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Author Topic: converting from prophoto to sRGB: tip  (Read 32845 times)
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« on: November 20, 2007, 11:57:13 AM »
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Someone correct this if inaccurate, but here is something I just discovered:

Problem- I like to edit in 16 bit prophoto.  However, when converting from prophoto RGB to sRGB in photoshop (for output to the web or for many offsite printers), the histogram will show the highlight and shadow clipping that occurs, forcing an "undo" of the sRGB conversion and a subsequent negative exposure compensation in prophoto, to allow headroom in sRGB.

Solution- Open files in camera RAW (RAW files, jpegs, whatever) and set the editing preference (in the bottom center of the screen) to 8 bit sRGB.  Correct the exposure and all other global adjustments in camera RAW that you wish.  When done, change the preference to 16 bit prophoto- watch the histogram become more compressed as this happens.  Now, open the image in photoshop from camera RAW (which will be 16 bit prophoto), and apply whatever local adjustments you want- masks, hue/saturation, whatever.  When done, convert back to 8 bit sRGB (if output is for the web or an offsite printer without ICC profile), or assign the printer's profile if available.  Watch the histogram jump back to what you saw in camera RAW under the initial 8 bit sRGB settings.  This will help prevent blown highlights (and crushed shadows) in your prints.

Any critique or comments appreciated.

John
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« Reply #1 on: November 20, 2007, 12:34:43 PM »
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hello John,

I really don't understand what you are exactly doing...but maybe it is only a problem I have.

What I do understand is that conversion from Prophoto to sRGB may be difficult with some images, because of the smaller gamut.; maybe you could explain it in a different way.

Pieter Kers
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« Reply #2 on: November 20, 2007, 12:59:07 PM »
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hello John,

I really don't understand what you are exactly doing...but maybe it is only a problem I have.

What I do understand is that conversion from Prophoto to sRGB may be difficult with some images, because of the smaller gamut.; maybe you could explain it in a different way.

Pieter Kers
[a href=\"index.php?act=findpost&pid=154439\"][{POST_SNAPBACK}][/a]

I don't understand either.

And I don't understand the issues of converting from ProPhoto RGB to sRGB. Well sure, its a lot smaller color space, you're going to use a RelCol conversion to map out of gamut colors into gamut, there's clipping. That's going to have to happen, you're starting out with a gallon of water and trying to pour it into a cup. The recipe (in this case, say the internet) calls for a cup. What's the problem?
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Andrew Rodney
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« Reply #3 on: November 20, 2007, 01:58:43 PM »
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The problem I encountered is that the histograms whose exposures I pushed to the right while editing in prophoto in photoshop looked balanced in that color space, but showed significant clipping when converted to sRGB for offsite printing.  The highlights that clipped on the histogram looked blown out on my prints.  My idea is to try to prevent this from happening by setting the exposure value in sRGB in camera RAW to give myself a preview of the maximum luminance headroom available for an sRGB print (preview a prophoto image in a program that's not color aware, such as Faststone, and see how crappy it is).  The reason to then switch to 16 bit prophoto is to allow more editting leway in photoshop.

I'm assuming that the switch from sRGB to prophoto in camera RAW is a non-destructive one that acutally reassings the color space, as opposed to starting off in prophoto in photoshop, switching to sRGB (which will chop off the out of gamut colors in a relative colormetric conversion) to check for clipping, and switching back to prophoto to edit (which will expand the now reduced color gamut, potentially causing posturization during editting).  Yes, you can simply switch from prophoto to sRGB in photoshop to preview the histogram and then "undo" it, but this becomes a problem after multiple edits and is cumbersome.  I haven't tried the "assign profile" command in photoshop, which may obviate these steps.

Please let me know if this makes sense.  

John
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« Reply #4 on: November 20, 2007, 03:10:29 PM »
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The problem I encountered is that the histograms whose exposures I pushed to the right while editing in prophoto in photoshop looked balanced in that color space, but showed significant clipping when converted to sRGB for offsite printing.

Forget the histogram for the time being. Just as two color spaces have differing numbers, they may have differing histograms. How's the color appearance?

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The highlights that clipped on the histogram looked blown out on my prints.

From simply going ProPhoto RGB to sRGB (and then to what output color space)? I've yet to see this. What output device and why are you converting from ProPhoto RGB to sRGB to output color space? Why are you not going ProPhoto RGB to output color space?  

