Photoshop Channel Mixer Question

richtersl

I converted a color photo to B & W with the Channel Mixer and got some really strange results. At first I thought there was something weird going on with my monitor so I asked someone else to take a look at it on theirs and the result is the same: the upper right corner of the photo looks mottled:



The photo was taken in RAW format and saved as a TIFF file but I resized it and saved it as JPEG for this forum (also, smugmug was not happy about me attempting to upload a TIFF file :cry ). This JPEG vesion looks just like the TIFF version.

On the original color version, that area of the photo looks fine to my eye but this resized one appears to have some JPEG artefacts. The full sized version looked great. Even the photoshop expert who looked at it was surprised at the difference between the two photos but he couldn't tell me why it was happening or how I could prevent it.



On the full size version that corner of the sky looks great in color but lousy with the channel mixer turned on.

Can someone please tell me why this is happening? :scratch It's driving me nuts! :hack

gluwater

The upper right corner is definitely darker in than the upper left corner. When you are using the channel mixer it is just excentuating that. If you use the eyedropper and take a color reading in both corners you will see the difference is quite large.

DudeX

Just by visual inspection, I see that the color picture looks mottled. In fact, I see a slight bit of green in the upper right corner.

Assuming the image recorded is faithful, you might be picking up natural phenomena or the effects of pollution or what have you.

You might have to to do some layers work and masking to hide the greater variations on the upper right corner.

Pamela

It almost looks like to me that the sensor is dirty?


Anyone else?

colourbox

What kind of brightness/curve adjustments were applied in the Raw conversion? The only thing I can think of is that there is a steep tonal adjustment applied to that area in the Raw converter, and it posterized. That's a guess, though.

richtersl

colourbox wrote:

What kind of brightness/curve adjustments were applied in the Raw conversion? The only thing I can think of is that there is a steep tonal adjustment applied to that area in the Raw converter, and it posterized. That's a guess, though.

Thanks. I'll have to check on those when I get home from work. I know for sure I am using a hue/saturation layer and that I applied a noise filter to the sky.

I did use a polarizing filter when I took the photo. I don't know if that has any bearing on things.

When we looked at the photo on the second computer, the color change in that upper right corner was "graduated" normally. Neither one of us saw any mottling. This was at the place where I went to get it enlarged to 11 x 14 for entry into a photo contest. :eek1 And it wasn't some kid helping me, it was the owner of the place. I'll find out tomorrow how it came out. :uhoh

I'm wondering out loud here if it would help for me to post a copy of the RAW image???

GraphyFotoz

Just me thinking out loud. :giggle

When you turned the polarizer it wasn't even across the sky just mostly the right corner?
Don't feel bad it's happened to me as well.
Looked good at the time then came the PP. Oooppss!

richtersl

GraphyFotoz wrote:

Just me thinking out loud. :giggle

When you turned the polarizer it wasn't even across the sky just mostly the right corner?
Don't feel bad it's happened to me as well.
Looked good at the time then came the PP. Oooppss!

....beats the hell out of me! I was just watching the clouds come fo life through the viewfinder.

I'll have to just figure out how big an OOPS I made and try to avoid it in the future. :giggle

wxwax

richtersl wrote:

I did use a polarizing filter when I took the photo. I don't know if that has any bearing on things.

That's exactly what it is.

A circular polarizer will not convert the entire sky, unless the sun with is within a very limited angle relative to your lens.

Usually, the more directly the sun is behind you, the better the CP works. In this shot, the sun was to your left. As a result, the CP only affected the right side of your piccie.

BTDT.

In my case, the sun was off to the right. See what the CP does to the sky to the left, versus the sky to the right?

richtersl

wxwax wrote:

That's exactly what it is.

A circular polarizer will not convert the entire sky, unless the sun with is within a very limited angle relative to your lens.

Usually, the more directly the sun is behind you, the better the CP works. In this shot, the sun was to your left. As a result, the CP only affected the right side of your piccie.

BTDT.

In my case, the sun was off to the right. See what the CP does to the sky to the left, versus the sky to the right?

Beautiful pic!!!! Was that taken in Yosemite?

I've seen that effect from the polarizer. That doesn't bother me as much as the mottling. How is it that your polarized area is not mottled? I'm using a Tiffen 58mm CP -- is there something in that polarizer itself that's causing it? Poor glass quality? Dirty glass?

colourbox

I stumbled across a thread, quite by accident, that possibly backs up what wxwax is saying. It discusses possible image quality degradation caused by a polarizer or underexposure or both...I need to read it a couple times to understand what they're saying, but here is the link:
Using a polarizer with a digital camera

richtersl

This is a JPEG copy of the original unedited RAW file


On the image with the PP, I did not use a curves or levels adjustment but did have a hue/saturation layer. But looking at this I can see where the channel mixer was just accentuating something in the photo as someone in an earlier post mentioned. But why is this happening?

