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MobiWorkshop MW5 Using the Histogram

FundyBrian

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There was a request to discuss the use of the histogram so that will be the focus of this week’s MobiWorkshop.

You will see a Histogram displayed in many camera apps. You may have to go into the settings to turn on the display of the histogram. It can be very helpful in making good exposures.

You will also see a histogram displayed in many image editing apps. I think of the histogram as fairly essential when editing to make sure I don’t accidentally push important image tones off-scale while using some of the adjustments.

That is what is coming up in this spot so if that interests you keep watching.
 

FundyBrian

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The Histogram is a simple graph that shows what is going on in your image.
C7F4BD3E-68AE-477B-8F7F-7F060D4E90F6.jpeg

#1 - I have added the bottom gradient bar to help you see what tones are represented across the width of the graph. We have black at the far left and pure white at the far right, and a progression of tones in between. So the horizontal scale represents the brightness or darkness of the tones in your image. You can visualize your image on the histogram as the darker tones are shown on the left and progressively brighter tones to the right, until you finally reach the brightest tones in your image on the far right. Once you get used to it you can tell at a glance a lot about what is going on in your image.

The vertical dimension of the graph simply shows how many pixels exist in the image of each tone. We don’t actually count the pixels. It just means the higher the graph the more pixels of that tone there are in the image. For instance the larger hump in the middle left represents a darker than middle grey tone. That would be something like green evergreen trees while the next big hump to the right of centre would be something like green grass (lawn). The large hump just shows there is a lot of that tone in the image We aren’t much concerned about the shape of the graph in the middle, except perhaps for curiosity sake. What we are primarily interested in is the width of the graph and how it is placed inside the graph. In this case we at least have all the image data inside the box. That’s where we want it. This looks safe, or OK.

One problem arises with some histograms in camera apps. The outer limits of the “box” are often not clearly defined so it is hard to tell when you have reached the end. That really is a poor design. I like to see the edges of the working space clearly defined.
538C258B-3215-4280-90DB-77930F4170AD.jpeg

#2 - Here is an important point to realize. The far ends of the graph right and left represent pure featureless white on the right, and solid black on the left. If you are hoping to see some detail in your brightest tones then the little vertical black line on the bottom right represents the farthest to the right the highlights should be. Otherwise they will have no detail. Likewise at the black end of the scale on the left. Anything beyond the little vertical black line will not have any shadow detail. Naturally, those little black lines are not even shown on the histogram. You just have to know more or less where they should be.

In simplest terms that means to be sure you record highlight detail in your image you should have a little safety zone on the highlight end. You also need to pay attention to what tones actually are in your image. It may be that your image doesn’t have any bright tones but has lots of low and middle values. In that case letting the middle tones creep into the area where highlights belong indicates over-exposure.

If you ever have to choose between the highlights or the shadows I would always opt to preserve the highlights. There’s nothing worse than burned out highlights.

Here’s another important point. On iOS devices the histogram shows JPEG values. If you are shooting RAW it is important to realize that the RAW file can contain a wider range of tones than a JPEG and that means it is considered “safe” to let your highlights bump up to the far right of the graph and you just have to have confidence that your highlight values really will be recorded. This is why shooting RAW & JPEG at the same time is a poor strategy. They have opposite exposure strategies. When photographing in RAW mode you want to minimize image noise which is more prevalent with underexposure. This is where “expose to the right” comes from.

F04499FD-0271-405B-8E6C-557D272CF918.jpeg

As I write I can now only see image file names so I have to remember which order the images come in.
#3 - This is what happens when you make an underexposure. All your image data is dragged to the black end of the scale. The data that has gone beyond the box will all be solid black -bye bye shadow detail.

5E3D7F03-E29C-406B-8750-74FA42DE4D31.jpeg

#4 - this is how the histogram actually looks when you have image data beyond the display region. The dark tones have piled up against the far edge. When you see a pile-up like this pay attention. You have an exposure problem. More about this when I talk about overexposure.

89BD1EE3-3291-40AE-9D4A-88FB22F94B99.jpeg

#5 This shows overexposure in action. All your image data is dragged to the right, some of it exceeds the limits of the histogram. Everything outside the box will be cut off and rendered as solid featureless white.

8C291554-0447-4D09-93ED-A1E14BA195E5.jpeg

#6 - The part of your image data that went beyond the sounds of the histogram will be cut off and displayed like this. Note the pile-up on the highlight end. Anything that goes beyond the perimeter of the box is simply not recorded - as if it never existed. You can see the significance of that with respect to exposure and image quality.

741880D6-A620-4A75-B678-E4BC0D610762.jpeg

#7 - People seem to think that anything can be fixed in editing. Here is what actually happens. You take your overexposed picture into an image editor and you adjust the exposure to darken it. All the data that was inside the box will respond well enough to the exposure adjustment. But the part that was outside the box is simple not there to be rescued. It never was recorded in the first place so there’s nothing to recover. You can darken your image until the white areas turn grey and still there will be no detail in them. The image is toast.

7A314FCD-27D1-434F-BD55-88D3468BDA50.jpeg

#8 - This is what you see when photographing in very low contrast conditions like fog. Increasing the contrast in editing can really help an image like this.

