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Author Topic: P45+ Highlight Exposure Latitude  (Read 4574 times)

Mort54

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P45+ Highlight Exposure Latitude
« on: August 25, 2007, 07:29:12 pm »

I just received my P45+ upgrade. One of the first things I do when I get a new camera is a test to see how much highlight exposure latitude it has (in other words, how many stops above midtone can I expose my highlights for and still preserve detail). One of the things I was quite disappointed in on my P45 was its limited highlight exposure latitude (it has tons of dynamic range in the shadows, but limited dynamic range in the highlights). On the P45, there is only about 2.5 stops between a properly exposed midtone and a blown highlight. Of course, knowing this, I can just spot meter a critical highlight and put it at +2, and my highlight won't be blown. But 2.5 stops isn't much of a range. Puffy white clouds are typically 3 to 3.5 stops over midtone, so if I put my cloud highlights at +2, my midtones end up underexposed by a stop or more. So that forces me to pull up my midtones in many of my shots, which pulls up any noise down in the shadows. Fortunately, the P45 has very clean shadows, so this isn't a problem. But still, it seems like a needless extra step, given that most modern DSLRs these days typically provide around 3 to 3.5 stops of highlight latitude.

Anyway, to make a long story short, I ran my usual test on the P45+, and it's no different than what I saw on my P45. Once again, I am quite disappointed. So, am I overlooking something? Does anyone else worry about the limited highlight exposure latitude on the P45 and P45+? Or does everyone just expose for the highlights and accept that they have to brighten up the midtones in post processing?

By the way, my test is simple. I simply photograph a grey card with different exposure compensations ranging from 0 to +4, in 1/3 stop increments, and note where the grey card becomes pure white. I spot meter off the card for the exposure. In my case, +2 and 1/3 stops yielded histogram values in the 250 range, just under pure white, while +2 and 2/3 stops yielded histogram values of pure white, i.e. a blown highlight.
« Last Edit: August 25, 2007, 07:33:54 pm by Mort54 »
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bradleygibson

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P45+ Highlight Exposure Latitude
« Reply #1 on: August 26, 2007, 12:05:57 am »

Hi, Mort,

One thing comes to mind--that you aren't actually metering where the back clips highlight data, but rather where the *post-gamma-corrected* rendering clips highlight data.

You can employ a different linear to non-linear curve which would provide you with more highlight headroom than you are getting now.  In the linear domain (the only place you can truly measure what the sensor is doing), you may find that your highlights are not overexposed at all.

If you use C1, I believe you can apply a linear profile to get this information (the image will be quite dark).  If you use ACR, you can download the Adobe's DNG SDK (this gets a bit geeky, sorry) and use the dng_validate  utility to get ACR to provide you with a linear conversion of your image.

From the Terminal/Command Line, try dng_validate -cs3 -3 stage3.tif -tif stage4.tif <input_file.dng>

(you'll have to convert a P45 raw to a DNG to get this to work).  Now you'll have 2 TIFF files both generated from the same raw file, from before gamma correction is applied (stage3.tif) and after it has been applied (stage4.tif).   Now you can see exactly what's going on.  I think this will be of interest to you, because in stage3.tif you will likely find that the RGB values of your white square likely haven't clipped or gotten anywhere near maximum in this rendering of your file.  

Once you know where the sensor's limits are, you can set up your favorite raw converter to apply a custom curve which preserves however much of the highlight headroom the sensor has available.

I suspect you will find that there is plenty of headroom, and that the default curves being applied are simply a bit aggressive in the darks to deliver more of the shadow detail that the sensor has to offer.  Nothing to stop you for dialing in the darks vs. lights tradeoff that makes you happy.