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I'm assuming that the switch from sRGB to prophoto in camera RAW is a non-destructive one that acutally reassings the color space, as opposed to starting off in prophoto in photoshop, switching to sRGB (which will chop off the out of gamut colors in a relative colormetric

Its not non destructive. You're converting color spaces right? And you're altering the data so there's data loss. What would make Camera Raw handle the conversions any differently than Photoshop (it doesn't other than it has to do yet another conversion into a linear encoded ProPhoto space before it can do any further adjustments or conversions).

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I haven't tried the "assign profile" command in photoshop, which may obviate these steps.
[a href=\"index.php?act=findpost&pid=154459\"][{POST_SNAPBACK}][/a]

There's no reason to use Assign Profile, that would really hose the color appearance.
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« Reply #5 on: November 20, 2007, 05:28:52 PM »
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I don't understand either.

And I don't understand the issues of converting from ProPhoto RGB to sRGB. Well sure, its a lot smaller color space, you're going to use a RelCol conversion to map out of gamut colors into gamut, there's clipping. That's going to have to happen, you're starting out with a gallon of water and trying to pour it into a cup. The recipe (in this case, say the internet) calls for a cup. What's the problem?
[a href=\"index.php?act=findpost&pid=154445\"][{POST_SNAPBACK}][/a]

I have discussed this problem before in some threads, but can't remember where. Here is an example of a highly saturated red flower which will not fit into sRGB, but which will fit into ProPhotoRGB.

sRGB without adjustment shows strong clipping of the reds

[attachment=3933:attachment]

One can decrease the exposure and eliminate the clipping, but the image becomes dark

[attachment=3934:attachment]

Or one can use ProPhotoRGB and get the full gamut

[attachment=3935:attachment]

Now if you want to convert to sRGB for the web, you will have to decrease saturation in the reds, either by manual editing or using a profile with perceptual rendering. This rendering does not exist for matrix profiles such as sRGB, but a forum member informed me of another profile for this purpose, but I can't remember the name. It used lookup tables.

Bill
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« Reply #6 on: November 20, 2007, 05:43:00 PM »
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I have discussed this problem before in some threads, but can't remember where. Here is an example of a highly saturated red flower which will not fit into sRGB, but which will fit into ProPhotoRGB.

I'd expect this to be the case with lots of images. They don't fit into the puny sRGB space. That's why we have larger color spaces like ProPhoto RGB.

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One can decrease the exposure and eliminate the clipping, but the image becomes dark

Obviously not a good solution....

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Or one can use ProPhotoRGB and get the full gamut

Fine.

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Now if you want to convert to sRGB for the web, you will have to decrease saturation in the reds, either by manual editing or using a profile with perceptual rendering. This rendering does not exist for matrix profiles such as sRGB, but a forum member informed me of another profile for this purpose, but I can't remember the name. It used lookup tables.
[a href=\"index.php?act=findpost&pid=154514\"][{POST_SNAPBACK}][/a]

Yes, it will be useful when V4 profiles of working spaces with perceptual tables are more widely available although having such tables alone doesn't at all guarantee that the problem is solved.

What you can do is convert and edit the sRGB version (or just live with it, doesn't seem to be a big deal), or soft proof the ProPhoto RGB image as sRGB and edit then convert.

So what's the real downside you see just doing the conversion from ProPhoto to sRGB, knowing doing this has to toss a lot of colors because you're demanding a document in a smaller color space. How is this different from converting from ProPhoto RGB to some CMYK process where everything gets less saturated or using a soft proof with a printer profile whereby the new color appearance isn't the same as the original? That's a tradeoff we all have to experience moving from a large gamut color space based on a display to a smaller gamut color space based on possibility a print.
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« Reply #7 on: November 20, 2007, 05:55:24 PM »
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Its not non destructive. You're converting color spaces right? And you're altering the data so there's data loss. What would make Camera Raw handle the conversions any differently than Photoshop (it doesn't other than it has to do yet another conversion into a linear encoded ProPhoto space before it can do any further adjustments or conversions).

