Histograms Purpose of this Presentation and Workshop is to demonstrate how to use your in camera histogram to improve your photos • to insure that you will end up with the best possible result for a given lighting situation • to be confident that you will end up with a photo that is acceptable to you • to reduce your post-processing time
The Exposure Triangle We are all familiar with the exposure triangle – the combination of ISO, aperture and shutter speed and how these three parameters work together to gather the amount of light needed for a correctly exposed photograph. If one variable changes, at least one of the others must also change to maintain the “correct exposure.” A correct exposure is one that strikes the best balance between the brightest and the darkest areas of the image.
But the correct exposure does not prevent some of the highlights from being blown-out (no detail available) or the shadows from being too dark (no detail available). Therefore, the settings your camera chooses for you in any given situation are not the correct settings – merely a suggestion of settings that could be considered the most common, middle-of-the- ground choice. The camera does not know what you want, i.e. what your creative vision is for the photo, but you do. You know which part of the photograph you want to expose “correctly” for you, and which parts will thus end up too bright or too dark. This is where the histogram comes in. The histogram provides information that you cannot see just by looking at the image in the small LCD on the back of the camera. Plus, what you are looking at on the LCD is a JPG interpretation of the image, even if shooting in RAW. The histogram may keep you from being disappointed by an image that looked good on your LCD but not when you bring it up on your computer.
What does a histogram represent? What determines a good histogram? What are your options when you do not have a good histogram?
What is a Histogram 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 (0 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. Source: https://digital-photography-school.com/how-to-read-and-use-histograms/
What Determines a Good Histogram Source: Histograms Exposed, Varina & Jay Patel https://visualwilderness.com/product/hec
The histogram is nothing more than a record of the tonal range captured by the sensor. It can’t tell you if the photo is right or wrong. To know that, we must know what histogram we would expect from the image for it to be correct. This is the histogram shape that we would consider a good histogram from this image. The image tonal values range from dark to light and the histogram shows there is no blown-out highlights or too dark shadows.
Likewise, this shows the histogram shape that we would want for this image. The image consists of primarily dark tones and there are no dark tones without detail. If we increased the exposure any further we would not get the creative intent that we want. Likewise, if we lower the exposure we will lose detail in the shadows.
This image has a wide range of tonal values. The tonal values from dark to light are fairly uniform. Therefore, this is the histogram that we would want. If the histogram was shifted to the right (over exposed), then we would expect to lose detail in the light areas of the leaf and if the histogram was shifted to the left (under exposed), then we would expect to lose detail in the dark areas of the leaf and the light areas would be grayer. This image shows us how important it is to be able to visualize the expected value of the histogram.
RBG vs Luminance Histogram So far, we have only discussed what is called the luminance histogram. Many professional photographers also rely on the RBG histograms to insure they have the correct color representation. In this histogram, it is apparent that the red colors are blown out although the luminosity histogram indicates a good exposure.
“An RGB histogram is essential if you who want the best, most accurate colors your camera is capable of producing. Why? One of the least talked about secrets of professional photography is this: great colors come from accurate exposures. If you miss the ideal exposure, the colors in your image will shift in different directions. You can correct the exposure in your computer but you can’t correct the color shifts. This is why professional photographers are obsessive about accurate exposures, and why they get better colors.” Source: http://www.blog.jimdoty.com/?p=10128 Another good explanation: http://www.cambridgeincolour.com/tutorials/histograms2.htm Before I prepared this presentation I only looked at the RBG histogram if I was photographing something like a red flower. Then I am interested in whether the red channel is burned out more than the luminosity of the photograph, as I want detail in the flower. Now I will be watching the RBG histogram more often.
Highlight Alert (Blinkies) When shooting especially bright objects or on a very sunny day, one of the things you’ll have to be aware of is overexposing or “blowing out” your image. This occurs when an area is just too bright or contrasty to be recorded in the image and instead you get a blank white space with no image data. Street lights, car reflections, snow, shine on faces are all common areas where overexposure occurs but it can happen anywhere. You can help avoid this by reading your histogram, but there’s also another camera feature that is helpful and that’s the Highlight Alert (nearly always referred to by its professional term, “blinkies”). Highlight Alert is viewed in playback mode, after you’ve taken the photo. If you’ve overexposed any areas, those areas will blink. If it’s significant portions of the photo, you’ll probably want to change your settings and reshoot. The histogram can tell you if you have blown out highlights but not where they are in the image. The blinkies will show you where so you can make the creative decision whether you are concerned about them. Some cameras come with this feature automatically enabled (the Canon Rebel line does) but you may have to turn it on. Check your manual for exact instructions. It’s a helpful, quick and easy way to view problem areas in your photos. But you should still learn how to read your histogram, too! Source: http://glimpsesofsoul.com/31-days-of-photo-tips/highlight-alert-blinkies/
Histogram and Contrast Histogram can tell you how much contrast is in your image and whether it will be a problem in your final image. This scene has a large contrast with the light above the clouds, the waterfall and the canyon walls
Zoomed in image has good contrast with no blown out highlights and no unrecoverable shadows. Can capture all the dynamic range in the image with a single exposure.
Broader dynamic range when zoom out. The problem with that is that now we have peaks on the right-hand side showing that part of the image is blown out with non-recoverable detail. But can capture detail in the shadow area.
Histogram and Exposure Histogram shows a proper exposure. Details in the shadows and in highlights.
As decrease exposure, the histogram gets narrower and narrower and has less contrast. Underexposed photo colors look dull and contrast looks flat.
As increase exposure, colors washed out and contrast is reduced. No details in the highlights.
Expose to the Right We have learned that for a shot to be well exposed we should aim for an even spread of tones across the histogram, peaking in the middle, and tapering off at the edges. When ‘exposing to the right’, the idea is to push the peak of the histogram as far to the right-hand side as possible, i.e. overexpose the image, without clipping any highlights. The resulting file, when processed back to the correct exposure, will contain more tonal information and less noise in the shadow areas, maximizing the image quality. Source: https://digital-photography-school.com/exposing-to-the-right/
Workflow 1.Determine expected histogram shape. If Low contrast scene, expect a very narrow histogram. If very bright, shifted to the right.
High contrast scene, broad histogram
If dark subject, expect a narrow histogram shifted to the left.
So, if shooting an image consisting primarily of snow. What type of histogram would you expect?
Snow is bright without much contrast. Therefore, you would expect a narrow histogram shifted to the right. But what happens if we shoot a “correct exposure?” When the camera is left to calculate the exposure by itself, it tries to read all the tones and colors in the scene and integrate to gray. What integrate to gray means is that, if you add up all the values of all the tones and colors and average them out you will arrive at a mid-gray tone . Therefore, if you do not know to expect a narrow histogram shifted to the right, you will end up with an underexposed image . To obtain a proper image, with white snow you must increase your exposure to obtain the expected histogram .
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