57472C37-43D4-47AD-8AC2-644402AA9894.jpeg

#9 - If you ever see this while photographing it means you’re in trouble. The dynamic range of the scene has exceeded the capability of the camera to record it. Both the highlights and shadows are off scale. It would be a waste of your time to make a photo like this - unless you switch to HDR. Preferably the real HDR with an app that makes 3 exposures, one normal, one overexposed by a set amount, and one underexposed a similar amount. Typically the exposure steps would be 1 stop, or 1&2/3 stops, or even more, on either side of your middle exposure. Then the three images are combined into one extended range image. Ahhh. That’s better.

I’m hoping you can see from this how useful the histogram can be in helping you be sure your exposures will be OK while making exposures with a camera app.

Next I will explore the usefulness of the histogram during editing.
 
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terse

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I’m hoping you can see from this how useful the histogram can be in helping you be sure your exposures will be OK while making exposures with a camera app.
And here I'd like to add that the problem for me with the histogram displays of most camera apps is that they're too small to be easily readable. If you can't tell where the endpoints of the histogram are, it's not going to help. The camera app I like best for this is ProCamera: The histogram is still quite small, but it turns red at the appropriate end when you are exceeding the limits for blacks or whites. In the screenshot below, I've overexposed, so the histogram is showing red at the right side (overexposed whites).

IMG_8877.PNG
 

FundyBrian

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And here I'd like to add that the problem for me with the histogram displays of most camera apps is that they're too small to be easily readable. If you can't tell where the endpoints of the histogram are, it's not going to help. The camera app I like best for this is ProCamera: The histogram is still quite small, but it turns red at the appropriate end when you are exceeding the limits for blacks or whites. In the screenshot below, I've overexposed, so the histogram is showing red at the right side (overexposed whites).

View attachment 120730
Yes, that’s why I mentioned the histograms don’t always have well defined edges. Without being able to see the ends clearly you can’t tell when you're Getting to close to the danger zones.
 

FundyBrian

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Here are 4 camera apps I found with the biggest histogram displays.
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In this one the dots remain even when there is no vertical component do you can always see the end points.

9A0F59B2-81C3-4471-B943-2D4673B59B61.jpeg

This one has pretty good contrast with the box.

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Not very clear, this one.

72122C13-5B74-4BBC-A0B0-DA7B270A1F70.jpeg

Maybe the best out there at the moment. Big enough some might find it intrusive.

Wouldn’t it be nice if we could tap on the histogram and have it pop up larger momentarily so we could have a better look.
 

FundyBrian

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Using the histogram during the editing process is most important when you are working with processes that can make basic changes to the tonality or dynamic range of an image. Exposure, contrast, and especially curves. There are many presets that cause disasterous consequences in the highlight areas on an image. Unfortunately, the histogram display is not often shows in that type of app.

The next problem is that only the more serious photo editing apps even have histogram displays so this leads me to suggest you do your serious work in apps that have histogram displays and leave the other apps for dabbling.

In many apps the histogram is only displayed in the curves adjustment area and some also in the adjustment sliders area.
9728B6A5-4057-42A5-9E3C-48271A052C1D.jpeg

You will find a small histogram in Snapseed in the adjustments area. One good thing is the outer edges of the histogram are clearly indicated. You may need to zoom and move your photo to make the histogram easier to see,

FDD2E51C-D4B4-44EC-89B7-F2C82C5728B8.jpeg

In the curves area in Snapseed the histogram is much bigger, but again, you may need to zoom and move your photo to make it easier to see.

00DEFA4D-1F0D-48CB-AA27-7726F5EEC52B.jpeg

MaxCurve displays a histogram in every curve displayed . It isn’t easy to see, depending on the image tones.

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Enlight displays a histogram in the curves area and it is easy enough to see since it is mostly outside the image area.

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RAW Power shows the histogram if you turn it on in settings.

On the iPhone I found these apps display histograms.
• Snapseed
• RAW Power
• Enlight
• Max Curve
• Fine
• Pixelmator

If you know of others please let us know.



I didn’t find a histogram in:
• iColorama
• Leonardo
• Superimpose X
• Darkroom
• Instaflash Pro
• Prime
• Photomatix FX (micro sized)
• MuseCam
• Pixomatic
• Wonderoom
• ColorStory
 
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FundyBrian

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Brian Townsend
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On the iPad things are looking much more favourable with the addition of Affinity Photo.

ACE4BFB5-9A31-4A30-A0FE-D55C749C3AE9.jpeg

Affinity Photo displays a good histogram all the time you’re in the development module. This is used mostly in RAW development.


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One of the most important areas where the Histogram is very important is during the development of HDR images. It also shows in the Tone Mapping module. It is important at all times to keep an eye on the histogram and make sure your highlights don’t creep off-scale. At this stage the histogram is my main guide in adjusting the main settings before moving on to the next steps in developing the image.


398467B9-3C88-4704-BD73-C97672676602.jpeg

At any time you can call up the Histogram by selecting the Metadata Studio.

20E75A86-F62C-4E1F-B854-234ED475C9BB.jpeg

RAW Power displays a small histogram. It would be nice to see it bigger but it is always available during adjustments.

461B428A-275D-47C7-8A5B-F8D663307860.jpeg

Fine has a small histogram in the upper left corner of the curves adjustment window.

Any more apps that display a histogram that I missed?
 
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