[a href=\"index.php?act=findpost&pid=154475\"][{POST_SNAPBACK}][/a]

Andrew,

Correct me if I'm wrong, but I think the bit depth and colour space choices offered in the menu under the image in CR 4.x are essentially instructions for rendering the image. If one were to select say 8 bit sRGB and render the image on that basis, that is all one would end-up with in Photoshop; hence from a pure Photoshop perspective one has lost 7 bits of bit depth and the gamut between pro-Photo and sRGB. BUT, if one were to revert to the raw file and change those settings back to 16bit ProPhoto, is it not the case that all the raw data is still there allowing one to make that reversion?

Now, turning to the OPs issue, his problem starts with his description of the problem. Sure, if the image begins life in 16 bit ProPhoto he needs to get into 8 bit sRGB for the web. But Photoshop has excellent tools for doing this seemlessly. He should do all his image editing in 16 bit ProPhoto; then use Photoshop's tools to prepare his images for the web. All the stuff on my website was done as follows, starting from 16 bit ProPhoto:

Image Size with resamplingto 96 ppi and 600~800 pixels on the large dimension; interpolation bi-cubic sharper;
PK Output Sharpen for the web; (set opacity to taste);
Convert to sRGB with RelCol Intent and BPC on;
Convert Mode to 8 bit;
Save for Web in JPEG format, 60% quality, Optimized, with embedded sRGB profile.

That menu almost always works. Where gamut clipping still appears to be problematic, it is necessary to experiment with Perceptual Intent, and/or reduce the saturation of the offending colour group.

No other gymnastics in the raw converter are really needed.
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Mark D Segal (formerly MarkDS)
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« Reply #8 on: November 20, 2007, 06:49:38 PM »
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That menu almost always works. Where gamut clipping still appears to be problematic, it is necessary to experiment with Perceptual Intent, and/or reduce the saturation of the offending colour group.

No other gymnastics in the raw converter are really needed.
[a href=\"index.php?act=findpost&pid=154521\"][{POST_SNAPBACK}][/a]

Mark,

As I understand the problem, there is currently no perceptual rendering when one is converting from ProPhotoRGB to sRGB. It is always RelCol, even though Photoshop gives no warning that the intent is not available.

The OP's technique is to temporarily switch to sRGB in the ACR preferences, perform the editing in ACR so that the gamut fits (apparently with exposure or perhaps saturation), and then change back to ProPhotoRGB before rendering the image. Since the gamut after the edit does not exceed that of sRGB, one could have rendered the image into sRGB and then converted to ProPhotoRGB in Photoshop. As Andrew pointed out, decreasing exposure is not a good idea, since the image darkens.

Bill
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« Reply #9 on: November 20, 2007, 07:26:15 PM »
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Andrew,

Correct me if I'm wrong, but I think the bit depth and colour space choices offered in the menu under the image in CR 4.x are essentially instructions for rendering the image.

Yes. By rendering, I'd prefer to do this from the Raw data of course.


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If one were to select say 8 bit sRGB and render the image on that basis, that is all one would end-up with in Photoshop; hence from a pure Photoshop perspective one has lost 7 bits of bit depth and the gamut between pro-Photo and sRGB.

Seems right to me. The potential of higher bit depth and gamut based on the same rendering settings.

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BUT, if one were to revert to the raw file and change those settings back to 16bit ProPhoto, is it not the case that all the raw data is still there allowing one to make that reversion?

Sure.

Quote
Now, turning to the OPs issue, his problem starts with his description of the problem. Sure, if the image begins life in 16 bit ProPhoto he needs to get into 8 bit sRGB for the web. But Photoshop has excellent tools for doing this seemlessly. He should do all his image editing in 16 bit ProPhoto; then use Photoshop's tools to prepare his images for the web.

Right. Now the question is, do you render both versions, one ProPhoto RGB, one sRGB (lots more work) or just work in ProPhoto, convert to sRGB? And as I asked, it seems that working on a high bit, wide gamut master is essentially something we do all the time for iterations that go out to various printing devices so why is the web any different here? Yes, we have less tables to work with in our profiles but I don't know that is necessarily going to solve a problem I'm not sure exists. Since every profile building product creates its own idea of an ideal Perceptual rendering, this could get more messy, not less so.

The main question remains. When you convert from ProPhoto RGB to sRGB, does this conversion produce issues with the subsequent color appearance? And if so, is it impossible or real difficult to correct this? The times I've done this, more recently out of Lightroom to make web galleries, I haven't found any real problems here with the subsequent color appearance of the images. I'm either looking at them on a display, or a print and the print is always different and that's why I soft proof the images and if necessary, try to make minor corrections for that final output media.

