/2.8 + Fa* 200/2.8 CA quite badly wide open.

To some degrees, Da* 16-50 Ca as well with direct sources.

Otherwise I had not seen much in the rest of lenses I tried...

01-04-2008, 07:25 AM   #5
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QuoteOriginally posted by skamaraju Quote
Somebody please educate me. With improvement in knowledge and technology, one should be able to overcome atleast few of the problems.

Most of the Pentax lenses like 16-45, 16-50 (supposedly professional grade), Limited Series seem to suffer from CA while cheap Pentax 18-55 or a Sigma 17-70 seem to be relatively free.

Please don't tell me that it can be easily corrected in PP and is not an issue at all. A correction is always a correction.
There are many compromises that must be made in the design of a zoom lens. ( even a prime lens) The emphasis of a design might be low geometric distortion or high resolution or low CA so a lens might have lower CA at the expense of lower resolution or higher distortion. To make a fully APO zoom lens may not even be possible, at least for a selling price that us mere mortals could afford.

The problem is that visible light is made up of many discrete frequencies and each of those frequencies have a differing amount of refraction through optical glass. To put it another way, the red frequencies come to focus at a different distance than do the blue frequencies and/or the green frequencies. So the trick is to try to make all those frequencies come to focus at the same distance.

Example:



This is a chart for a moderately expensive (~ 6500 Euros) APO optical tube assembly (basic telescope). This is a 3 element APO 980mm f/7.0 'prime lens' that still doesn't bring all colors to focus at the same point. (they spec it as .02% of the focal length which is VERY good)

So all in all I think modern zoom lenses do a pretty good job given the limitations of physics.
01-04-2008, 09:21 AM   #6
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Thank you Rick for a simple explanation. Reviews in Photozone.de seem to show it for almost all the lenses in pentax stable. I did not check other brands in detail to know if they are proportinately there too.
01-04-2008, 10:34 AM   #7
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QuoteOriginally posted by skamaraju Quote
Thank you Rick for a simple explanation. Reviews in Photozone.de seem to show it for almost all the lenses in pentax stable. I did not check other brands in detail to know if they are proportinately there too.
They are.

The Pentax kit zoom though is a stand out in that it's performance is quite a bit ahead of similar kit lenses of other brands ...

Quote from Photozone.de rs: Pentax 18-55mm kit lens

"Lateral chromatic aberrations (color shadows at harsh contrast transitions) surprisingly well controlled with an average pixel width around 1px at the image borders at 18mm and less so beyond. Very few standard zooms show such a moderate CA characteristic."

I've found that if I set my 18-55mm kit lens to 35mm at F/5.6 - F/8.0 I can nearly get 'prime lens' performance. Not bad for a $90 lens.

One note: You can not directly compare CA performance between different mfg on Photozone.de as he uses different camera bodies and his results are based pixel widths which vary from camera to camera. You would need an optical bench to test the lenses independent of the sensor for comparative results.

Read this page for more info ...

Lens Test FAQ

Last edited by Rickster; 01-04-2008 at 10:50 AM.
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