What's the purple haze?..C&C please.

[Steve U]

Hi everyone,


Still playing around at wide open on the 85mm. This one is an afternoon shot with an ND4 and a ND8 stacked. The picture is as shot no post, I am wondering why the chrome on the canopy support of background boathas a purple haze in places. When viewed full size via link there is purple around the windows of the main boat. Is this a product of the Sigmalens? Is it just what the sun and chrome do? Is it the ND's? Why are the blacks a bit purple?


I know 1.4 is not the usual aperature for this type of shot, but I am experimenting still with trying to get separation(pop) from the background and I think there is a bit of that(no sharpening)with the top part of the main boat. (excuse obvious lack of nautical knowledge)


[img]/resized-image.ashx/__size/950x0/__key/CommunityServer-Discussions-Components-Files/14/7558.IMG_5F00_6706_5F00_001.jpg[/img]





http://www.flickr.com/photos/steves_life/5391161160/in/photostream/


@85mm, f1.4 1/320sec,ISO100


Thanks for viewing,


Steve

[barba]

chromatic aberration

[Steve U]

Thanks, I hadn

[neuroanatomist]

More specifically, it's longitudinal CA (LoCA, as opposed to lateral CA). LoCA occurs only in high contrast areas with fast lenses (not just your Sigma!) shot wide open, occurs across the frame (lateral CA affects the edges more), and is worst in out of focus areas. LoCA is also called 'bokeh fringing' and is much more difficult to correct in post than lateral CA (often, LoCA cannot be corrected).


However, unlike lateral CA, LoCA is eliminated by stopping down the lens. It also helps to minimize strong contrasts (e.g. chrome) when shooting wide open, or if they are in the image, it's best if they are in focus, as shown in a previous post where I did some testing of my 85L for LoCA.


Hope that helps...


--John

[Andy Stringer]

Steve,

The combination of high contrast and wide open aperture are resposible for the purple fringing in this shot. It's a problem that afflicts many prime lenses. It would be less obvious with a narrow aperture, but that wouldn't separate the boat from the background.

If you can't get satisfactory results with the chromatic aberration tools in Lightroom (including the "Defringe" options), try changing the luminance of the blue and/or purple colour channels. It wasn't perfect when I tried it on your jpeg, but the raw file might respond better.

Your neutral density filters are not responsible. Some ND filters transmit more light in the red part of the spectrum, but this is not normally noticeable with less than ND64, and you used just ND32 for this shot. The effect would be a very slight warming of the whole image, which is not the problem here.

[erno james]

Man, reading this post is like going to photography college. You guys rock!

[Steve U]

I second that Erno.


John and Andy thanks for the time and explanations. The LoCA I had heard about, but never took the time to try and understand. Your posts in the link above make it very easy to grasp, even for a rookie.


You are a big help, thank you again, gents.


Steve

[wickerprints]

Is there any particular reason you were shooting at f/1.4 with ND filters for this type of subject? Just guesstimating from the image, and assuming you are shooting full frame, I would say you are at a subject magnification of about M = 1/500, which means that your blur circle at f/1.4 for background objects near infinity is going to be quite small, about only 0.12mm. That

[Sheiky]

Offtopic a bit, but:

I know 1.4 is not the usual aperature for this type of shot, but I am experimenting still with trying to get separation(pop) from the background and I think there is a bit of that(no sharpening)with the top part of the main boat. (excuse obvious lack of nautical knowledge)

Haha Steve, when I saw this shot it reminded me immediately about your topic on 3D-effect[] I do agree there's a little bit of "pop", however it's not directly present. I think the shadow on the boat pulls the attention away from the parts that you'd like to show for the 3D-effect.


It does remind me that I should try the "3D-effect" as well. If you get more close to the secret, keep me posted []


Jan

[HDNitehawk]

Steve


What is the Purple Haze?


Wikipedia says "Purple Haze is actually a glorified term that was adapted from the Jimi Hendrix song, which most believe was created about the "brand" of super potent LSD created by Owsley Stanley"


It also can refer to a certain type of Cannabis.


If this is the Purple Haze you are talking about I am sure you would get some kind of "3D effect" but probably not the one you are looking for.


Rick