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derstanding Your Digital Camera's Histogram -- A Nikonians Guide http://www.nikonians.org/html/resources/guides/digital/histogram_101...

2 24/07/2007 12:43

home > resources > Guides > Understanding Your Digital Camera's Histogram

UnderstandingYour DigitalCamera'sHistogramby Digital Darrell

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Introduction

Image & histogramshape

Image & histogramshape... continues

Computeradjustment ofimages

In conclusion

INTRODUCTION

Digital photography offers certain benefits for thinking photographers that areonly realized with a basic understanding of a metering feature called the"histogram."

Using the histogram in your camera's image viewing LCD will guarantee you a

much higher percentage of well exposed images. It is well worth working tounderstand the histogram, and is not overly complicated. I will try to cover this

feature with enough detail to give you a working knowledge of how to use thehistogram to make better pictures. If you are deeply interested in the histogram,there is much research material available on the Internet. This article is onlycovering a small amount of the information that is actually available, but willpresent enough knowledge to improve your technique immediately.

Nikon D2X rear view

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LIGHT RANGE

A digital camera sensor can only record a certain range of light values.

Remember how with film the light range was limited to only a few f/stops of

range? With transparency film the limitation was about 3 or 4 "stops" of light,while with negative film it could go on out to 5 or 6 stops. Today's digital sensorsseem to be able to record about 5 or so usable stops of light. Most will agree thatthe digital camera can record a little more than transparency film, and a little less

than negative film. This will change, since some digital cameras are now offeringextended range sensors.


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derstanding Your Digital Camera's Histogram -- A Nikonians Guide http://www.nikonians.org/html/resources/guides/digital/histogram_101...

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Unfortunately, many of the higher contrast subjects we shoot can contain over12-stops of light values. This is quite a bit more than it is possible to capture withfilm or a digital sensor today. It is important to understand how your digitalcamera records light, so that you can better control how the image is captured.

Look at Figure 1 below. The gray rectangular area is a representation of anin-camera histogram. Examine it carefully! Think about it for a minute beforereading on.

The histogram basically is a graph that represents the maximum range of light

values your camera can capture, in 256 steps. (0 = Pure Black, and 255 = PureWhite) In the middle of the histogram are the mid-range values that representmiddle colors like grays, light browns, and greens. The values from just abovezero and just below 255 contain detail.

The actual histogram graph looks like a mountain peak, or a series of peaks. Themore of a particular color, the taller the peak. In some cases the graph will berounder on top, or flattened. The left side of the histogram represents themaximum dark values that your camera can record. The right side represents themaximum white values your camera can capture. On either end of the histogramthe light values contain no detail. They are either completely black, or completelywhite. The top of the histogram (top of mountain peak) represents the number ofdifferent colors, a value you cannot control, so it is for your information only. We

are mostly concerned with the left and right side values of the histogram, since

we do have much control over those. (Dark vs. Light)

So, basically, the histogram's left to right directions are related to the darknessand lightness of the image, while the up and down directions of the histogram(valleys and peaks) have to do with color information. I repeated this foremphasis! The left (dark) to right (light) directions are VERY important for your

image making. If the image is too dark, the histogram will show that by clippingoff the light values on the left, or, if too light, by clipping on the right. This will

become easier to understand as we look at well exposed and poorly exposedimages..

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derstanding Your Digital Camera's Histogram (2) -- A Nikonians Guide http://www.nikonians.org/html/resources/guides/digital/histogram_101...

2 24/07/2007 12:44

home > resources > Guides > Understanding Your Digital Camera's Histogram (2)

UnderstandingYour DigitalCamera'sHistogramby Digital Darrell


Introduction

Image &histogram shape



In conclusion

IMAGE AND HISTOGRAM SHAPE

Look at the image below (Figure 2). It is well exposed with no serious problems.The entire light range of this particular image fits within the histogram window,which means that it is not too light or too dark, and will take very little or noadjustment to view or print. It contains no more than four or five stops of lightrange.

Look at the left side of the histogram graph above (Figure 2), and see that itdoes not cram itself against the dark value side. In other words, the dark valuesare not clipped off on the left. This means that the camera recorded all the darkvalues in this image, with no loss of darker detail. Then look at the right side ofthe histogram graph, and note that it is not completely against the right side,although quite close. The image contains all the light values available. Everything

in between, such as the blues and grays, are all exposed quite well, with fulldetail. A histogram does not have to cover the entire window for the exposure to

be fine. When there is a very limited range of light, the histogram may be rathernarrow.

