Odd color -- Purple Fringing

TonyCooper

I don't know where to ask this, but I guess "Technique" is close enough.

This shot was taken at an outside "biker bar". Mid-day on a very bright day, but you don't go hunting for biker shots in the early morning hours. The day was a record-high day in temperature with a cloudless, pale, sky.

What causes that blue-green fringe around the trees in the right?

This was shot in RAW and converted to a .jpg with no adjustments.

craig_d

The blue-green fringe is what you'd expect to see where a totally blown-out sky meets something dark, like trees. The sky probably has both its green and blue channels clipping, and red nearly maxed out, resulting in a slight blue-green tint that isn't obvious except where the anti-aliasing filter introduces intermediate tones to smooth edges.

TonyCooper

craig_d wrote:

The blue-green fringe is what you'd expect to see where a totally blown-out sky meets something dark, like trees. The sky probably has both its green and blue channels clipping, and red nearly maxed out, resulting in a slight blue-green tint that isn't obvious except where the anti-aliasing filter introduces intermediate tones to smooth edges.

This is not a photo that I'm at all concerned with saving or improving, but it does demonstrate something that I often see in my photos. If I shoot mid-day in Florida this time of year, I'm sure to get blown-out sky.

What can be done, in post using CS4, to alleviate this?

SLRdude

Craig said it better, but I am dumb and do not understand most of that stuff, so this is how I usually get rid of that.

Most digital cameras have a hard time going from very dark to very bright areas and that's when you experience that purple fringing.

About the only way to fix that properly is to shoot a bit underexposed so you do not lose the sky definition, and that usually gets rid of that issue.

The image is also a bit cold for me (at least on my crappy work monitor). Warming it up some reduced that purple just a little bit, but there isn't a whole lot you can do now since the image is a bit over exposed.

Cool shot though! Hey... when all else fails, that's what B&W if for! clap

craig_d

I don't find blown-out skies to be acceptable under any circumstances.

Others may have better suggestions, but I'd shoot this scene in RAW at ISO 100 to get maximum dynamic range, underexposing enough to make the sky come out blue. An ND filter might be necessary if the scene is so bright that even at ISO 100 your shutter isn't fast enough to reduce the exposure enough. Then I'd produce two uncompressed TIFF images from the RAW at different EV levels, and combine them with layer masks (blue sky from the darker image, everything else from the brighter one) or HDR. If the scene will sit still (probably not in this case, since there is a person in it and the trees might be affected by wind), then you can do bracketed exposures and combine them.

A GND filter might be helpful sometimes, though not in this particular case because this shot isn't divided in a straight line between the bright sky and everything else.

craig_d

SLRdude wrote:

Cool shot though! Hey... when all else fails, that's what B&W if for! clap

Ha, good point! You can always get rid of color fringing by removing all the colors.

Btw, something I discovered a while back is that with some cameras at moderately high ISO levels, you can almost completely eliminate digital noise by reducing the image to B&W using the "maintain luminance" method (whatever that may be called in whatever software you use) rather than color channel mixing. If the noise in the image is mostly in chroma rather than luminance, then simply throwing away the chroma information throws away the noise too.

pathfinder

If the issue is the difference in brightness, between the proper exposure for the face and for the sky, why not add enough fill flash to lighten the exposure for the face, and thus expose the background sky more correctly?

Adding fill flash is almost always easier than graduated NDs, at least for me anyway.

craig_d

pathfinder wrote:

If the issue is the difference in brightness, between the proper exposure for the face and for the sky, why not add enough fill flash to lighten the exposure for the face, and thus expose the background sky more correctly?

That would be fine if you didn't mind having the trees turn to black mud. The trees on the left look to be far enough away that fill flash on the biker's face wouldn't do them much good, though I could be wrong.

TonyCooper

While all of the suggestions have merit in certain situations, this photo was a candid taken at an outdoor biker bar. It's not a situation where where a lot of adjustments can be made on individual shots, and not a place where it's appropriate to fire off a flash in the subject's face. The idea is that the photographer has to be inconspicuous and shoot on the fly as people turn in the right direction. Most of the time I sat with my back to a trash can and shot upwards at people walking by in order not to include distracting background clutter.

This is the only one of the group I particularly like.

craig_d

That one's really nice!

pathfinder

craig_d wrote:

That would be fine if you didn't mind having the trees turn to black mud. The trees on the left look to be far enough away that fill flash on the biker's face wouldn't do them much good, though I could be wrong.

If you think flash will make the leaves black, I do not think you understand how fill flash works.

In Av mode in Canon land anyway, with the flash in ETTL, the exposure will be set appropriate for the background, and you flash will emit the proper amount of light to properly expose the subject. In bright sunlight, you will need to enable High Speed Synch. I see nothing here which suggests under exposure.

Or you can shoot in manual mode, and let the flash light the subject correctly, while you choose the desired exposure ( over or under ) for the background.

TonyCooper

I did try the biker in black and white. The problem is that the photograph really isn't that interesting in the first place. The hair on top of the head is blown out and has no detail, the sky/tree area has funky edges either in color or black and white, and the subject is only somewhat interesting.

That's not noise on his shirt...that's beard dandruff.

craig_d

pathfinder wrote:

If you think flash will make the leaves black, I do not think you understand how fill flash works.

In Av mode in Canon land anyway, with the flash in ETTL, the exposure will be set appropriate for the background, and you flash will emit the proper amount of light to properly expose the subject. In bright sunlight, you will need to enable High Speed Synch. I see nothing here which suggests under exposure.

Or you can shoot in manual mode, and let the flash light the subject correctly, while you choose the desired exposure ( over or under ) for the background.

Look at the original picture again. The trees look fine, but the sky is blown out. To avoid the blown-out sky (which was the point, in the first place), you have to reduce the exposure significantly, which will make the trees and the biker dark. Fill flash will illuminate the biker in the foreground more than the trees in the background. Hence, dark mud for the background trees, unless they're closer than I think they are.

The photographer tells us that flash would not have been appropriate for the situation, so it's a moot point. But I think blended exposures would produce a better result anyway.