Exercise: Histograms

As you have seen, workflow has a special importance in digital photography, because you are now in charge of the entire production of each image, from shooting to printing. A very useful indicator of how the images looks ‘Technically’, and how it will process, is the histogram, and becoming familiar with this will help a lot.

A histogram is simply a column graph. Pixel brightness is plotted from left to right, whilst the number of pixels of each level of brightness, are plotted vertically. The scale is the standard 8bit kind of 256 levels, and runs from 0 (Black) at the left hand side, to 255 (White) at the right hand side.

The purpose of this exercise is to increase your familiarity with histograms by relating each one to the image you have just shot. Histograms will appear twice in a normal workflow. The first time is on the cameras LCD screen, the second time is when the image is being processed. It is valuable to make use on both occasions.

For this exercise, you will need to find and shoot the three most basic categories of scene by contrast.

Low Contrast: Something which is flat in appearance, with a low dynamic range.

Average Contrast:

High Contrast: Something with a dynamic range greater than that of the camera.

For each of these picture situations, shoot not only an average exposed version, but also one that is approximately one f-stop darker, and one which is one f-stop brighter. This will therefore make 9 images in total.

As you shoot, make a point of checking the histogram as it appears on the cameras LCD screen.

The lower the contrast, (the flatter), the more the values are squeezed together in the histogram, whereas a high contrast image will be spread right across the histogram, right up to the edges. Varying the exposure will shift the bulk of the values either left with (Under Exposure) or right with (Over Exposure)

When you upload your images onto the computer. Open up your processing software. I will be using Photoshop Elements 9. And screen shot the histogram or any shadow clipping warnings that appear on the images. Write a note for each image and histogram, and explain how the histograms vary with exposure.

Low Contrast Image:

With this image, I noticed in the LCD screen, that the histogram showed it as being of Average Contrast as the histogram levels were mainly in the middle. However, when I looked at the final images here on the computer, they were infact of low contrast. The image is shot for low contrast (Not this), when I processed on here, showed it as being of average contrast. But I will discuss that below.

Average Exposure:


Average Exposure

With the average exposed image. The histogram is almost level. It only touches the sides towards the black, mainly because the cat is fairly dark furred.

1 F/Stop Brighter:


Fstop brighter

With the one f stop brighter, the histogram leans more towards the white, but the rest is still quite low.

1 F/Stop Darker:


fstop darker

With the one f stop darker, the histogram is almost the same as the average exposed image. However, the mid tones have become higher and more towards the black.

Average Contrast Image:

When I was shooting this image, the histogram in the LCD screen showed this as being of low contrast. However, when processing, this was infact of average contrast.

Average Exposure:


average contrast

The histogram for this image is quite central. It is touching part of the right hand side towards the white. I think this is due to the brightness towards the top of the image.

1 F/Stop Brighter:


fstop brighter

The histogram shows that the bulk of it has moved towards the right hand side towards the white, simply because it is 1 f/stop brighter.

1 F/Stop Darker:


fstop darker

The histogram here shows a dramatic change. The bulk of it shifts towards the left hand side into the black, and the amount of pixels has risen quite large.

High Contrast Image:

Average Exposure:


average contrast

As you can see from this histogram, the levels are quite high, and there is a large bulk towards the black, left hand side.

1 F/Stop Brighter:


fstop brighter

This bulk in the histogram has began moving more central.

1 F/Stop Darker:


fstop darker

With the 1 stop darker, there is a sharp point, almost triangular bulk, on the right hand side by the black. The rest of the histogram has now smoothed out.

Overall opinion:

I have to admit, I had some issue with this exercise. I got quite annoyed because everything I chose to photograph, didn’t produce what I wanted it too in the histogram. I chose really detailed coloured items for the high contrast images and plain blank items for the low contrast images, however, they didn’t have the required results on the histogram. Therefore I had to use lighting to give me the desired effects.

The one thing I found quite odd was when I was photographing the cat, it showed on the LCD screen that it was of average exposure. The histogram showed most of the bulk in the middle of the histogram, and touching the sides, however, when processing it on here, it then showed that it was only just touching the sides, and was infact of low contrast as the histogram was quite rounded and smooth.

The union flag cushion was supposed to be the low contrast image, as like with the previous images I mentioned, when I looked in the LCD screen, the histogram showed everything being smooth, low and central, however, when I processed it on here, the histogram was stretched across the graph, touching the sides.

This exercise has definitely taught me that histograms are very important for making sure your images are correctly exposed. I have noticed that the histogram changes dramatically with just one movement of the camera, and the lighting. I have learnt to take more notice of what my histogram tells me, as even though the image may appear correct on the LCD screen, it may not look the same once processed. Histograms also show that sometimes, you may have to under or over expose your image by one f./stop, in order to produce the correct exposure. I will definitely read up more on histograms, as I did have some trouble using them, and I would like to re assure myself some more, so I can take advantage of them in the future.




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