Digital Photography - Histograms - Page 12

One of the best tools available for underwater digital SLR photographers is the histogram. It is especially important for jpeg shooters. The exposure latitude (how much the exposure can be off and still get a good image) of a jpeg is less than one f-stop. Digital SLR's have an option for a histogram to be superimposed over the image or placed next to the image in the camera's LCD. This histogram will provide exposure information. The image below shows a histogram.

Histogram - A histogram shows the number of pixels of each color from black at the left side of the histogram to white at the right side. The horizontal placement of pixels show the tonal range from black on the right to white on the right. The vertical pixel values show the quantity of the colors in the various tonal ranges in the photo. The first point to remember about a histogram is that you should avoid having the pixels climb either the right side or the left side of the histogram. If the pixels climb the right side, you are overexposing. If the pixels climb the left side, you are underexposing. This is a general rule of thumb but there will be times that it cannot be avoided.

Example Histograms :


Normal Exposure


The histogram on the left shows underexposure. The pixels are bunched up on the left side of the histogram and there are very few pixels on the right side. I would add exposure and reshoot the image. The center example shows a correctly exposed photo. The pixels cover the entire range of the histogram. The right example shows an overexposed image. There are very few pixels in the dark areas (left side) and the pixels on the white side (right side) are climbing up the right hand border. This is typical of overexposure. I would reduce the exposure and reshoot. It is important to understand that if you do err, it is best to err on the side of underexposure. If you have underexposed areas, you may be able to fix the image in an image editing program. If you overexpose, there will not be much useable data in the overexposed areas to work with.

The Greatest Practice Tool Ever For Underwater Photographers : In all of the years that I have been doing underwater photography, it has been difficult to practice above water and see results that I could use underwater. A histogram changes all of that. You can now practice your photography above water and use what you learn underwater and it is absolutely free ! I will explain what I do and how I drive my wife crazy.

My wife will be sitting at the table and I will come in with my camera and start taking pictures of her and then sit down and evaluate the histograms. I do this a lot and it drives her nuts. She doesn't like having her picture taken and I love taking it. I try to have a specific routine when I take the photos so that it is easy to evaluate the results. I will shoot with a flash and without a flash. I will shoot subjects (like my wife) as close-ups or as wide angle images. I will shoot midtone subjects, dark subjects and light subjects. I change exposure on all the images and see how the histogram changes. If I shoot in an auto mode, I use exposure compensation to change exposures. In a manual mode, I use f-stop or shutter speed. The following is a sequence that I use with exposure compensation:

-1.0, -0.7, -0.3, 0.0, +0.3, +0.7, +1.0

Each change in exposure compensation changes the exposure by 1/3 f-stop. I look at each image and see how the histogram has been changed with the exposure change. This helps me to learn how to make changes underwater based on histogram information. After I finish practicing, I delete the images from my compact flash card and the lessons have been essentially free. Please note that I only use this sequence when I am practicing. Another sequence that I use is to take the first photo with no compensation and then by looking at the histogram, see what changes I have to make for a better exposure. I then change exposure and see if my choice is the correct one. It is a great way to learn.  If you are not doing this before your dive/photo trip, you are missing a great learning experience.

Samples - The following are examples of underwater digital images that I took and the histograms that they created. I will supply explanations of the histograms as I see them.

Anemones & Mussels in New Jersey: This images has a lot of pixels in the dark area (black background, mussel shells) and a lot of pixels in the midtones (anemones, interior of mussel). The pixels cover the entire range of the histogram. This is a well exposed photo.

Anemones in New Jersey: This is a strange looking histogram but it makes sense when you dissect it. The big spike  on the left is in the shadow (dark) area. It represents the black background. The smaller mound in the center are the midtone pixels (anemone body and tentacles). I would not have changed the exposure on this image.

Winter Flounder in New Jersey: This image has mostly midtones and very few dark tones or light (white) tones. The dark tones come from the shadow area under the front of the flounder and the light tones come from the light areas on the flounder and the white in the rocks. This is a good exposure.

Island Frogfish in the Bahamas: This island frogfish has a lot of midtone values (gray skin, background, sponge in the foreground. The dark areas are evident (algae, dark areas on skin, etc.). There are not many pure white pixels in the photo. The pixels cover the entire range of the histogram and that is good.

Southern Stingray in the Bahamas: Here is a good example of overexposure. The light (white) pixels are climbing the right side of the histogram  and there aren't many pixels in the shadow (black) area. The overexposure is evident in the photo in the sand. I should have taken another photo and underexposed to get correct exposure.

Robin Reed in the Bahamas: This histogram shows a good exposure. There aren't many pixels in the highlights and I suppose that I could change the white point in an image editing program but I am happy with the photo as is and will not make changes.

Practice Before You Dive
Make your next photo trip more successful. Whether you shoot raw or jpeg, learn how histograms work and use them to your advantage. PRACTICE - PRACTICE - PRACTICE. You will be happy that you did.