Histograms

[Chuck Gardner]

With John Jackson's permission I'm posting an e-mail exchange about histograms....

On Sep 10, 2007, at 5:38 PM, John Jackson wrote:

Chuck, I followed your tutorial on editing with photoshop the different levels. I’m still learning but I do believe some of my pictures are looking better after going through the transformation. Here’s my question. In your tutorial your curve on the levels looks nothing like mine. I’ve attached a screen capture of mine so you can see what I’m talking about. Is this because of the way my camera was set up or is it normal?

Hi John,

No two histograms look the same. The RGB histogram also really doesn't tell the story of what actually happened during capture in the camera. That's why its important to look at the histogram for each of the channels separately.

One way to visualize what a histogram is telling you is to just use a gray scale like this one:

If you save that and open it in levels it will look like this:

The lines on the histogram correspond to the tones on the scale. They are a bunch of lines instead of a bell curve because there are only a few tones on the scale. All the line are the same height because in terms of surface area covered all the tones cover the same amount.

Now go up to Image > Adjustments > Brightness / Contrast and change the contrast to about -30. The shadows go gray and so do the highlights. Thats similar to what happens on an overcast day when there are not any strong shadows and a photo is underexposed -- like my rabbit example. Now open levels and look at the histogram:

It has the same number of lines because it has the same number of tones in it, but they have moved in on both sides creating a gap... The gap on the right tells you the highlights are underexposed. The gap on the left tells you there aren't any pure black shadow areas. So you see its the width of the graph and the gaps (or not) on each end that tell you what's happening.

Now without exiting Levels, move the sliders in...

When you move the sliders in to where the curve ( or lines in this case) start, you will increase the contrast of the image. If you compare it with the orginal grayscale you started with you'll find they look almost the same. Moving the sliders in increases the contrast of the image end to end. If you move the middle slider it affects the "internal" contrast but not the how light/dark the end points are.

Now lets consider your photo. It has the opposite problem from the one in my example -- too much contrast!

The first problem is that there are blown, overexposed highlights. Not so much in the subject in the foreground (except the arm) but in the sky, water, etc. The problem is that the histogram is showing all the tones and the blown areas are making it run off the side to the right...

To get a better idea of what is happening in the important areas of the photo just select a small area and then open Levels. The histogram will only reflect the area that is selected:

Do the same thing on your original. Then change channel to Red, Green, and then Blue. See how they are all running off the right side, indicating over exposure? Especially the Red channel. Since all that is selected is a skin tone area its a sure sign the photo is over-exposed.

Here's a color scale similar to the colors and contrast in your photo:

Here's how it looks in Levels:

Open up that color scale in Levels on your machine and look at the individual channels. Much different than the RGB version.

So what to do in those situations?

#1 - Avoid overexposure when shooting -- adjust exposure compensation (minus)

#2 - Keep the flash on - the fill light from the flash helps lower the overall contrast of the scene

How to correct this one?

The top sliders control the input side. Notice there is another set of sliders below it labeled "Output Levels". Slide those in a bit and watch what happens to the image:

The histogram up top doesn't change, but the image appearance in the editing window does. The shadows become more open and the highlights darker. Overall the lighting looks flatter, more like an overcast day.

But that makes some parts of the photo better and others, like the fish worse. But you can have it both ways!

Cancel out of Levels without changing output.

Open the layers window and drag background layer down and duplicate it, like in the second part of the tutorial so you have two copies.

Click top copy, open Levels, and adjust the output to make the shadows lighter and highlights darker.

Add a black mask to top layer (see below) the erase it in the areas you need less contrast / darker highlights...

You can see in the mask icon where I erased the mask to apply the flat copy on top of the contrasty one. But it looked too flat, so I moved the opacity slider back and forth until it looked about right -- 62%

It's the same general idea as using a duplicate layer set to screen mode to lighten as in my post processing tutorial.

When you duplicate and stack layers with masks in Photoshop you can blend the various layers together seamlessly by simply erasing the mask of the layer you wish to reveal. I find it is much easier to control what is happening to specific areas that way. Here I retained the vivid color and contrast in the fish but made the tone on the face a bit softer.

Also FWIW, the photo would be improved by cropping out the distractions when shooting. The fewer distractions you put in the photo the more visual impact what is really important will have. My "acid" test for whether something is a distraction or not is this question -- if it wasn't in the photo would you say, "Heck, where is that car on the shore?"

Whenever you have two centers of interest, like the fish and face in this photo, put them close together so the viewer can see both at the same time without ping-ponging between them...

Compare this crop edited to hold the fish closer to the face so you can see both together...

Better yet get a super wide angle lens and stick the fish out closer than the face -

the near / far perspective will increase the apparent size of your catch :-)

Chuck...

[Terry Beeson]

MY EYES!!! MY EYES!!!

TOO MUCH JOHN!!!! TOO MUCH JOHN!!!

But great information, Chuck... Thanks for posting... Good lesson...

[Kansas Fly Fisher]

Now I thought that was a pretty handsome model! This fish that is! I especially like the last touch up - man's that's a big fish!

John

[Chuck Gardner]

Here's another less complicated Histogram / Levels example:

Here is a photo which is about 1/3 stop underexposed...

How do I know its 1/3 stop underexposed?

First I used the eyedropper measurement tool to measure the brightest areas in the photo. It is necessary to open the "info" window to see the readings.

The brightest spot on the feeder measured 238, 238, 238 (red, green, blue). 255.255.255 is pure white without detail like you might want in the reflection off water or chrome. 238 is about where you'd want to see a textured highlight.

Next I opened Levels and saw a gap between the right side of the histogram window and the base of the curve on the right. Anytime there is a gap it means the brightest area in the scene is being reproduced in the photo as darker than pure white. From experince seeing photos which are correctly exposed and those which are 1/3, 2/3, 1 stop under I can tell this is a 1/3 stop gap.

Since I wanted the photo to have a bit more contrast and snap I decided to brighten the highlights by moving the levels highlight point to the left. When I did that a second set of number appeared in the info readout. As I moved the slider to the left I watched the #1 reading and stopped when it equalled 255 - pure white.

The highlights in the photo were made brighter and the contrast is improved. It's a subtle difference, but one which can be seen in a side-by-side comparison:

I also sharpened the "after" photo on the right which also improves the "snap" and sharpness, particularly in detail like the tongue.

This is 100% crop (pixel for pixel) which was processed the same way:

It really doesn't matter if you are using a $1,000 DSLR or a $300 point and shoot, the approach is the same. Take a shot and set the playback so it displays the histogram. If you see a gap between the curve and the right edge its telling you the photo needs more eposure. Open the exposure compensation (usually a button labeled "+/-" and add 1/3 stop (one-click) plus compensation and check again. If there is still a gap add another click of + EC until the right side base of the histogram curve kisses the right side of the window and looks like the one below:

If you see a histogram that is running off the right side with a gap on the left on the shadow side it indicates the photo is overexposed.

It's difficult to tell how much it is overexposed just by looking at the histogram, so often the quickest way to a correct exposure is to first cut it in half with a - 1 (minus 1) EC correction and take a second shot to see if it is still over, a bit under, or just right.

With a bit of practice you'll be able to look at the histogram and use its right side to see at a glance if the exposure is over / under. Cameras which show a live view in the LCD screen on back will usually also display a "live" histogram so you can see the effect EC adjustments have as you make them.

Hope you find this sort of info helpful for taking better shots, regardless of what you shoot with.

Chuck Gardner