The JPEG version of the photo in its original size is here. Exif info is here.

Thanks for your input so far, everyone.

richtersl

colourbox wrote:

I stumbled across a thread, quite by accident, that possibly backs up what wxwax is saying. It discusses possible image quality degradation caused by a polarizer or underexposure or both...I need to read it a couple times to understand what they're saying, but here is the link:
Using a polarizer with a digital camera

Thanks so very much! It sounds like that's what's going on in my photo because the original came out slightly underexposed and I was using a polarizer. I'll see if I can figure out what they're saying.

wxwax

Thanks! Yes, it's Yosemite.

I'll try a channel mixer on your photo, to see if I can duplicate the effect. It may simply be the result of making the rest of the shot look good - the darker sky may have responded poorly to the settings you used.

pathfinder

Richters -

I think that if you examine the blue channel in your original color image that you will find the problem is there. Most of the noise is usually in the blue channel, and I bet that is true for this image also. The trip through channel mixer is then accentuating what is already there in the blue channel.

You did not describe the settings you used in Channel mixer. Try just using the Red and Green channels at + 50 each, with the Blue channel set to minus 50 and see if you still see this phenomenon.

Addendum

When I downloaded the original size jpg of yours, I see lot of artifacts in the red and the blue channels in the upper right corner. I suspect these are related to underexposure due to the polarizer with the increased noise as a result.

There are some folks who recommend NOT using a polarizer with digital cameras for this reason. I am not one of them, but they do work better with lenses that are less wide than 28mm on a full frame body.

richtersl

pathfinder wrote:

Richters -

I think that if you examine the blue channel in your original color image that you will find the problem is there. Most of the noise is usually in the blue channel, and I bet that is true for this image also. The trip through channel mixer is then accentuating what is already there in the blue channel.

You did not describe the settings you used in Channel mixer. Try just using the Red and Green channels at + 50 each, with the Blue channel set to minus 50 and see if you still see this phenomenon.

Addendum

When I downloaded the original size jpg of yours, I see lot of artifacts in the red and the blue channels in the upper right corner. I suspect these are related to underexposure due to the polarizer with the increased noise as a result.

There are some folks who recommend NOT using a polarizer with digital cameras for this reason. I am not one of them, but they do work better with lenses that are less wide than 28mm on a full frame body.

Unfortunately I can't resurrect the channel mixer settings I tried. I have PSE and used a Photoshop channel mixer plug-in from Earthbound Light for the conversion and it's read-only: I can't make edits to settings. All I can do is delete the layer and create a new one. But it looks like I may just have kept the default setting which was Red at 100%. I just applied a new channel mixer layer to it with your settings and that took care of the problem in the sky. It actually made the photo really dark. To lighten it, I added a Levels adjustment layer. Using those channel mixer settings certainly helped! Thanks for suggesting them!!!

I'm happy with the sky but now there's a problem with the grasses in the foreground.... I guess I just need to tweak the channel mixer settings a bit more to get that right???


Some days you just can't win!



Actual size

pathfinder

Problems with polarizers are hard to exorcize.:D

richtersl

pathfinder wrote:

Problems with polarizers are hard to exorcize.:D

But perhaps a local shaman may want to try in exchange for a small token.

I forgot to ask you this earlier, but would you mind posting a crop of the area where you found that there was too much red and blue in the sky? I'm not sure what to look for in the full sized image to find it.

pathfinder

I said the jpg artifacts were in the upper right quadrant of the image. You can look at the channels individually - the Red, the Green, and the Blue channel.

In Photoshop, CTRL-1 displays the red channel in RGB images, CTRL-2 the Green channel, and CTRL-3 the Blue channel, so it is very fast to survey each of them before doing any editing to an image. CTRL-~ returns the image to full color .

I think this will also work in Elements, but Nightingale is not here to answer that question for me right now.

Probably the easiet way to exorcise your image is to use the Green channel. This was not shot in RAW I gather??

richtersl

pathfinder wrote:

I said the jpg artifacts were in the upper right quadrant of the image. You can look at the channels individually - the Red, the Green, and the Blue channel.

In Photoshop, CTRL-1 displays the red channel in RGB images, CTRL-2 the Green channel, and CTRL-3 the Blue channel, so it is very fast to survey each of them before doing any editing to an image. CTRL-~ returns the image to full color .

I think this will also work in Elements, but Nightingale is not here to answer that question for me right now.

Probably the easiet way to exorcise your image is to use the Green channel. This was not shot in RAW I gather??

