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One of my personal goals as a photographer is to take pictures of a high enough quality that I never open one on my computer and find it unusable. Whether for diagnostic purposes or as part of a patient presentation the exposure is a critical factor in the accuracy of a photograph. A critical factor in taking good photographs is displaying the histogram and being able to interpret what it is communicating about the exposure of the image. Different camera systems allow you to display the histogram along with the image on the back of the camera. Your owner’s manual or quick start guide should show you how to have the histogram display.
This graph displays how the pixels of a photograph are distributed from dark (black) to light ( white) in quantity in the image. The left side of the histogram is the darkest tones and they move to lightest at the ride side of the graph.
Histogram Peaks
The vertical axis shows you the quantity. Many histograms will have one dominant peak showing where the largest portion of the photograph resides from light to dark. This distribution is common to all of the dental images we take with the exception of the full face shots, which often appear to have multiple peaks. What we are looking for in dentistry is to have an image exposed so that the peak on the histogram occurs very near the center of the graph. Some dentists prefer it to be just right or just left of center, but that is more a matter of taste than image quality.
Image Underexposed
If an image has the peak of the histogram to the left, it is underexposed, or will appear dark. On the contrary an image with the peak to the right side is overexposed and will appear very bright. Either way we need to make some corrections in order to properly expose the image. How you make these adjustments will depend on the way you shoot your images and have the camera set. I shoot with both my flash and my camera on manual. This allows me to adjust the exposure by changing the F-Stop (aperture). When I see an image with the histogram peak to the left I simply change the f-stop to a lower number. When I take a photo and the histogram is pushed to the right side of the graph I need to increase the F-Stop number and decrease the amount of light.
Image Overexposed
I think about my photos in four groups, full face, close-up, close-up retracted and mirror. Each f these groups has different light properties and therefore will require an F-Stop adjustment to gain an accurate exposure. I check the histogram and make any alterations on the first image of each group. Once I have the correct exposure I can take the rest of the images in that group without further adjustments.
If I may make a couple suggestions to try – I believe it may help in the quest for better shots.
1) Shoot in RAW – instead of jpg. The reason is RAW gives you a couple stops latitude without hurting the image in case you do have to make any changes in an editing program like Photoshop. It just carries more information and makes it available.
2) Shoot to the right – you want to have the peak of the histogram slightly to the right of center with the right side trailing off into but not to the end of the last section on the right. The reason again is that there is a lot more useable data in the highlights than there are in the darks. Ever try to fix an underexposed image and seen the grain pop out? That’s because the data is lost. You can darken an image and actually get back some blown highlights – but not the other way around.
3) Its ok to shoot in Aperture priority. Use the aperture to control depth of field and let the shutter float for proper exposure. If you need a little more light on the image – you simply adjust the exposure compensation a stop or so to the right. ISO:200, AV priority ƒ5.6 full face and ƒ22 intral oral.