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The histogram is a useful but often misunderstood tool that your camera provides to help you get the correct exposure on your images.
In this article we’re going to look at how to read it and use it to your advantage to help you do just that. Getting the best exposure (there is not such thing as the “correct” exposure, as it’s all subjective) in camera should be your goal every time you click the shutter. Using these tips should help you increase your success rate.
Dictionary definition: A bar graph of a frequency distribution in which the widths of the bars are proportional to the classes into which the variable has been divided and the heights of the bars are proportional to the class frequencies.
HUH?! Anyone else confused? But what does it do? How do you read it? Let’s have a look!
A histogram is a graphical representation of the pixels exposed in your image. The left side of the graph represents the blacks or shadows, the right side represents the highlights or bright areas and the middle section is mid-tones (middle or 18% grey). How high the peaks reach represent the number of pixels in that particular tone. Each tone from 0-255 (o being black and 255 being white) is one pixel wide on the graph, so imagine the histogram as a bar graph all squished together with no spaces between each bar. Have a look at the diagrams below:
There are many things we can learn about an image just by looking at the histogram.
We can tell an image is well exposed if it reaches fully from edge to edge without a space on one side of the graph, and isn’t heavily going up one side or the other. In an ideal world, it should just touch the left and right edges, and not spill up the sides, with a nice arch up in the center. However that doesn’t always apply in every situation, for every scene. Here are a few examples:
Gaps on either end indicate you are missing information and your exposure can be shifted safely without losing detail. When your graph is shifted too far in one direction or the other so that it does not even touch the other edge – that means you can safely shift your exposure to cover more of the range of tones. Let’s look!
Spikes up the left or right edge indicate “clipping” of that tone and loss of detail in that area. Clipped areas are often unrecoverable, especially in the highlight area but it is generally advised to expose so you your graph just touches the right edge and keep your highlight details. It is usually easier to recover some shadow detail and retain a decent image, than try and create highlight detail that isn’t there on the file.
In some scenes, however, it may not be possible to keep the graph within an acceptable range. For example, if you are photographing a scene with extreme contrasts such as: a sunset; bright sunlight and deep shadows; or an inside a building where you show outside the windows as well. In all of those cases you will not be able to keep from clipping either your blacks, or whites, or even both.
This graph shows an image with extreme contrast, lots of blacks, a spike of white and not much in the middle.
Is it wrong? Can you correct for it?
No it’s not wrong. You can’t really “correct” for it but you do have a decision to make when you see something like this. Do you shift the graph left and maintain highlight detail, or shift it right and keep shadow detail?
There is no right or wrong here, it’s how you interpret the scene before you. If in doubt, shoot both and decide later. The graph above comes from the image below, so as you can see it is not the incorrect exposure at all.
There are no mid-tones in this scene.
Here’s another example of a scene that will potentially go off the graph on both ends.
Using advanced techniques like image merge/blend, HDR and processing in Lightroom 4 (or PS CS6) you can compress the contrast range of the scene to fit within the histogram and therefore have details in all areas.
In the image above, I’ve used 4 bracketed images (taken 2 stops apart), and the HDR tone mapping process to bring the dynamic range of the scene down within printable range.
To help you establish how far to go in the image brightening direction, most SLR cameras have a setting called “highlight warning”. It will make any overexposed highlights “flash” or blink when you preview your images on your camera screen. Many people affectionately call this, “the blinkies”.
To do this on a Nikon, preview an image and press the Up or Down buttons (near the OK button) until you see the highlights flashing or outlined. This is the “highlight mode”. If you choose this setting, the camera will remember your setting for the next image you preview. You may need to activate this feature “highlight warnings” in your settings menu first.
To do this with a Canon press “Display” or “Info” button (depends on your model), until they show up on the screen when previewing images. You also may need to turn on this feature in the menu settings. Check your camera’s manual if you aren’t sure where to find it.
By using the tools your camera provides for you, it is easier to see how to adjust your image exposure. There is a lot more to know about the histogram, and you can use it when you process your images in Photoshop or Lightroom as well. Keep in mind that if you shoot JPG format, nailing the exposure in camera is even more critical. If you shoot RAW format you have some leeway to make adjustments later, but it’s still a better idea to get it right in the first place. If you are still on the fence about shooting RAW perhaps this article will help you decide: “Why shoot in raw format“
June 20, 2013 11:37 pm
Very informative. I learned a lot. Thank You... :)
February 2, 2013 01:04 pm
Thanks for a clear explanation of histograms and its application.
