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If you’ve used a digital camera at all, you’ve already discovered that the LCD on the back of the camera is fairly useless as an aid in judging exposure. Its brightness varies considerably with ambient light, and in some cases, user-controlled settings. Its optical quality is minimal, and it lacks tonal detail, displaying a very contrasty image. Fortunately, camera manufacturers are aware of this (yes, sometimes they do pay attention…) and they provide you with an alternative, which actually does a great job of helping you judge exposure— The Histogram. Highlights, shadows, mid-tone, tonalities, dark, light, high key, low key, black, white–all are terms that describe in words what the histogram shows pictorially. It is essentially a simple graph that charts the tonal values in your image. The position of tones along the horizontal X-axis tells you about the distribution of tonalities, with pure black on the left and pure white on the right. The absolute numbers don’t matter as much as their relative position along the line. (Originally, the X-axis was divided into 256 segments because the original black and white histogram was made for 8-bit images that contain 256 shades of gray). For our purpose, however, you can divide the X-axis into thirds, with the left third representing shadow detail, the right third representing highlight detail, and the middle third representing mid-tones. This is a crude approximation, but it will help you visualize the tonal values and where they fit into the graph. Photography and Text © 2009 Michael Lustbader BEING COMFOR BEING COMFOR T T ABLE ABLE WITH WITH YOUR HIST YOUR HIST OGRAM: OGRAM: Not a Gynecological Concept Not a Gynecological Concept A light image will show the bulk of its tones grouped towards the right side of the histogram. Dark tones are found on the left side of the graph. A mid-toned image will have most of its tones in the middle of the histogram.

Photography -- Being Comfortable with your Histogram

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Page 1: Photography -- Being Comfortable with your Histogram

If you’ve used a digital camera at all, you’ve alreadydiscovered that the LCD on the back of the camera isfairly useless as an aid in judging exposure. Itsbrightness varies considerably with ambient light,and in some cases, user-controlled settings. Its opticalquality is minimal, and it lacks tonal detail,displaying a very contrasty image.

Fortunately, camera manufacturers are aware of this(yes, sometimes they do pay attention…) and theyprovide you with an alternative, which actually does agreat job of helping you judge exposure—The Histogram.

Highlights, shadows,mid-tone, tonalities,dark, light, high key, lowkey, black, white–all areterms that describe in

words what the histogram shows pictorially. It isessentially a simple graph that charts the tonal valuesin your image.

The position of tones along the horizontal X-axis tellsyou about the distribution of tonalities, with pureblack on the left and pure white on the right. Theabsolute numbers don’t matter as much as theirrelative position along the line. (Originally, the X-axiswas divided into 256 segments because the originalblack and white histogram was made for 8-bit imagesthat contain 256 shades of gray). For our purpose,however, you can divide the X-axis into thirds, withthe left third representing shadow detail, the rightthird representing highlight detail, and the middlethird representing mid-tones. This is a crudeapproximation, but it will help you visualize the tonalvalues and where they fit into the graph.

Photography and Text © 2009 Michael Lustbader

BEING COMFORBEING COMFORTTABLEABLEWITHWITH YOUR HISTYOUR HISTOGRAM:OGRAM:

Not a Gynecological ConceptNot a Gynecological Concept

A light image will show the bulkof its tones grouped towards theright side of the histogram.

Dark tones are found on the leftside of the graph.

A mid-toned image will havemost of its tones in the middle ofthe histogram.

Page 2: Photography -- Being Comfortable with your Histogram

The position of values along the vertical Y-axis simply tells you how many pixels are present atthat particular tonal value.

It is important to remember that there is no “correct” histogram, just as there is no “correct”image. The shape of the histogram will vary according to the distribution of tonal values (light,dark, mid-toned) within the image. It is however, more than a graphic representation includedto impress you with another flashing light–the histogram is a valuable tool that helps you create

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This crab spider on a Datura blossom would beconsidered a “high-key” image, with itstonality mostly towards the right side of thehistogram. I used fill-flash for this image, andpaid careful attention to the histogram so I didnot “clip”, or over-expose the highlights.Several exposures were required to “fine tune”the flash so none of the highlight values werelost.

Personally, I find blown-out highlights moreobjectionable than blocked-up shadows. If Ihave to compromise and make a choice betweenthe two, I will usually choose to sacrificeshadow detail rather than highlight.

This photograph of a marine iguana in the Galapagos Islands(referred to by Charles Darwin as “imps from hell”) containsmostly darker tones which are grouped towards the left side ofthe histogram. The shadows under the claws and chin did need abit of a boost during the RAW conversion but paying attentionto the capture histogram ensured that the correction needed wasminimal.

This luna moth on wild geranium shows mostly middle tones,from light to darker green. Your meter rejoices when it sees animage like this and will nail the exposure with little, if any, helpfrom you. A mid-toned image will have most of its tones, asdemonstrated above, in the middle of the histogram.

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histogram will vary according to the distribution oftonalvalues (light,dark, andmid-toned)within theimage. It is,however,more thanjust a graphic

representation included to impress you with another flashing

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When the image has a preponderance of lighttones, the histogram is shifted to the right. Thefact that the peak of the histogram runs off thegraph is not important–it just means that thereare more light tones than the graph has room toshow. These tones are not lost, just not visibleon the graph.

