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playinhockey
28th of January 2004 (Wed), 10:42
Can any one tell me what causes halos around images. It seems to happen when I have underexposed shots that I try to fix in PS. When I get the highlights and shadows the way I want them a halo appears around the image. Is there a way to correct this other than taking the shot correctly? I would include the picture, however, I have no idea how to do that.

robertwgross
28th of January 2004 (Wed), 11:52
"Fixing in PhotoShop" is a standard occupation for some people. Unfortunately, some do not understand the subtle effect on things that they are not fixing. Then it becomes difficult to un-fix it.

Changing contrast often has the result of producing edge artifacts such as a halo.

If you edit by looking at the entire image on-screen, then you might miss the edge artifact that you just created. Often it is wiser to edit by looking at a 100% view of a small area.

Back when I was younger and more stupid, I would apply a sharpening brush to many areas of an image. Then I would step back and admire my work. But it didn't look right. Later on I discovered that there are ways to perform sharpening without screwing it up.

---Bob Gross---

maderito
28th of January 2004 (Wed), 11:52
Can any one tell me what causes halos around images. It seems to happen when I have underexposed shots that I try to fix in PS. When I get the highlights and shadows the way I want them a halo appears around the image. Is there a way to correct this other than taking the shot correctly? I would include the picture, however, I have no idea how to do that.

Are you talking about the purplish or greenish fringes around image elements - like tree branches against a bright sky?

If yes, that's chromatic aberration which becomes more obvious as you adjust levels and brighten an underexposed image.

If yes, it's a bigger problem with wide angle lenses and zooms, especially at large apertures. It is partially corrected by more expensive lens designs and glass.

If no, I don't have a clue what you're referring to. :? What lens are you using?

playinhockey
28th of January 2004 (Wed), 12:43
Yes, the fringe effect was against the bright sky. I was shooting ISO 800, 17-40 f4L. The camera was set on TV @ 1/500. The subject was underexposed against a cloudy but bright sky.First I set the highlights and shadows the way I wanted them, then, I filtered the noise using neat image. Next, I sharpened the picture using high pass set at radius 1.9 followed by USM 500%. radius 1.2, threshold 19. The result was a good picture with slight edge artifacts where the subject meets the bright sky. I am new to all this. Maybe I'm just to picky. The reason the camera was set at such a high shutter speed was that earlier I was trying to catch geese in flight. I changed the lens to take a picture of my son and forgot to change the other settings. I thought it woud be a good picture to play around with and hopefully learn something using PS CS that I bought yesterday!

robertwgross
28th of January 2004 (Wed), 12:57
First of all, I'm curious. Exactly why were you using a wide angle lens to shoot geese in flight? A telephoto lens might get better results.

Lots of photographers wander around using either optical filters or else digital process filters to attain certain intended effects. However, it is better if we can study up on the actual filter theory from engineering school to understand better what we are actually doing, and what the side effects may be.

It is a pretty standard problem for a digital camera to put a purple fringe around a dark object against a bright sky.

---Bob Gross---

maderito
28th of January 2004 (Wed), 13:27
Yes, the fringe effect was against the bright sky. I was shooting ISO 800, 17-40 f4L. The camera was set on TV @ 1/500. The subject was underexposed against a cloudy but bright sky.First I set the highlights and shadows the way I wanted them, then, I filtered the noise using neat image. Next, I sharpened the picture using high pass set at radius 1.9 followed by USM 500%. radius 1.2, threshold 19. The result was a good picture with slight edge artifacts where the subject meets the bright sky. I am new to all this. Maybe I'm just to picky. The reason the camera was set at such a high shutter speed was that earlier I was trying to catch geese in flight. I changed the lens to take a picture of my son and forgot to change the other settings. I thought it woud be a good picture to play around with and hopefully learn something using PS CS that I bought yesterday!

OK. I have the same lens (17-40L), and the fringing only becomes really noticeable at the widest angle (at or near 17mm).

There are techniques and plug-ins for removing the fringe - but it's a hassle.

I've found that chromatic aberration of this type is seen in all wide angle zoom lens - some much worse than others. I didn't know that digital cameras were more susceptible? It was especially noticeable on my Elan IIe using a 28-105mm Canon zoom lens.

High pass filtering as a technique to sharpen images will have its greatest effect on portions of the image that have high spatial frequency - that is, where color and brightness changes rapidly over a small area. That's where the fringing occurs, so you're probably bringing it out with the sharpening technique. But...you'd also emphasize it using an edge filter mask. Therefore, I would be very gentle with any shaprening when you have problems with chromatic aberration. Alternatively, you can mask out the sky (where the problem is usually most noticeable) while sharpending.