January 3, 2013 06:16 am
I liked the info in the article but it seems like you didn't even use a spell checker... I'm more of math guy than english and I could have edited this better.
October 10, 2012 06:18 am
My conclusion from this discussion:
- I continue taking RAW only (memory is cheap nowadays and a nonrecurrent expense, besides I enjoy the post-processing of my pictures).
- I forget that there is a button "picture style" on my camera.
- I use the camera histogram to see if the exposure is in the ballpark
- If the histogram indicates very high contrast (as explained in this article), I use the automatic bracket function. From these three exposure I'll decide "in the lab" which is best suited. I can think better at home than on the scene (I do most pics on safaris)
P.S. I tried the UniWB approach, but I do not think it's worth the trouble
October 9, 2012 11:28 pm
Actually I'm not very knowledgeble at all. I just started reading about this stuff once I began wondering if the in camera histogram I was looking at came from the raw data or from the jpeg elaboration.
And yes, you are right. I acknowledged that these technical issues are likely not very useful to most people and photographers out there. My first post was more of a "warning" message meant to increase people awareness about an issue with in camera histograms, but then I got carried away... :)
About how to set UniWB.
The procedure is slightly different for each camera model so it's not something I really know how to do in general. It involves three steps: defining the WB setting, importing it into the camera and telling the camera to apply it as the WB setting currently in use. The first step is the hardest but people have been uploading custom UniWB profiles for a while now so there is a good chance that you can find the file and detailed installation instructions just by googling "UniWB + [specific camera model]".
I don't think I would be able to do a better job at explaining the procedure than the people who wrote detailed articles about UniWB and that's why I am just telling potentially interested readers to use google and find tutorials tailored to their specific camera models. These tutorials usually also explain how to define and import the custom tone curve that is needed to correct the second main source of distortion for the jpeg histogram. Using UniWB alone is not enough.
October 9, 2012 05:35 am
@Leonardo wow you seem to be a wealth of highly detailed and technical information. That's all very interesting. I think though that for the average camera user that's a lot further than they want to take it, or need to know.
So how do you actually set the UniWB?
October 7, 2012 06:26 pm
First of all let me clarify a point. By saying that there is a HUGE problem with in camera histogram I was really exaggerating. What I said is true but it's not that big of an issue for most people in most situations. By following your good advice it's possible to get very good expositions in all the relevant shooting conditions and that's because as rough an approximation as the in camera histogram might be with respect to the raw data histogram, it's still accurate enough for the purpose of getting well exposed shots.
It becomes an issue for people who would like to squeeze every bit of performance from their hardware by maximizing the dynamic range of their shots.
About your question.
Well, I'd say you are right but I will expand a little on the subject because I think there are a few nuances that are worth taking into account.
Like I said, the issue with the histogram is that you are shown the jpeg histogram and to produce a jpeg the camera has to apply some sort of post-production to the raw data. This post production is based on the settings you chose including white balance, tone mapping, sharpening, saturation, contrast and the like. So you are right: you can minimize distortion by choosing the "right" sharpening, saturation and contrast values. The problem is that I'm not sure there is necessarily such a thing as "no picture styles" applied.
I only own a Nikon camera so I will be talking about it. Other manufacturers' products might be different. Nikon's cameras menu usually rate the amount of sharpening, contrast and saturation on a arbitrary scale that ranges from a few negative units to a few positve ones and is centered on zero (example: -3 to +3). These numbers mean almost nothing, being only arbitrary values. Now, choosing to leave the sharpening setting on zero doesn't mean necessarily that no sharpening is applied by the hardwired post-processing software; it just means that we are applying what Nikon has deemed to be the default amount of sharpening for that camera model. The default amount is what Nikon's engineers think is the most pleasing in the most common shooting conditions.
So it's certainly true that you can control and minimize histogram distortion by choosing the right settings but that depends on how the software works and it could require you to actually apply a picture style in order to reduce the sharpening, contrast and saturation that your camera might apply by default (if it does).
In my previous post I didn't mention saturation, contrast or sharpening just because the white balance setting and the tonal curve mapping have a much more dramatic impact on the histogram's appearence. But you are right: they have to be taken care of.
Because of the way digital cameras work, choosing a white balance setting simply equates to multipling the raw RGB values by a set of three coefficients that are determined by the camera manufacturer and depend on the color temperature of the light source. For example:
Canon 350D -> Daylight: multipliers (2.132483 1.000000 1.480864)
That means that red channel values are more than doubled, green ones are not changed and blue ones are increased by almost 50% with respect to the raw data (I think this all happens before applying the tonal curve)
UniWB is nothing more than a pretty clever way to trick the camera into applying a custom WB setting whose coefficients are (1,1,1). That's why it's called "unitary". The reason it tends to produce green tinted jpegs is that the Bayer filter of most cameras usually contains 2 green "pixels" for each red and blue ones. So if you don't account for this peculiarity (and white balance does) you get an image where green is dominant.