This, however, is an accurate representation ofthe image, and does not require adjustment in-camera. That doesn’t mean that I may not stilldo some dodging and/or burning in Photoshop.As we will see later, an image may still need tobe optimized for maximum aesthetic impact; thehistogram just assures that all of the tonalitiesare present in the original capture.

The black areas of this Amazonian poison dart frog’sskin are mucoid and truly without detail. Even thoughthe histogram shows clipping of the dark tones, it wouldbe fruitless to attempt a salvage of tones that are notpresent in the original capture. I bracketed this image inthe field, and even in the over-exposed images, there wasno detail in most of the black areas.

The remainder of the image is quite middle-toned, and Iwas able to use the camera’s recommended exposure.A reflector provided the catch light in the eye. I wasafraid that fill-flash, even at low intensity, would causetoo many distracting reflections.

When the image contains mostly dark tones, its histogramis shifted to the left.

In this image of two oystercatchers, there is slight“clipping” (which we will discuss shortly) on each end,but it is acceptable to me, since it represents areas thattruly contain no detail. Analysis of the image shows thatthe areas where detail is lost are located in the blackshadows in the lava and in the white pebbles. No detail islost in the black or white feathers of the birds. Attempts atcorrection might lead to excessive lightening of shadows,burning out of highlights, or both, so I chose to expose “asis”. Again, some burning and dodging took place in-computer, but all the important tonalities were preservedin the original exposure..

More examples:

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a final image containing all the information your camera iscapable of capturing.

There is no “correct”

histogram, just as there is no “correct” image; there is aunique histogram for each unique image. The shape of the

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OK, so that’s a lot of theory. How does the histogram work for you in the real world?Essentially, it significantly decreases the need for bracketing exposures. It helps you choose acombination of f-stop and shutter speed which will yield an optimal exposure and identifiesexposure problems at the time that you are best able to correct them--in the field.

What is Clipping?

Each end of the histogram should taper off and approach the baselineBEFORE it “hits the wall”. A graph that runs into the wall BEFORE ithits the X-Axis on either side indicates “clipping” and subsequent lossof available information.

If clipping occurs on the left, detail is lost in the shadow area of the image. If you simplyincrease the brightness after exposure (with Levels or Curves in Photoshop, for instance), allyou will accomplish is an increase in NOISE in the affected areas. (Noise being the digitalequivalent of grain). You can lighten the area in question, but you cannot recreate detail that hasbeen lost. This is a problem you must try to improve before exposure, although you do havemore leeway if you are shooting RAW.

Shadow Clipping

You can correct clipping of shadows by using a slower shutter speed or a larger aperture--in other words--MORE LIGHT.To this end, you can also use fill-flash or a reflector. You can usually judge the result of using the reflector through theviewfinder, but if you use flash, be extra careful not to burn out your highlights. You will learn to rely on your histogram.

Fill-flash was used to photograph this crab spider. I used a two-headed Nikon R1C1 ring flash, with one flash straightahead at 2/3 power and the other from the side at 1/3 power. Just enough light to open the shadows, and not enough toburn out the white highlights.

Shadow clipping occurs when the histogram “hitsthe wall” on the left side. Detail is irretrievably lostif this is not recognized in the field, although somesalvage is possible if you are shooting in the RAWformat. Change your exposure parameters to shiftthe histogram to the right.

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OK, so that’s a lot of theory.How does the histogram work for you in the real world?Essentially, it significantly decreases the need for bracketingexposures. It helps you choose a combination of f-stop andshutter speed which will yield an optimal exposure, onewhich contains all the tonalities of the scene that your camerais capable of capturing.

Eachendofthe

graph should taper off and approach the baseline BEFORE it“hits the wall”.

A cliff-edge pattern on either side indicates “clipping” andsubsequent loss of information. If it occurs on the left, detailis lost in the shadow area of the image. If you simply increasethe brightness after exposure (in Photoshop, for instance), allyou will accomplish is an increase in NOISE in the affectedareas. (Noise being the digital equivalent of grain).

If the clipping takes place on the right, highlight detail islost. If highlight or shadow detail is compromised in the

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If highlight or shadow detail is compromised in the original exposure, you cannot restore it inyour editing software. Remember the saying that you only get one chance to make a good firstimpression? The same holds true of capturing an optimally exposed image. The only chanceyou get to capture a full range of tonal values is in the original exposure. If you are capturingRAW files, you do have a second chance at salvage during your RAW conversion, but even thebest RAW conversion cannot replace detail that has been lost.

So, you see the pattern of the graph and interpret it. Now change your exposure settings(aperture and/or shutter speed) to compensate for the clipping and recheck the histogram afteryour next exposure.

Is clipping always a “bad” thing? Not necessarily. It depends upon the content of the image. Insome cases, clipping may represent areas of pure black, which contain no shadow detail. Inother situations, it may represent specular highlights (reflections off water, metal or glass), andlikewise may not contain any highlight detail. The histogram gives you warning of potentialtrouble and allows you to make adjustments before you see on the monitor that you can’t “fix itin Photoshop”.