Bob suggests that you go back to engineering school and concentrate on signal processing theory. Not a bad idea if you have the spare time. Good luck. :)

playinhockey
28th of January 2004 (Wed), 14:13
Bob, I started out using my100mm f2 prime and found that the geese were too close, 10-15 feet away! My Son and I were hunting out of a pit. We are able to call them in to the decoys at very close range. This makes for a lot of fun with a gun or a camera! It sounds like this cromatic aberation thing is pretty normal. It's the first I've noticed it. Ive got some cool pictures of the geese!

Tom

playinhockey
28th of January 2004 (Wed), 14:29
Mask out the sky! That sounds like the answer. It will probably take me all day to figure that one out.

robertwgross
28th of January 2004 (Wed), 14:30
I, too, shoot photos of wildlife. Unfortunately, we have little control over the beasties, so they stay far and come close with much contempt for the photographer. As a result, I always use zoom lenses for wildlife (every lens I own is a zoom, except for one).

I had to study all of that electrical filter theory many decades ago in school. But then I didn't use much of it for a long time, so I forgot it. Then as I started getting deeper into photography and image editing, all of that filter jazz started coming back to me. I suddenly realized the relationship of electrical filters to glass filters and digital editing filters. Then I realized that my classes had not been a waste of time.

You know how you can study a histogram and know how the different intensities are distributed from dark to bright? Well, think about how you would study the spectrum of *light frequencies* from low to high (which is related to *light wavelengths* (colors). Then when you have to edit an image and you are staring at one line edge, think about the filtering. It gets more fascinating to figure out how to build a Singh-Ray filter.

---Bob Gross---

Lesmac
28th of January 2004 (Wed), 14:33
It could be neatimage that's causing the halo effect, I've noticed it a few times I've used it.

robertwgross
28th of January 2004 (Wed), 14:46
I use Neat Image also.

What part of it do you think is causing the problem with a halo?

---Bob Gross---

playinhockey
28th of January 2004 (Wed), 14:56
It's the first time I've used neat image with an outdoor shot. I use it all the time for my ice hockey pictures and have never had a problem. In fact it works relly well on those shots. I don't use the wide angle lens for ice hockey. I guess Neat image could have an effect on the wider lenses. I think it has more to do with trying to fix an underexposed image shot with the wide angle lens. I guess I will have to sign up for that class Bob suggested!

maderito
28th of January 2004 (Wed), 15:04
Mask out the sky! That sounds like the answer. It will probably take me all day to figure that one out.

Try this is PS 7 (should be similar in PS CS)

On a COPY of your original image:

- Duplicate background layer
- Do your sharpening thing... and end up with a sharpened image layer above the original background layer
- Make the sharpened layer active
- In the layers palette, click "add layer mask" icon (or menu bar: layer->add layer mask->reveal all)
- Hit "D" key to set background/foreground colors to black/white (make sure black is foreground - hit little arrow to reverse if necessary)
- Select Gradient tool (nested in paint bucket)
- "Pull" a vertical line from the top of the image to just below the horizon. (You're painting a black to white gradient on the layer mask. If you goof, just do it again)
- Merge this masked/sharpened layer with the background

The only confusing thing (which might make this a 1/2 day instead of 30 sec project) is figuring out how to work on the layer mask rather than the layer image.

maderito
28th of January 2004 (Wed), 15:48
It gets more fascinating to figure out how to build a Singh-Ray filter.

Bob -

I'm not an engineer but I've been known to associate (cautiously) with a few. While you're here, perhaps you can tell us who Mr. (?) Singh-Ray is and why his name is associated with graduated ND filters?

Incidentally, a graduated digital filter design would be a challenge since the gradation occurs in 2-D space which is not referenced in the frequency domain.

robertwgross
28th of January 2004 (Wed), 16:14
No, I don't know the man. But the Singh-Ray filter was very interesting to me until I thought about it. I think of Singh-Ray in the context of a color filter, not with graduated neutral density filters.

For example, what is a red filter? It is a bandpass filter for red color. It lets more red through compared to the rest of the spectrum.

What is a blue filter? OK, you have that figured out.

The opposite would be a bandstop filter. It would stop all of the red and let everything else through.

I think in a Singh-Ray filter, a rare earth is used in the glass to create multiple bandstop filters. So, for example, it might trap out the orange while letting both red and yellow through, and it might simultaneously trap out purple while letting blue through. This would give the effect of accenting the red, yellow, and blue, but it would trap the orange and purple. The late Galen Rowell seemed to use this for sunrise and sunset shots. Interesting stuff if you are an aging engineer.

---Bob Gross---

playinhockey
29th of January 2004 (Thu), 07:47
Wow, Thanks everyone for your help! I just read in the WSJ that Nikon expects to gain 40% market share wiht the release of it's new D70. Do you think the point and shoot crowd will figure all this out? My In-Laws still can't figure out how to get the pictures out of the camera!