A good website that explains all of this in detail is for example: http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm
It's pretty technical but not that hard to follow.
October 7, 2012 07:39 am
@Hans - no you aren't doing anything wrong. I'm not sure why the histogram is changing based on the picture style. I'm just guessing here but the camera may be showing you the histogram for the jpg if you are shooting both. I'm not really sure, read the manual and if you aren't sure maybe submit the question on a Canon user forum.
@leonardo, interesting I did not know that, seems to answer Hans' questions. But if you are shooting raw with no picture styles applies won't you get a more accurate reading of what your raw file actually looks like on the histogram? Unitary WB, never heard of that. Now I have to go google that.
Laura, Tom and everyone else that has found this article helpful, I'm so glad.
October 7, 2012 06:53 am
@Andrew "Why don’t you show the actual pictures that generated the “poor” histograms?" - well truth is I didn't have any like that so I just created them to look that way. I see your point though, next time I will find some images to illustrate that. But really the "bad" histograms are just images that will be either under (too dark) or over exposed (too light).
October 6, 2012 11:41 pm
This is maybe the clearest article I've ever read on how to actually use and understand the histogram. Thank you for leaving out all the technical jargon. Now maybe I'll be able to use mine!
October 6, 2012 11:32 pm
The HUGE problem with in camera histograms is that the camera actually shows the histogram for the post-processed jpeg: either the one you get if you decide to "shoot jpeg" or the one embedded in the file when raw data are saved. This histogram is heavily influenced by the in camera post processing, especially by the white balance setting and the tone curve mapping.
What all of this means is that the histogram you see in camera is only a rough approximation of the real sensor response distribution. How rough it depends on the actual camera settings and shooting conditions. For example, most of the times the sensor gives you more leeway on the right end of the histogram than what you would think by looking at it: sometimes the highlights appear to be burned while in fact they aren't (at least not all three channels), being easily recoverable during the post production of the raw file (that's why the Recovery slider in Camera Raw works so well). The end result is that exploting the full dynamic range of modern cameras is not that easy.
People have been asking for the main camera manufacturers to provide the option for the camera to show the raw data histogram for years now, but sadly all requests appear to have fallen on deaf ears.
There is at least one method to minimize the distortion caused by the in camera post processing on the histogram: Unitary White Balance. It introduce issues of its own though, mainly the fact that jpegs produced by in camera post processing become utterly useless when the UniWB setting is applied (they are strongly green-tinted). But people who care about the raw histogram tend to shoot raw and personally perform post-production, so they are usually willing to trade the in camera jpeg usability for a greater accuracy in histogram evaluation.
October 6, 2012 06:12 pm
Thanks for the hint that picture style only applies to jpg. I said that I only use RAW anyway. However, I did some test shots (manual exp., low contrast object, RAW only). In picture style -> user definition I bracketed contrast between -3, 0, +3. As you correctly stated, the histograms look practically identical in Photoshop. BUT on the camera display the +3 setting was definitely much wider than the -3. This is why, in the past, I always fiddled with that contrast setting on the camera although I was taking RAW. Am I doing something wrong? (camera is 50D)
October 6, 2012 12:14 am
Great article and comments. I have been trying to analyze histograms for some time now. This has helped to make sense of it all.
October 5, 2012 01:52 pm
@hans - the picture style setting only applies if you are shooting jpg. However I've never used those settings, I do my adjustments in Lightroom afterwards. Those settings are primary for how you want the picture to look and the camera "processes" it for you. Contrast, sharpness and saturation.
When you shoot raw the file remains unprocessed by the camera. So like I said those "picture styles" only apply to JPGs out of camera.
October 5, 2012 11:34 am
Very helpful info here.
Thanks for sharing.
October 5, 2012 11:03 am
Thanks for the lovely article. I have started to understand the Histogram a bit now. I am sure to keep studying the same for further improvement.
October 5, 2012 05:29 am
Good article. However, in this context it should have been mentioned that many cameras have a "contrast" setting (on my Canon 50 D it's called Picture Style). I think this is often overlooked. My guess is that it directly affects the gain of the picture sensor's amplifier(s). If I have a high-contrast scene I set it to -3 and it a low-contrast scene to +3. This way I almost always can get a nicely distributed histogram. Always taking RAW format this gives me all the freedom to do the best possible post-processing.