In this photograph of a milkweed bug, the initial histogram showed two problems. First, the milkweed silk wasoverexposed (highlight clipping was evident on the initial histogram). In addition, the blacks were also blocked-up andwithout detail.

The solution here was to underexpose to restore detail to the seedhead. Then in Photoshop, I was able to manipulate theRAW file to restore some of the detail to the black areas on the bug’s wing. When I have to choose, I will alwaysoptimize highlights first--To me, a burnt-out highlight is much more distracting than a blocked shadow. In this image,I could have lived with the overly-dark blacks, but the burnt out highlights would have ruined the image for me. Thisis a subjective call--everyone has a different tolerance level.

Highlight Clipping If the clipping takes place on the right side of thehistogram, highlight detail is lost. Change yourexposure to shift the histogram to the left. Commonwisdom states that, to minimize noise (the electronicequivalent of grain), the histogram should be shifted asfar to the right as possible without clipping thehighlights.

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Some general rules of thumb:A. Avoid clipping at either end unless you are comfortable that it represents either deep black without detail or specular highlights.B. If the contrast range is too great to avoid some clipping, then clip shadows rather than highlights. There is software available and “workarounds” that can help you minimize shadow noise, but once detail is lost, you cannot replace it. Blown-out highlights are more objectionable than blocked-up shadows.C. Move the body of the histogram as far to the right as you can without clipping highlights.

(No, I have not forgotten about High Dynamic Range techniques or layer masks for contrastadjustment, but those are topics in their own right and deal with issues which may be unfixablewith a single exposure in the field).

Become comfortable with the histogram. It may appear several times during the life andprocessing of your digital image:

A. In-camera, as just described, you can use it to optimize the original capture.B. In your conversion software (Camera Raw, Nikon Capture, etc.) if you are shooting RAW.C. In some scanner software (if you are still shooting slides or scanning your archive into your editing software).D. In your photo-editing software.

Remember, the more optimal your image is BEFORE you bring it into your photo-editingsoftware (Photoshop®, Photo Elements®, etc) the more information will be available for youand the better the final quality of your image will be.

An adjunct to the histogram is the “highlight blinky” warning. This can be helpful in letting you know that you areclipping your highlights, but it’s like the idiot lights on your dashboard. The oil warning light will tell you when yourengine is about to melt down because of lack of lubrication, but an earlier warning would be more helpful. Likewise,the blinky warning will tell you that your highlights are blown, but the histogram gives you the details of whichtones and by how much your image is compromised.

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This was a slightly tricky histogram tointerpret, because the dark shadow of thegrasshopper did contain detail, but the holesin the leaf did not.

The histogram reflects this, with one peak inthe middle, representing the greens, anotherpeak on the far right, representing theshadow, and a clipped region off the left sideof the histogram, representing the detail-lessblack hole in the leaf.

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light– it is a valuable tool that helps you interpret thedistribution of values for each image and create a final image

thatcontainsall the

information your camera can capture.

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Many cameras have outgrown the simple black-and-white (luminance) histogram and now offerthe same graph, but in RGB, with each color channel being represented by its own curve (alongthe same axes).

The importance of the three-color histogram wasdemonstrated recently, when I photographed the wingsof a morpho butterfly suspended in a spider web in CostaRica. The luminance histogram looked fine in the field,but when I uploaded the images into Photoshop®, Idiscovered that the blue channel was burnt out, withNO detail in several areas of the bright blue wings. RayKlass, a Photoshop®-savvy friend had showed me atechnique called “channel blending”, where detail can beoptimized in one channel and then re-applied to theimage, and I was able to bring out some detail in theburnt out areas, although not all.

Even after correction, the histogram still doesn’t lookperfect, with both ends of the graph showing clippingof the blue channel. The image, however is acceptable.When I tried to completely eliminate the clipping inPhotoshop®, I lost the iridescence of the blue in theright hand wing. Sometimes, you have to trust youreyes rather than the electronics.

Just an expanded view of the histogram inPhotoshop®, showing the clipping on both endsof the blue channel, as well as in the red channel.

When in doubt, this is one of the rare times that“shoot now, fix it later in Photoshop®” is alegitimate plan.

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originalexposure,

youcannotrestore itin youreditingsoftware.

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Channel blending is not always successful, however.The luminance histogram (black and white) on anolder digital camera indicated a good exposure of ofthis yellow warbler, with no clipping. On the monitor,however, the loss of detail in the bright yellow breastfeathers was obvious. I was not able to restore thisdetail in Photoshop®. A color histogram would haveled me to underexpose and bracket in the field, andthen use either blending techniques, cloning, or layermasks to enhance and/or restore the color detail. Liveand learn.

The histogram of this Indigo Buntingagain showed a normal luminancegraph, but the RGB histogram showedclipping in the blue channel. Detail wassaved in the final image byunderexposing slightly, being carefulnot to clip the shadow detail. These, ofcourse, are the Photoshop® histograms,not the ones from the camera LCD, butthe principal remains the same.