The image in Figure 2 is a relatively bland image with smooth graduations oftone, so it makes a nice smooth mountain peak looking histogram graph. This willnot happen all that often, since most images contain quite a bit more colorinformation. Each prominent color will be represented with its own peak on the

histogram graph. The most prominent colors will have higher peaks, while theless prominent will have lower or no peaks.

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As we progress into images with more color or light information, we will see thatthe histogram looks quite different. Look at the image in Figure 3 below, whichis one that far exceeds the range of the camera's digital sensor.


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Notice that, overall, this image is dark and underexposed looking. The clouds arepretty well exposed, but the image is not very usable unless the clouds are the

primary subject. See how the histogram above (Figure 3) is crammed to the left,effectively being clipped off there? There are no gradual climbs like on amountain range, from valley to peak and back to valley. Instead, the image

shows up on the left side in mid-peak. It is "clipped." (Remember that word) Ifthis is confusing, refer to the histogram graph (Figure 1) at the very top of this

article, and notice that it has unclipped peaks and valleys like a mountain range.

If you don't fully grasp this yet, do not worry. The most important thing to knowis that when you see a histogram like in Figure 3 above, with part of the peakand valley clipped off on the left, THE IMAGE IS TOO DARK. This problem could

be corrected on this image by using a neutral density filter on the sky, whichwould have compressed the light range enough that the image could be more

fully recorded. This image above is even clipped a little on the highlight side(right). You can see why when you look at the rays of light shining between theclouds. The light is too bright, so it exceeds the light range of the sensor and isclipped.

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Understanding

Your DigitalCamera's

Histogram

by Digital Darrell


Introduction


Image &histogram shape...continues


In conclusion

IMAGE AND HISTOGRAM SHAPE ... continues

Now look at a similar image below (Figure 4). In this image a larger aperture

was used and more light was allowed in. We can now see the ground, but, once

again, the range of light is too great for the sensor, so it is now clipped off on thehighlight side (right). The dark-side graph value is not clipped; instead the graph

extends right to the left dark-side edge but stops there.

This image in Figure 4 above shows more detail in the ground area, but it is notprofessional looking, and will win no awards. The range of light is simply too great

to be recorded fully. The clouds and light behind them are overly light, and that

can be seen by the histogram's clipping on the right side. The most importantthing to remember with this image's histogram is that when you see a histogram

graph that is crammed all the way to the right and clipped, THE IMAGE IS TOO

LIGHT. Overall, a great deal of the image in Figure 4 is recorded as pure whiteand is gone permanently. (It is "blown out")

Also notice in the Figure 4 above that there are few mid-range values, as

represented by a big valley in the mid-range area. It has two peaks, the leftrepresenting darker values and the right representing lighter values. There is no

strong mid-range peak. Of course, other images will have multiple peaks and bejust fine. The important thing is that you prevent the image's light values frombeing clipped on the left or right. This is not always possible, but do your best totry. In other words, if you try to center the histogram, your images will be better

exposed. If you take a picture, and the histogram graph is shifted way left or

right, well, you can then retake it, exposing in the direction of the opposite lightvalue.

If there is too much light to allow centering the histogram, you must decide whichpart of the image is more important, the light or dark values. Does that make

sense? You must expose for the highlights, or you will lose detail in the lightareas. Which is more important, the dark areas, or the light areas?

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.

HOW DOES THE EYE REACT TO LIGHT VALUES?

The camera, with its lenses, film, or sensor is only a weak imitation of our

marvelously designed eye and brain combination. There are very few situationswhere our eye cannot adjust to the available light range, and we can see well. So,


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as photographers, we are always seeking ways to record even a small portion ofwhat our eye and mind can see.

Since our eye tends to know that shadows are black, and expects that, it isusually better to expose for the highlights. If you see dark shadows, that seems

normal. We're simply not used to seeing light that's so bright that all detail is lost.

An image exposed for the dark will look very weird because all highlight detail isgone. Your eye can see a HUGE range of light in comparison to your digitalsensor. The only time you will ever see light values that are so bright that detail

is lost is when you are looking directly at an overwhelmingly bright light, like the

sun. So, in a worse case scenario, expose the image so that the right side of the

histogram graph just touches the right side of the histogram window, and theimage will look more normal. (See the next image for a view of this)

This is no different than shooting with film, since we have always fought with onlybeing able to record a limited range of light. But, with the digital camera and its

histogram, we can now see a visual representation of the light values, and can

immediately approve of the image, reshoot it with emphasis on lighter or darkervalues, or see that we must use a filter to capture it at all.