I have a (German) book about the 50D and that Picture Style contrast setting isn't even mentioned!
October 5, 2012 03:53 am
@Mike - sometimes when you change the White balance on your camera, you will see it affect the different color histograms differently. So you can use it there if you see one color going off the charts. Inside LR you can pull down the brightness of each color independently using the sliders in the HSL section under the Luminance tab.
@Scott - exactly, couldn't agree more! Sometimes you WANT blow out highlights to create a certain look you're going for. This article is more about understanding how to read it, then use it how you want. If you don't know what you're looking it, then it's useless.
@albin "The “other” way to drag in those clippings is to raise ISO, but depending on camera, there’s often a price to pay in sensor noise or denoised plasticky smoothing." - actually raising the ISO isn't going to correct your exposure if it's over exposed. All raising the ISO does is makes your camera sensor more sensitive to light. If your image were underexposed it might help if you're on the edge of being possible to even make an image. But raising it won't fix clipped highlights or shadows. Only adjusting the actual amount of exposure being given to the camera will do that and ISO is just one piece of 3 things that control that (aperture and shutter speed being the others).
@Jim "I get the idea that there are a lot of folks out there who are trying to make digital photography seem difficult..it is not.." that is not my intention here at all. The thing with viewing your image on the screen only is the same as doing your editing visually on your computer monitor without first having calibrated it. Electronic screens have brightness settings that you can turn up or down. So if for example you have your camera brightness turned all the way up to see it outdoors, your image may actually be too dark in reality. Then let's take that a step further and you now open that dark image on your computer monitor that is also too bright and it looks just fine. Then you go to print and you get a dark muddy image and can't figure out why. Or if your monitor is too dark you may see a really dark image and brighten it too much. My point is that screens aren't trustable for viewing correct exposure. Check cropping and composition - absolutely! And if you just wanted to do photography for fun and don't edit your images, that's totally fine - but likely you wouldn't be here on this site.
@Jay I don't really use the color histogram on camera because I can't really do much to control it. If red is off the charts I could change my white balance to make the image cooler and bring the reds down but my color balance may be off then. I can do more to control individual colors inside LR so I do that there. My personal preference, that's all.
Bottom line is each of us has to find a system that works for us. If turning it all off and going visually is working for you, then keep doing it. If using the histogram and blinkies works, then keep doing that.
October 5, 2012 03:23 am
Usualy, I use the "blinkies".
October 5, 2012 03:04 am
Thank you ... enjoy your explanation ... very versatile ..
October 4, 2012 02:08 am
You're most welcome. Like I said it's one of the most understood things on the camera.
October 4, 2012 12:52 am
Nice post, thank you.
I request a similar article on color histograms.
October 3, 2012 11:58 pm
Nicely explained. I use these features as important gauges. They tell me the limits of my camera. Then a decision can be made to adjust or go with the readings. The exceptions included in your post are well suited.
Thanks, for the tips.
October 3, 2012 10:10 pm
Helpful article but every time I see this type of a discussion I'm left with the same question
Why don't you show the actual pictures that generated the "poor" histograms? Describe what you did to compensate, re shoot and show the new pic and new histogram so we noobs can see before and after.
October 3, 2012 12:04 pm
Histograms and Blinkies. Boy how I fought with these two for my first couple of years with digital..Then I I got a D300 with a large enough screen so I could actualy see my images. I turned off the Blinkies and never look at the histogram again.. I look at my image..I either like it or I reshoot until I get what I want, I get the idea that there are a lot of folks out there who are trying to make digital photography seem difficult..it is not.. But thats just my opinion.
October 3, 2012 10:22 am
The "other" way to drag in those clippings is to raise ISO, but depending on camera, there's often a price to pay in sensor noise or denoised plasticky smoothing.
October 3, 2012 08:51 am
By far the best treatment of histograms I have read.
October 3, 2012 06:39 am
Highlighting, of course, that there is essentially no such thing as a "wrong" histogram taken from photographic data. There are beautiful photographs with what some would call "bad" histograms, but we typically put the photographs on display -- not the histograms produced from them.
October 3, 2012 03:33 am
Good article. Both my Nikon D7000 and LR4 have color histograms. Any thoughts on how best to make use of that info?
October 3, 2012 02:28 am
Thanks for the tips. I often would think that the perfect one is always to strive for. But thanks for pointing out that there could be legitimate situations when the bar could be a little off.
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