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Weekly Chatter: Histogram question with 1Ds
Wednesday October 15, 2003

Section 1: Histogram question with 1Ds





Digital Capture > Image Processing & Workflow > Histogram question with 1Ds




AuthorSubject: Histogram question with 1Ds  
Mark Tomalty
Posted on Wed Sep 3, 2003 3:11 PM


Hi guys/gals

This is my virgin post on the forum so please go easy. I'll try to get up
to group speed asap :>))

Could someone explain to me why the histograms I'm getting on images
shot with my 1Ds are slightly different than what I see in the Levels
histogram of the same image.

One practically all of the images I have processed in AdobeRaw (assuming
the image is correctly exposed and not dominated by high or low key
subject matter) the initial histogram is generally smooth and centered.
Opening the Levels dialogue box shows,in a vast majority of cases'a very
similar histogram but with the addition of a small spike,usually nudged
up against the 'dark side',occasionally on both sides simultaneously even when
the image doesn't have any apparent small areas of blown highlights or
plugged shadows.

Is this something to be concerned about?

Thanks
Mark




Andrew Rodney
Posted on Wed Sep 3, 2003 3:25 PM

on 9/3/03 2:15 PM, IR - Mark Tomalty wrote:

> Could someone explain to me why the histograms I'm getting on images
> shot with my 1Ds are slightly different than what I see in the Levels
> histogram of the same image.

My understanding (and Jeff will correct me if I'm wrong) is the Histogram is
based on the matrix you pick on the camera. So if you're shooting RAW but
you have a matrix setting for sRGB, what you're seeing is sRGB capture (and
at that, who knows how accurate that is). When you see a Histogram in Camera
RAW, again you're seeing the processed data (what settings you currently
have in effect and the colorspace you're telling the ACR you want to convert
into). Kind of makes you wonder just how useful that Histogram on the camera
really is!


Bruce Fraser
Posted on Wed Sep 3, 2003 3:26 PM

It's something that's useful to understand.

The Camera RAW histogram is a luminosity Histogram like the one you get from the Image>Histogram command.

The RGB Histogram in Levels just shows the aggregate values per channel. So while a 0 or 255 in the luminosity histo indicates black or white, respectively, the Levels Histogram shows a spike if there's a zero or a 255 in any channel.

The one cause for concern might be that you're clipping some saturated colors. In that case, you may want to choose a larger working space (if it's a concern at all.)


Mark Tomalty
Posted on Wed Sep 3, 2003 4:18 PM


HOLY COW BATMAN ! YOU GUYS ARE FAST!!

Thanks for the quick responses.
Just to clarify a couple of points as,typically,I was more enthusiastic
than clear.

I shoot RAW in the Adobe RGB matrix and process out into the same
space.

The comparison histograms I was looking at were both from the final processed
images in Photoshop and not from either the camera LCD nor the histogram
in Camera Raw

Assuming that I am seeing clipping of some saturated colors immediately upon viewing the image after Raw conversion is there a remedy at the conversion stage that is worth exploring,like moving the Saturation slider a little to the left,for example?

Mark


Jeff Schewe
Posted on Wed Sep 3, 2003 4:30 PM

Mark Tomalty wrote:

> I shoot RAW in the Adobe RGB matrix and process out into the same
> space.

You may "THINK" your are shotting raw in Adobe RGB, but you would be fooling
yourself. Shooting raw ignores ANY camera settings other than ISO. You're
shooting raw, not Adobe RGB. . .

> The comparison histograms I was looking at were both from the final processed
> images in Photoshop and not from either the camera LCD nor the histogram
> in Camera Raw


So, what histograms EXACTLY are you comparing? Or, are you just looking at
various histograms?


Jeff Schewe
www.schewephoto.com - www.pixelgenius.com - www.imagingrevue.com




Mark Tomalty
Posted on Wed Sep 3, 2003 5:00 PM


> > So, what histograms EXACTLY are you comparing? Or, are you just looking at
> various histograms?
>

I am comparing TWO different histograms from the SAME processed image.

ONE is the Image>Histogram,
the OTHER is the Levels RGB histogram.

My question was why did the Levels RGB histogram sometimes show a small
spike that was not evident on the Image>Histogram version.

Mark




Bruce Fraser
Posted on Wed Sep 3, 2003 5:15 PM

As I said, the Luminosity histogram in the Histogram command shows luminosity, while the composite RGB histogram in Levels shows pixel values.

The individual channel histograms are identical in both.


Jeff Schewe
Posted on Wed Sep 3, 2003 5:30 PM

On 9/3/03 5:05 PM, "IR - Mark Tomalty" wrote:

> My question was why did the Levels RGB histogram sometimes show a small
> spike that was not evident on the Image>Histogram version.

As Bruce indicated, the Histogram command produces either a luminosity
histogram or a single channel histogram.

The levels command (BTW do you have your preferences set to use cache for
histogram or not) by default shows the composite. . .

The odds of the luminosity ever matching the composite is nil and even less
if you have cached histograms turned on. . .

Jeff Schewe
www.schewephoto.com - www.pixelgenius.com - www.imagingrevue.com




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