An sRGB version versus a ProPhoto RGB version, especially considering most people are working with an sRGB output device (the display) seem nearly identical visually to me. There is a far bigger disconnect in the ProPhoto version and the output version on print. We've dealt with that for years. They are just vastly different output mediums.
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« Reply #10 on: November 20, 2007, 07:56:33 PM »
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The main question remains. When you convert from ProPhoto RGB to sRGB, does this conversion produce issues with the subsequent color appearance? And if so, is it impossible or real difficult to correct this? The times I've done this, more recently out of Lightroom to make web galleries, I haven't found any real problems here with the subsequent color appearance of the images. I'm either looking at them on a display, or a print and the print is always different and that's why I soft proof the images and if necessary, try to make minor corrections for that final output media.

An sRGB version versus a ProPhoto RGB version, especially considering most people are working with an sRGB output device (the display) seem nearly identical visually to me. There is a far bigger disconnect in the ProPhoto version and the output version on print. We've dealt with that for years. They are just vastly different output mediums.
[a href=\"index.php?act=findpost&pid=154549\"][{POST_SNAPBACK}][/a]

Re your main question above, from my experience the answer is "generally not". And when it does, correcting it in PS is not that big a deal for the reason you state. In the workflow I described, all those steps are performed before Lightroom picks up the images to make a gallery. To start from scratch in my case, the images are optimized as much as possible in 16 bit ProPhoto in CR4.x, then residual work is done in PS to get them to print properly. Then to repurpose them for the web I go through the steps I outlined in my previous post, perhaps with a Curves tweak or two because of the re-purposing; then I pick the JPEGs up in Lightroom to create the galleries. In this workflow Lightroom is being used mainly to generate the macromedia code and the display format, not to re-adjust colour and luminosity. Hence if the OP does not have LR and doesn't want to buy it, but has some other application for generating the web galleries, the PS workflow will serve his purposes too. And I agree with you completely - it is much more challenging to work a print from ProPhoto than a web-purposed image for the reason you suggest.
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Mark D Segal (formerly MarkDS)
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« Reply #11 on: November 20, 2007, 08:20:02 PM »
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Mark,

As I understand the problem, there is currently no perceptual rendering when one is converting from ProPhotoRGB to sRGB. It is always RelCol, even though Photoshop gives no warning that the intent is not available.

The OP's technique is to temporarily switch to sRGB in the ACR preferences, perform the editing in ACR so that the gamut fits (apparently with exposure or perhaps saturation), and then change back to ProPhotoRGB before rendering the image. Since the gamut after the edit does not exceed that of sRGB, one could have rendered the image into sRGB and then converted to ProPhotoRGB in Photoshop. As Andrew pointed out, decreasing exposure is not a good idea, since the image darkens.

Bill
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Bill,

When you "Convert to Profile" in the same dialog box where you select sRGB, down below you can select the Rendering Intent and can select Perceptual, RelCol, etc.

I wasn't recommending to decrease exposure for dealing with OOG - rather decreasing the saturation of the OOG colour(s), if the Perceptual Intent doesn't look after it sufficiently.
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« Reply #12 on: November 20, 2007, 09:53:08 PM »
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When you "Convert to Profile" in the same dialog box where you select sRGB, down below you can select the Rendering Intent and can select Perceptual, RelCol, etc.
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Actually, for transforms between working spaces, there's really only one intent at play, Relative Colormetric...there are some ICC v4 profiles out there that DO have perceptual intents...there's a v4 sRGB beta space. See: [a href=\"http://www.color.org/srgbprofiles.xalter]sRGB v4 Preference.ICC Profile[/url] but then of course, you really should only use v4 profiles with other v4 profiles...and I don't think there is a v4 Pro Photo profile.

So, when you do a Convert to Profile command, it doesn't matter WHAT intent you choose, you'll end up getting RelCol.
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« Reply #13 on: November 20, 2007, 10:14:57 PM »
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Actually, for transforms between working spaces, there's really only one intent at play, Relative Colormetric...there are some ICC v4 profiles out there that DO have perceptual intents...there's a v4 sRGB beta space. See: sRGB v4 Preference.ICC Profile but then of course, you really should only use v4 profiles with other v4 profiles...and I don't think there is a v4 Pro Photo profile.

