A question was posted on Wetpixel about what an in-camera histogram should look like for a blue water shot. This sparked my interest so I did a bit more research on the topic.

What is a Histogram?

The histogram is commonly used in image editing programs like Adobe Photoshop to evaluate photos and determine what might be needed to correct them. In a digital camera, the histogram can be used for the same purpose. With digital cameras on the market today, you cannot see the histogram until after an image is shot, but this will tell you whether or not you've got an acceptable exposure or need to make adjustments for your next shot.

A histogram illustrates how pixels in an image are distributed across the tonal range. It will accurately show you where and how many pixels fall on a scale between the RGB value of 0 and 255, 0 (left side) being the darkest shadow possible and 255 being pure white (right side). The midtones fall in the center.

So am image that has a histogram with pixels concentrated on the left side will likely be too dark. If all the pixels are sitting on the right side, the image will likely be over exposed. Pixels too close to either end of the histogram will likely have blown out the highlights or plugged up the shadow detail. The height of the black lines in the chart illustrates the number of pixels there are in each individual tone.

In general, one should attempt to achieve a histogram that is evenly distributed across the tonal range, but not all images will fall into the "evenly balanced" category. High key or low key images will display pixels concentrated on the right side or left side of the histogram. An image with dramatic lighting might have the majority of the pixels in the shadow areas with a few highlights and specular highlights. What you want to avoid is to cut off any pixels on either end of the histogram.

For blue water shots, the histogram can also be anywhere on the scale, depending on whether or not the shooter has captured the sun and/or the deep blue colors of depth in good visibility. Poor visibility will create a flatter, smoother histogram.

The Test

I was curious to see whether the histogram on the camera displayed exposure exactly the same as the histogram I was already familiar with in Photoshop. If it was the same, I felt I could use the camera's histogram to help me to achieve the perfect exposure before it ever got to my computer.

Since I shoot with the Nikon D100, I've been using the flashing highlights feature to show me if and when I've blown out my highlights, but had not used the histogram. The first thing I noticed when using the histogram instead, was that the tiny yellow line that indicated I had pixels in the midtone to highlight section were so small, I could hardly see them, especially through a mask and housing window. I was concerned that because I didn't see pixels in the upper third of the tonal range, I was under exposing. When I opened the image in Photoshop, the exposure was good and I did have pixels in the highlights and even specular highlights, but there just weren't enough to make more than a single line in the histogram. This images tones were concentrated primarily in the midtone.

Simulation of the D100 flashing highlights display option

The Nikon D100 histogram is nice and bright but is still barely big enough to see the tiny detail captured between the midtones and the highlights. Note that that simulation to the left is bigger and clearer than what is seen in the LCD of the camera. The D100 does not allow the display of individual RGB channels like the Fuji S2 Pro. When you do blow out your highlights, you can see it, but its not as easy to notice as using the flashing highlights feature the D100 offers.

When the same image is brought into Adobe Photoshop, the histograms are nearly identical to what is seen on the LCD histogram, however PS did crop off the top peaks (which isn't really an issue, since it's the left and right side of the scale that is important, not the top and bottom).

Fuji S2 Pro

The Fuji S2 doesn't have the same flashing highlights feature as the D100, but does expand the histogram feature to display the individual red, green, and blue channel histograms. The histogram screens are available when you select the POSTVIEW w/Histogram option from the IMAGE DISPLAY option in the SET-UP menu. If you enable the POSTVIEW option, the last image captured is displayed on the color LCD and you can display the histogram in the lower right corner of your LCD. Press the histogram button to display the MASTER, then again to display the RED, GREEN and BLUE channels.

The Bottom Line

The histogram on both cameras are very small and difficult to use alone to judge your exposure. You will not get the exposure as perfect as you can make it in Photoshop. You can determine whether or not you've seriously over or under exposed your image. You will still need to rely on your photographic experience to pick your f-stop, shutter speed, ISO, and flash power based on the subject you are shooting and conditions you are shooting in.

Avoid running the right side of the histogram off the chart. This will definitely blow out your highlights and will ruin that shot. Stop down, change the shutter speed, or lower the power of your strobes, which ever is appropriate. Play it safe and don't attempt to run the histogram right up the right side. There may be pixels there, but sometimes you can't see them on the LCD histogram. You won't see that you've over exposed until you open the image in Photoshop. Same rules apply for the shadows. Leave a little room to be safe.

Especially for flat contrast or some blue water images you may not have a good range of contrast in the shot. In this case, its OK to allow a larger gap on the highlight end of your histogram. You might find that an image with a perfectly distributed histogram isn't what you wanted at all.

If you really want to pursue this further and figure out what the perfect blue water histogram should look like, experiment a bit in Photoshop on a number of different images taking into consideration different conditions. Then, next time you shoot in those conditions, try to match that histogram. For example...

During this trip there were many underwater photographers on board who contributed to the discussion. All agreed that they would not rely on the histogram to judge exposure. Whether or not you find it useful will depend on your own set of techniques and tricks that makes your style and images unique to you.