Is this a CA problem?

[zhaowei]

Hi everyone,

I'm relatively new to DSLR photography and I'm still trying to pick up all the jargon that fly around photo sites and forums. One term I thought I understood was Chromatic Aberrations (CAs).

Up till recently, I've only shot with my Nikkor 16-85mm VR and the double zoom kit lenses, ie the 18-55mm and 55-200mm VR. I usually do some slight retouching of photos after I upload them to the computer, and I usually zoom in to within the 25% to 100% range while doing that.

One thing I don't think I've seen is CAs in my pictures. I think it's mostly because the three aforementioned lenses are not known for high CAs, plus the fact that I don't shoot wide open with them much anyway.

Recently, I bought a new lens - the Nikkor AFS 60mm. Eager to test it out to make sure I got a good copy, I've been shooting random objects all over the place. However, I noticed that in a few shots I took at the camera store, there seems to be something resembling CAs, with greenish and purplish fringes on the metal screw in the picture. From what I understand, CAs can only be noticed if you zoom in quite a bit, but for this picture, even at 20% or less the purple and green fringes are impossible to be missed.

Original shot, f/3.8, 1/80sec, ISO 400, -1/3EV, and yes, I shot a tripod handheld

50% centre crop.

Can anyone help verify what's the problem here? I'm honestly clueless about what's wrong here. I'm wondering if it's simply because I'm shooting a metal surface.

Thanks in advance for replying!

Zhaowei

PS. I'm actually very happy with my 60mm, especially with the excellent bokeh I could never achieve with the other lenses I have now. At the end of the day, I'm just trying to ensure I got a good copy. I think anyone who paid good money for expensive equipment has that basic right. The store I bought from has a 2-week exchange policy, so I can always go back if there's anything wrong. ;p

 

[icebox]

Hm I've not seen CA manifested in such a manner (but I may be wrong! ). It's usually around the edges of objects with light coming from the back. Could it be some funky lighting in the camera shop, or there's some red/purple/pink object hovering around?

 

[-ReignOfComputer-]

looks like a reflection.

 

[Rashkae]

Yup, that's known as lateral CA. It's normal for that lens. It should mostly disappear by f/4

 

[zhaowei]

Hi, icebox and -ReignOfComputer-, thanks for dropping by. I think it's unlikely to be caused by reflection because I've observed this in other shots under different lighting as well.

Yup, that's known as lateral CA. It's normal for that lens. It should mostly disappear by f/4

Hi Rashkae, thanks for pointing this out. I guess I can sleep easy now.

Just another question, is lateral CA a normal "flaw" for macro lenses in general?

 

[Rashkae]

Hi, icebox and -ReignOfComputer-, thanks for dropping by. I think it's unlikely to be caused by reflection because I've observed this in other shots under different lighting as well.



Hi Rashkae, thanks for pointing this out. I guess I can sleep easy now.

Just another question, is lateral CA a normal "flaw" for macro lenses in general?

For older lenses and some newer ones, yeah. It depends on the lens coatings; from what i've seen, lenses with coatings designed to reduce flare to a great job at that, but usually have lateral CA. Lenses with "glare issues" tend to have the more traditional blue/purple CA.

 

[camoptics]

From my understanding, this may be longituginal chromatic aberration, which means the on-axis focal point for different color is not same. And this aberration is in the center of the photo which differs position from Lateral chromatic aberration (at corner).

In this shot, the center object is highly reflective material with shining surface, this enhance the CA.

 

[ST1100]

i've seen CA, but i've never seen both purple and green CA so close together, so severe, and smack in the middle of the image (where the lens is most "corrected" - it's the edges that are CA-compromised). i have no experience with this lens. but i would go back to the shop and try another one, and if this phenomenon is "normal" for this lens, i would return the lens or sell it off. You would spend hours correcting CA on software if that performance is "normal". Not quite acceptable, IMHO.

 

[bigpond]

Think you have a point here. Does not look too much like CA to me.

Why not try and shoot some trees against a bright sky at various apertures -- this will give you a much better idea of the CA handling of this lens.

 

[zhaowei]

Hi camoptics, ST1100 and bigpond, thanks for joining the discussion.

From my understanding, this may be longituginal chromatic aberration... In this shot, the center object is highly reflective material with shining surface, this enhance the CA.

Why not try and shoot some trees against a bright sky at various apertures -- this will give you a much better idea of the CA handling of this lens.

I tried taking some shots of houses against the bright sky just now. At f2.8 wide open, there are some green fringes in some parts (near the centre) visible when I zoom in on my camera LCD. At f5.6, the green fringes disappear.

Also, I took a few snaps of a newspaper from a slanted angle. At f3.2 (couldn't go faster due to the focus distance) there were some purple tinges in the out-of-focus foreground and some green tinges in the out-of-focus background. The centre was free of any CA. At f5.6, the green tinges disappeared but some vestiges of the purple remained, though I think it can only be seen at close scrutiny.

From this mini-test, I think the CA might be super-enhanced by the metallic surface in the photo in the original post, as camoptics suggested.

i have no experience with this lens. but i would go back to the shop and try another one, and if this phenomenon is "normal" for this lens, i would return the lens or sell it off. You would spend hours correcting CA on software if that performance is "normal". Not quite acceptable, IMHO.

Actually, is CA performance something that varies from sample to sample? Lens tests sometimes mention that they have bad samples, which affect sharpness results but usually they don't mention anything about CA variation. In terms of sharpness I think I got a good copy though.

 

[camoptics]

Also, I took a few snaps of a newspaper from a slanted angle. At f3.2 (couldn't go faster due to the focus distance) there were some purple tinges in the out-of-focus foreground and some green tinges in the out-of-focus background. The centre was free of any CA. At f5.6, the green tinges disappeared but some vestiges of the purple remained, though I think it can only be seen at close scrutiny.

From this mini-test, I think the CA might be super-enhanced by the metallic surface in the photo in the original post, as camoptics suggested.

A bit confused on the testing because not sure which CA you are talking, center or edge. They may be different in this case.

I would like to further confirm if this is longitudinal chromatic aberration. But I need some shot condition of the posted photo like: the thickness of the screw and the tilt angle when shot. Thanks.