So, when you do a Convert to Profile command, it doesn't matter WHAT intent you choose, you'll end up getting RelCol.
[a href=\"index.php?act=findpost&pid=154581\"][{POST_SNAPBACK}][/a]

Jeff, if that's the case, why does PS give us the option to select the three others?
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« Reply #14 on: November 20, 2007, 10:21:16 PM »
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Actually, for transforms between working spaces, there's really only one intent at play, Relative Colormetric...there are some ICC v4 profiles out there that DO have perceptual intents...there's a v4 sRGB beta space. See: sRGB v4 Preference.ICC Profile but then of course, you really should only use v4 profiles with other v4 profiles...and I don't think there is a v4 Pro Photo profile.

So, when you do a Convert to Profile command, it doesn't matter WHAT intent you choose, you'll end up getting RelCol.
[a href=\"index.php?act=findpost&pid=154581\"][{POST_SNAPBACK}][/a]

Jeff,

Very interesting. What would happen if you used the new profile with a non-V4 ProPhoto profile? I guess one could download the new profile and give it a try.

Bill
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« Reply #15 on: November 20, 2007, 10:34:09 PM »
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Jeff, if that's the case, why does PS give us the option to select the three others?
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To screw with your heads...

:~)

Actually, the Convert to Profile SHOULD dim out non-available rendering intents, but that logic hasn't been added to the app...yet.
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« Reply #16 on: November 20, 2007, 10:35:41 PM »
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Very interesting. What would happen if you used the new profile with a non-V4 ProPhoto profile?
[a href=\"index.php?act=findpost&pid=154589\"][{POST_SNAPBACK}][/a]


I have no idea...I haven't had time to test the v4 profiles...(been kinda busy)

:~)
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« Reply #17 on: November 20, 2007, 11:21:45 PM »
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It looks as though the OP has assumed that the changes reflected in the histogram when converting from ProPhoto to sRGB are also reflected in the appearance of the converted image on screen.

What springs to mind here is the fact that no monitor can fully display the gamut of ProPhoto. Most monitors are closer to the sRGB gamut are they not, with just a few really expensive monitors being able to display the full gamut of ARGB, which is not as wide as ProPhoto.

Here's an image I took recently in poor lighting for the purpose of checking out the noise reducing capability of the 'stacking' feature in CS3E and the auto-alignment capability with 7 hand-held shots at 1/20th sec and ISO 1600.

There's no doubt that the histogram is radically altered when converting to sRGB, but the appearance of the image seems to remain the same.

[attachment=3937:attachment]  [attachment=3938:attachment]

You will notice that even Buddhist statues are now paying homage to the histogram  .
« Last Edit: November 20, 2007, 11:27:47 PM by Ray » Logged
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« Reply #18 on: November 20, 2007, 11:30:50 PM »
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John,

I noticed the same and I am doing the same thing you do. Now, whether what we are doing makes sense is a different matter, hence your totally legitimate call for discussion. Unfortunately, it seems that your question provoked mostly smarty-pants comments. Some posters either pretend they do not understand, or did not bother to read carefully your post (and I exclude here those who really have no idea what you are talking about). Yet they try to win the argument by barking up the wrong tree (i.e, by refuting something you did not say). Hopefully, further discussion will return to your original question.
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« Reply #19 on: November 20, 2007, 11:47:36 PM »
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It looks as though the OP has assumed that the changes reflected in the histogram when converting from ProPhoto to sRGB are also reflected in the appearance of the converted image on screen.[a href=\"index.php?act=findpost&pid=154598\"][{POST_SNAPBACK}][/a]

Actually, I do not notice a change in the appearance of an image when I switch between pro photo and sRGB, despite the histogram changes.  I think this observation strikes at the heart of the problem- when I soft proof, I don't see a histogram change, and so I don't yet know what the limits of the output space are.  Only when I assign a profile to the image (ex: a costco ICC profile, or as a default, sRGB) for finalizing do I see a change in the histogram, which usually shows clipping.  I "Preview" the image in sRGB in camera RAW because an sRGB image seems to yield decent results from most offsite RGB commercial printers that don't provide ICC profiles (Walgreens, etc).  If I could preview with an ICC profile, I'd use that instead if I wanted to print the image from that printer.

John
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