Hope that helps,
Brad
« Last Edit: August 26, 2007, 12:09:11 am by bradleygibson »
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Mort54

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P45+ Highlight Exposure Latitude
« Reply #2 on: August 26, 2007, 01:18:54 am »

Quote
Hi, Mort,

One thing comes to mind--that you aren't actually metering where the back clips highlight data, but rather where the *post-gamma-corrected* rendering clips highlight data.
Hi Brad. Thanks for the very detailed reply. It's going to take me a little time to digest the DNG portion of it. I did try the linear profile in C1, and it didn't seem to make any difference. At + 2 and 1/3 stops above midtone, the highlights were almost blown, and at + 2 and 2/3 stops the highlights were fully blown.

I do understand that I can use a custom curve to give me more highlight headroom at the expense of the shadows, however I've never done anything like that. Do you have any suggestions on where I might read up on how to do that? Also, I'm not familiar with C1 (although I'm going to have to get familiar with it, since the plusses don't open in anything else). Can a custom curve be applied within C1? Also, does Phase One provide any alternate curves to their default that I could try, or am I going to have to create my own?

Mort.
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eronald

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« Reply #3 on: August 26, 2007, 06:59:02 am »

Mort,

 Open an image in C1, go to the tab where you adjust exposure, below there's a button which says "Curve". Click on that, then click on the curve in the box to drag it around directly.
 When you are done, save the curve out by clicking on the "downarrow" icon above the controls. You can then load it back in for another image with the "uparrow" icon.

Edmund


Quote
Hi Brad. Thanks for the very detailed reply. It's going to take me a little time to digest the DNG portion of it. I did try the linear profile in C1, and it didn't seem to make any difference. At + 2 and 1/3 stops above midtone, the highlights were almost blown, and at + 2 and 2/3 stops the highlights were fully blown.

I do understand that I can use a custom curve to give me more highlight headroom at the expense of the shadows, however I've never done anything like that. Do you have any suggestions on where I might read up on how to do that? Also, I'm not familiar with C1 (although I'm going to have to get familiar with it, since the plusses don't open in anything else). Can a custom curve be applied within C1? Also, does Phase One provide any alternate curves to their default that I could try, or am I going to have to create my own?

Mort.
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bradleygibson

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« Reply #4 on: August 26, 2007, 11:10:04 am »

Quote
Also, I'm not familiar with C1 (although I'm going to have to get familiar with it, since the plusses don't open in anything else). Can a custom curve be applied within C1? Also, does Phase One provide any alternate curves to their default that I could try, or am I going to have to create my own?
[{POST_SNAPBACK}][/a]

It's been a while since I've used C1, but I believe you can put your own custom curves into it.  Besides, Edmund says you can, and he does seem to know his way around the low-level details of image processing, so I believe!  

Quote
It's going to take me a little time to digest the DNG portion of it.
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The weird dng conversion step happens because the tool (dng_validate) is designed to check the validity of DNG files being written by third parties (C1 4.0, DxO optics, etc...)  The conversion to DNG shouldn't affect your measurements at all.  Let me run through it step by step.

1) Process the file to be examined with Adobe's [a href=\"http://www.adobe.com/products/dng/]DNG Converter[/url] tool.  Be sure "preserve raw image" is checked under preferences to ensure that our raw data isn't processed during the conversion step.  (That would mess up the readings you'll do later.)

2) Download the dng_validate tool.  It's a part of Adobe's DNG SDK.  (Once you unzip the package, you'll find dng_validate under dng_sdk_1_1/dng_sdk/targets/mac/release/dng_validate (or ...win/release/dng_validate.exe).

3) Run dng_validate from Terminal/the command line as follows on the DNG file you created in step one.  Using the following command will give you both the pre- and post-gamma-corrected image:
dng_validate -cs3 -3 stage3.tif -tif stage4.tif <name_of_file_you_made_in_stage_1.dng>

Stage3.png (downsized and saved as png for web with no JPEG artifacts)
[attachment=3084:attachment]
Note the strong green cast of the stage3 file. This is because it was outputted before white balancing was performed.  Also, gamma correction, color space conversion, noise reduction, and so on have not occurred yet either.  This way you're getting a good look at the actual sensor data.  My white square on this image averages to around 135 (on a 255 scale).  Here's a look at the very same image post-gamma corrected:

Stage4.png (downsized and saved as png for web with no JPEG artifacts)
[attachment=3085:attachment]
This white square measures about 235/255--very near the top of its useful range.  Apply a different curve, and you can control how much this highlight is boosted.