The original was shot in RAW. When I tried to upload a resized TIF version smugmug wouldn't let me do it so I automatically assumed it would not let me upload a RAW (CR2) file from Canon either. When I get home from work I'm going to try increasing the exposure by +1 to see how your suggested settings will work. The RAW image editor that came with PSE sucks but Canon's is not too bad and it will let me work with individual channels to an extent.

Ctrl-~ doesn't do anything in PSE. :cry

wxwax

I think the shot is underexposed for the ground. It's fine for the sky, except for the polarized bit.

Playing with the Channel Mixer, I see the same noise that PF found in the top right corner on the blue and green channels.

Time to employ the power of Layers, R.

richtersl

wxwax wrote:

I think the shot is underexposed for the ground. It's fine for the sky, except for the polarized bit.

Playing with the Channel Mixer, I see the same noise that PF found in the top right corner on the blue and green channels.

Time to employ the power of Layers, R.

:yikes Oh noooo....not the dreaded layers....:giggle

But I think you're absolutely right.

I have PSE and don't have as much liberty with them as you do with PS. But I do have that capability. The current TIF version had a separate layer for the sky in which I attempted to remove some of the noise. But thanks to everyone's help so far I think I can diddle around with the channels on a sky layer similar to the one that I had created and see what happens.

pathfinder

You MIGHT get by with a Gaussian blur in the Red and Blue channels after selecting the sky first.

I think you MAY be able to decrease the jpg artifacts in the Red and Blue channels, by using a more conservative approach in RAW conversion.

If you have access to PSCS 2, you might try bringing the image from RAW to Photoshop as a 16bit image in the ProPhoto RGB space and then convert from there after editing down to 8bit and sRGB finally.

Smugmug will ONLY accept 8bit sRGB images for correct color rendition.

richtersl

pathfinder wrote:

You MIGHT get by with a Gaussian blur in the Red and Blue channels after selecting the sky first.

I think you MAY be able to decrease the jpg artifacts in the Red and Blue channels, by using a more conservative approach in RAW conversion.

If you have access to PSCS 2, you might try bringing the image from RAW to Photoshop as a 16bit image in the ProPhoto RGB space and then convert from there after editing down to 8bit and sRGB finally.

Smugmug will ONLY accept 8bit sRGB images for correct color rendition.

Thanks. These can certainly be done "a la PSE" though it does look like I'll eventually have to ditch it in favor if its bigger brother.

BTW, my original goal had been to enter that photo into an contest/exhibit as a B&W. When the camera shop owner and I saw the artifacts on his copy of PS we went with the color version (which looked perfect on his monitor, BTW : ). Then I started this thread and learned a whole bunch more since that day so I wasn't shocked to see the artifacts in the printout but he was after I pointed them out :giggle. I think he's going to start playing with those red and blue channels.... Anway, a crop and a custom-cut mat solved the problem. So now it's a panorama. That photo plus two more are being dropped off today. Wish me luck!

01af

richtersl wrote:

I have PSE and used a Photoshop channel mixer plug-in from Earthbound Light for the conversion ...

Oh no! Why didn't you mention this earlier? Seems the plug-in does the channel-mixing calculations in 8-bit mode internally. Create a 16-bit TIFF file from your raw file and try giving it to someone who has Photoshop CS or CS2. See if true 16-bit processing will also create that mottled sky! I'd be surprised if it will.

-- Olaf

richtersl

01af wrote:

Oh no! Why didn't you mention this earlier? Seems the plug-in does the channel-mixing calculations in 8-bit mode internally. Create a 16-bit TIFF file from your raw file and try giving it to someone who has Photoshop CS or CS2. See if true 16-bit processing will also create that mottled sky! I'd be surprised if it will.

-- Olaf

Interesting!

PSE lets you load 16 bit TIFF files, but you can't do much with them.

But what you menitioned makes sense!

Thanks!

Linda

Tanuki

Mottle effect
I imported your color photo into PSE, and discovered the mottled effect is due to clipping of the red channel at the dark end of the histogram. You can see this for yourself in PSE if you select just the area with the mottling and view the histogram in colors. The blue and green channels are fine, but the red channel is getting clipped.

I had the converse problem when I tried to photograph a red flower against a green background, because the red channel was getting clipped at the bright end of the histogram.

Armed with this information you might be able to go back into the raw file and salvage some more information out of the red channel. Or you could do a different channel mix for the sky (with less red channel influence) and use it to replace the sky in your other b&w using layer masks. Please let me know if either of these suggestions works.

If you were to go back in time and retake the photo, I would say to either reduce the polarizer or increase your overall exposure (as long as you don't clip the highlights somewhere else in the photo.