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View Full Version : Using the Histogram in Photoshop CS2 or other Processing Software



convergent
08-03-2006, 01:08 PM
I picked up this tidbit yesterday while watching the Ron Reznick DVD series. I typically use levels to make corrections in Photoshop CS2, but I rarely ever bring up the histogram display, even when making corrections to the RGB channels individually. I use the histogram on the camera, but wasn't really using it on the computer. Well, I learned something yesterday that opened up some areas I had been missing.

One of the reasons I hadn't been using it, was well... I thought the Levels display looked close enough to the histogram for me. Well, I was wrong. I was inherently doing some of the same things as this technique teaches to do, but I was doing it totally by eye and not fully understanding what was going on... thus repeatable results were more difficult.

I'll show a few screen shots here to illustrate.

Here we open the file and the histogram window. You open the histogram window by going to >Window and checking the Histogram selection. If it is already checked, then it is open and maybe tabbed or hidden by another window. Once it is open, you can use the little right arrow below the X (in the histogram window) to select All Channels View. While this particular image has pretty good alignment of the channels, many images may be blowing one channel and the composite histogram would look good. You may also have a color cast that the All Channels view would help you uncover.

http://www.northeastfoto.com/gallery/files/3/2/2/histogram1a.jpg

We can see that the histogram hits the X axis well before the left margin. This indicates that we are giving up some dynamic range, and that we need to adjust the shadow tones. This is something that I've always done with Levels, so we go to >Image>Adjustments>Levels

http://www.northeastfoto.com/gallery/files/3/2/2/histogram2a.jpg

In this dialog, we are going to take the adjustment handle at the left margin of the histogram and move it to the right until it aligns near the point where the data starts. Again, this has been my normal approach.

http://www.northeastfoto.com/gallery/files/3/2/2/histogram3a.jpg

Once you make this adjustment, the histogram looks much better in the Levels display. This is where the Histogram window is very useful. In the Histogram window, we can see that it shows the before and after view. We see then that we not only adjusted the shadow levels, but we also greatly effected the midtones, or Gamma levels as well. The same thing would have happened if we adjusted the highlights to account for under exposure. We'd move the Gamma as well. I have in the past just kind of eye balled things and made Gamma adjustments that way. But, with this histogram display, you can see very well how to fix the Gamma problem created by the Shadow adjustment.

http://www.northeastfoto.com/gallery/files/3/2/2/histogram4a.jpg

In this case, with the visual indicator on the histogram of where the midtones were at before, we simply grab the midtone adjustment handle and move it until the right edge of the slope in the middle aligns with the before view.

http://www.northeastfoto.com/gallery/files/3/2/2/histogram5a.jpg

And this completes the adjustment. Now, if we wanted to make further Gamma adjustments, we could, but we'd be starting from the "as shot" midtone.

Here are a close-up of the histogram before and after the Gamma adjustment


http://www.northeastfoto.com/gallery/files/3/2/2/histogram6.jpg

http://www.northeastfoto.com/gallery/files/3/2/2/histogram7.jpg

The histogram is an invaluable tool in processing. After seeing this, I took a look at making adjustments with Curves, Shadow Highlights, and other methods and found that the histogram gives you a clear indication of what is going on. I always struggled with using Curves for adjustments, and I think the Histogram window is the missing key for me.