Here is an example where I used my own curve to create more headroom.  This was pretty quick and dirty, so note the extra contrast, density in the shadows, etc.  These will be the tradeoffs for more headroom, and you can design your curve to balance all of these aspects to your taste.

[attachment=3086:attachment]

Note that the white square measures around 200/255 in this image.

Quote
I do understand that I can use a custom curve to give me more highlight headroom at the expense of the shadows, however I've never done anything like that. Do you have any suggestions on where I might read up on how to do that?
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How about right here?  

(As you mentioned, now that you're on your + back, you probably can't use ACR for raw development until the next rev unless you're tethered, but for the purposes of your testing, you can use your P45 files to build the curve you would like.)

Using ACR (screenshots below are from ACR4.1), go to the Tone Curve tab (second tab from the left under the histogram display at the right of the screen) and click the Point tone curve sub-tab.

[attachment=3087:attachment]

Now ACR will let you create the curve you'd like and will apply your curve to the curve being used to apply gamma to the sensor's linear data.  Or, in plain English, you can use your curve to get more headroom.

Here I used the TAB key to move between the points on the medium contrast curve, and droped them all about 25 ticks (press the down arrow when you're on a given control point).  I generally don't use the mouse when manipulating these control points if I need to be precise.  

[attachment=3089:attachment]

Using the little doohikey at the top right of the panel, you can save your curves, make them default, load others, etc. to your hearts content.  In the end, you can customize this to your curve automatically whenever you're bringing in new files, so your workflow goes right back to normal, but now your back is giving you however many stops of headroom you require.

[attachment=3088:attachment]

The bottom line is to remember that the results you're seeing aren't the hardware, but rather the culmination of some set of tradeoffs someone at your raw converter's company made.  The nice thing about many of these raw converters is that they let you override those tradeoffs for your own preferred set of tradeoffs.

Hope that clarifies it for you.  Enjoy your back!

P.S.  It's a shame we will have to wait until at least ACR 4.2 to get proper + support for our backs.    I will be in precisely the same boat once my + back arrives.  I will look at trying to figure out why tethered + files work in ACR 4.1 and unthethered ones seem not to.  Perhaps I can patch the untethered files to look like tethered ones, so that the will work in ACR 4.1.  If you wouldn't mind sending me a P45+ untethered raw file I'll take a look and see if this is feasible.  (My e-mail can handle large attachements.)

Best regards,
Brad
« Last Edit: August 26, 2007, 11:32:13 am by bradleygibson »
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eronald

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« Reply #5 on: August 26, 2007, 06:03:16 pm »

Of course, Mort could just ask me to build him a profile which has a steep curve in it. This would take me about 5 minutes and give him an easy seamless workflow

Mort, email me as below, and I'l send you an edited profile back.
You will have to send me a Raw image though so I can check out the effect.

edmundronald at gmail dot com

Edmund
« Last Edit: August 26, 2007, 06:28:45 pm by eronald »
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Mort54

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« Reply #6 on: August 26, 2007, 07:12:10 pm »

Quote
If you wouldn't mind sending me a P45+ untethered raw file I'll take a look and see if this is feasible.  (My e-mail can handle large attachements.)
Hi Brad. Thanks again for that incredibly detailed answer, and for taking the time I know if took to pull all that together. I tried emailing you a P45+ test file (an exciting shot of a grey card :-), but my email objected. So I've uploaded the file to my website, and I've emailed you a link. Hope that helps.