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2 24/07/2007 12:47


Understanding


Histogram

by Digital Darrell


Introduction




In conclusion

COMPUTER ADJUSTMENT OF IMAGES

Looking at the image below (Figure 5), taken in mid-day overhead sunshine, we

see an example of a range of light that is too great to be captured by a digital

sensor, but is exposed in such a way that we can get a usable photo later.

Notice in the image above (Figure 5) how the dark values are indeed clipped off,and dark detail is lost. But, look to the right side of the histogram and notice how

the light values are not clipped off. So the camera recorded all the light values,

but lost some dark values. Since our eye sees this as normal, this image looksokay.

If we were standing there looking at the cabin ourselves, our eye would be able

to see much more detail in the front porch area. But, the camera just can't record

that much light range. If we want to get a bit more detail in the shadows thanthis image seems to contain, we can do it. Normally, the camera does not give us

enough control to add light values on the fly, so we use the histogram to get thebest possible exposure, and then adjust the image later in the computer. Some

cameras can be "profiled" to capture light ranges more effectively in one directionor the other, but when you push one area, the opposite area must give. So, we

need a way to take all this light and compress it into a more usable range.

We are now entering the realm of "post-processing," or of out-of-camera

(in-computer) image manipulation. Look at the image below. This is the exact

same image as above, but it has been adjusted in Photoshop to cram more imagedetail into the histogram by compressing the mid-range values. Notice that theentire histogram seems to be farther right toward the light side. Also notice that

the mid-range peaks are basically gone. We removed a good bit of the

mid-range, but since there was already a lot of mid-range there our image didnot suffer greatly.

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A side note: How this computer post-processing was done is outside the scope of

this article, but it is not very hard. Buy a program like Photoshop, Photoshop

Elements, Micrografx Picture Publisher, or another fine graphics programdesigned for photographers. Your digital camera and your computer are a

powerful imaging combination; a digital darkroom, where you are in control fromstart to finish, from clicking the shutter to printing the image. But, retreating

from philosophy, let's continue with histogram exploration.


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In the image above (Figure 6), see how the histogram edge is just touching the

highlight side of the histogram window? A small amount of clipping is takingplace, and you can see the slightly blown out area on the peak of the cabin's roof.

Sometimes a very small amount of clipping does not seriously harm the image.The photographer must be the judge.

The greater apparent detail in this image is the result of compressing themidrange of the light values a bit in the computer. If you compress or make the

mid-range light values smaller, that will tend to pull the dark values toward the

light side, and the light values toward the dark side. So, you will have moreapparent detail in your image. It's like cutting a section out of the middle of a

garden hose. If you pull both of the cut ends together, the other two ends of thehose will move toward the middle, and the hose will be shorter overall. If you

compress or remove the mid-range of the histogram, both ends of the graph willmove toward the middle. If one of the ends of the graph is beyond the edge of

the histogram window, or is clipped off, it will be less so, when the mid-range is

compressed.

We are simply trying to make the histogram fit into the frame of its window. If wehave to cut out some of the middle, to bring both ends into the window, well,

there is usually plenty in the middle to cut out, so the image rarely suffers.Remember, this is being done outside of the camera, in a computer. You can't

really control the in-camera histogram to compress values, but, you need to be

aware that it can be done in the computer, so that you can expose accordinglywith your camera's histogram. Then you will be prepared for later

"postprocessing" of the image in your computer.

In fact, now that we have compressed the mid-range values the above imagemore closely resembles what our eye normally sees, so it looks more normal to

us.

This is a strong benefit of digital vs. film photography. Most do not realize it, but

a RAW digital image contains an adjustable range of light. In a sense, the camerarecords a bracketed light range inside of a single image. Each RAW image

contains about a three stop bracket. It would be like taking three slides; one

exposed a stop under, one in the middle, and one a stop over, then combiningthem into one image. With a RAW image you can use a slider in Photoshop, or

your camera's included RAW image computer conversion software, to select fromthe bracketed range of light within the big RAW image file. It is like moving the

histogram window to the left or right over all that wide range of raw image data.You select a final resting place for the histogram window, capture the underlying

RAW data, and then your image is ready for use. This is a serious

oversimplification of the process, but, I hope, makes it more understandable. Inreality, the digital sensor records a wider range of light than you can use in one

image. While you might be able to use about 5 stops of light range in a normalimage, the digital sensor is recording probably about seven stops of light range.