Thanks again,
Mort.
« Last Edit: August 26, 2007, 07:12:32 pm by Mort54 »
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Mort54

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« Reply #7 on: August 26, 2007, 07:18:01 pm »

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Of course, Mort could just ask me to build him a profile which has a steep curve in it. This would take me about 5 minutes and give him an easy seamless workflow
Edmund, thanks for your responses on this, and for the very generous offer to create a profile. What kind of image would be best for generating a profile? I could shoot a Gretag-Macbeth color checker, if that would work (I need to shoot that anyway to adjust my ACR calibration). The image I put on my website for Brad is simply a shot of a grey card. I'm assuming you need something more than that?

Regards,
Mort.
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eronald

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« Reply #8 on: August 26, 2007, 08:10:27 pm »

Quote
Edmund, thanks for your responses on this, and for the very generous offer to create a profile. What kind of image would be best for generating a profile? I could shoot a Gretag-Macbeth color checker, if that would work (I need to shoot that anyway to adjust my ACR calibration). The image I put on my website for Brad is simply a shot of a grey card. I'm assuming you need something more than that?

Regards,
Mort.
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Mort,

I need a few images of your favorite subject matter. In other words some of the Raw files you have already been working with. A Colorchecker shot if you have one is a plus, but it's not the main thing. The best way to send me Raw files is www.yousendit.com

Edmund
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Mort54

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« Reply #9 on: August 26, 2007, 08:50:58 pm »

Quote
Mort,

I need a few images of your favorite subject matter. In other words some of the Raw files you have already been working with. A Colorchecker shot if you have one is a plus, but it's not the main thing. The best way to send me Raw files is www.yousendit.com

Edmund
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Hi Edmund. It'll take me a few days to pull some suitable shots together. I got the back late yesterday and I've only taken various test shots so far, none of which are the type of shots you're looking for. Would it help if I shoot various exposures of the same shot - a normally exposed shot, and a version with blown highlights, for example - or is that not necessary? I'll also include a ColorChecker shot. Or for that matter, do the shots even have to be from this back? Could they be shots from my P45 back? As far as highlight lattitude, they were essentially identical.

Regards,
Mort.
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eronald

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« Reply #10 on: August 26, 2007, 09:23:45 pm »

Below, for a demonstration of what a profile does, a portrait by Ric Agu, with the original P25 profile, and with the one I made. Color balance is on the white cloth, has not been touched between shots - just profile assigned in PS then conversion to sRGB and Jpeg. First is orig, second is mine. In case you wonder, the one where you can see she has red hair is mine

I recommend downloading and viewing the images in Photoshop, the web browsers always do strange things to image colors.

Edmund[attachment=3091:attachment][attachment=3092:attachment]
« Last Edit: August 26, 2007, 09:32:59 pm by eronald »
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bradleygibson

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« Reply #11 on: August 27, 2007, 02:23:19 am »

Thanks, Mort54, I got the file.

I wrote more detail about what I found in another thread, but the quick answer is that P45+ files are written in one of two legal ways.  When written in one way, 100% of the files I have (6 of them) decode properly in ACR 4.1 and LR 1.1.  When written in the othe way, 100% of the files I have (5 of them, including the one you provided) fail to decode properly in both applications.

Since the file you provided was written by the back itself, it stands to reason that all backs are producing files that the current version of ACR can't read (just our luck).

Also, it is the entire file is affected by this 'way of writing'.  Assuming this to be the cause of the issue in ACR, there is no way to simply patch a header to get these files to work (they have to be completeley re-written).

Actually, there's a small chance that the way of writing isn't actually the cause of the issue, but I would need files shot tethered on a PowerPC Mac, an Intel Mac and a PC to have a chance at determining that.

Unless someone happens to have these files handy, at this point, I'm hoping Adobe will bring out an update with support for + backs before long.

Best regards,
Brad
« Last Edit: August 27, 2007, 02:24:55 am by bradleygibson »
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