You just can't get all of that range into the final image. It is there in the RAW fileas a selectable range. I prefer to think of it as a built-in bracket, since it works

the same way.

This bracketed light range within the image is present to a very limited degree in

JPEG, and a bit more so in TIFF images, but is the most pronounced in pure RAWimages. That is why many choose to shoot in RAW mode, instead of JPEG or TIFF.

If you are unsure about these modes, please read this linked article for a better

understanding.

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home > resources > guides > Understanding Your Digital Camera's Histogram (5)

Understanding


Histogram

by Digital Darrell


Introduction




In conclusion

IN CONCLUSION

Most digital cameras have a rocker switch of some sort that is used to scroll

through the images you have already taken. Most of these rocker switches will be

pressed up or down to scroll through the images, and then left or right to selectvarious informational modes, such as the histogram mode. So, when you take a

picture of an important subject, use the little thumb rocker on the back of most

digital cameras to select the histogram view of your image. (Check your manual,if you can't figure it out)

Your camera meter will be used to get the initial exposure only. Then you will look

at the histogram, and see if the image's light range is contained within the limitedrange of the sensor. If it is clipped off to the right or the left, you may want to

add or subtract light with your +/- EV compensation button, or use your manual

mode. Expose for the light range with your histogram. Let your light meter getyou close, then fine tune with the histogram.

In Figure 7 above is an actual histogram as seen on the back of a digital SLR.

Note that the histogram is not clipped on either the dark or light sides. The yellowhistogram graph is well centered, so the image is well exposed.

Notice also that there is not much mid-range in this image (Figure 7). It is rather

high-key, since it is a direct flash shot with a light background. See how the

histogram shows this by peaking the mountain range very sharply in the lighterside of the histogram window. The tallest peak is probably a representation of the

white background, while the smaller peak to the right is most likely a

representation of her skin tones. The young lady's hair is mid-range to dark, soyou see a smaller peak to the left side. See how nice it is to be able to use thehistogram to control your exposures?

Most of us will try to expose for the bright light, since our eye craves to see detail

in the light areas, but could care less about detail in the dark areas. Keep yourhistogram's bright light values side from being clipped off, but get it really close,

and your image will probably look a lot better. If the light is too contrasty, thenyou must resort to filters and reflectors, or overcast skies, like in the good oldfilm days.

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There are other LCD viewing modes that you can use along with the histogramgraph, such as the blinky blinky mode for blown out highlights, which most digital

cameras have today. What this does is to blink from light to dark in the blown out

highlight areas. This is a rough representation of a highlight-value clippedhistogram, and is quite useful for quick shooting. Using your camera's light

meter, histogram, and the highlight burnout blink mode together is a verypowerful method to control your exposures. If you master this method you will

have a very fine degree of control over where you place your image's light

ranges. This is sort of like using the famous Black & White Ansel Adams "Zone

System," but it is represented visually on the LCD of your digital camera.

You can even use your histogram to imitate your favorite film type. For instance,let's consider transparency film. It is limited to only a few stops of light and isvery sensitive to overexposure. A digital sensor is somewhat similar. So, if you

use the histogram to expose "for the light" side of the graph, not allowing

blowout (clipping) of light detail, and then you ignore the left side of thehistogram graph, allowing the dark values to fall where they may, you will

achieve a look very similar to a well exposed Fuji Provia slide. Then, if you takethe image into your computer and boost the saturation of the colors a bit...you

now have achieved a super-saturated Fuji Velvia look.

Your histogram is a very valuable tool, and this article just touches on its

capabilities. There are many discussions out there about how the histogramrepresents a combination of the three RGB channels (Red-Green-Blue), or only

the green channel of the three in the RGB, or even only a combined luminance

channel. It varies according to the camera. Your manual will tell you what type ofhistogram your camera has, if you are interested.

The manipulation of the histogram "levels" in Photoshop is a detailed study in

itself. Learn to use your computer to tweak your images, and you will be able toproduce superior results most of the time. But, first learn to use your histogram

to capture a nice image in the first place!

Your histogram is simply a graph that lets you see at a glance how well your

image is contained by your camera. Too far left, and the image is too dark, toofar right, and the image is too light. Learn to use the histogram well, and your

images are bound to improve!

K e e p o n c a p t u r i n g t i m e . ..

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