Understanding the histogram and how to use it can greatly improve you solar captures. In solar photography histograms can you tell you a great deal about the image you are capturing such as exposure, tonal range, and even if the image is in focus. Most digital cameras have live histograms that can be used during the photo shoot and you should get accustomed to using them. If you use a computer to capture and control your digital camera a live histograms is especially helpful since monitors are not created equal. How many times have you had a glare on your computer screen making it difficult to see the solar image or increased the exposure until you could see detail only to find out that you overexposed other parts of your image? Learning to use a live histogram can not only help improve your images but also reduce your stress levels so you can spend more time viewing the sun and spend less time on the computer. Histograms are a representation of dynamic range or tonal range from 0 (black or underexposed) to 255 (white or overexposed) and the amount of image in each tone that can be captured by the camera. At both ends of the tonal range details has not been captured and are either underexposed and there is insufficient light to capture any detail or overexposed and the detail has been blown out. The ideal image will fall in between 0 and 255 but just short of either end. Shown in figure 1 is a typical histogram of the solar disk in white light or H-Alpha using a log scale. Log scale display is better to use because this classic cartoon whale is easier to see and observe shape changes as you make adjustments. A linear scale can be used, however, it can be more difficult to use mainly if you are using the histogram to also help you focus.
Figure 1 - Histogram
You should notice several things about the histogram. One is that the sun is not centered on the middle of the graph and that the tonal range is below 255, and finally that there is a dip where the sun meets space. Let’s discuss each of these and why they are important.
Figure 2 - Tone Ranges
In photography dynamic range is used to capture the most detail out of an object and solar photography is no different. To order to maximize the disk details your tonal range of the histogram should be as close as possible to the right without hitting 255 while spreading the graph across the entire available range. This allows you to capture very bright, bright, middle, dark, and very dark tones. In figure 2 I have approximately marked the tonal zones in the histogram.
If you are using a DSLR camera you will probably see similar line markings on your histogram display. Visualizing these tonal range zones are extremely helpful especially if you are trying to bring out specific details in your image. You can see that if the sun is centered in the histogram will you be capturing mostly middle tones and missing out on other details that are visible. Post processing can help increase detail but cannot create detail that was not captured.
The second observation is that the histogram is below 255 on the scale. This is because cameras are linear devices that convert analog, in this case optical, signals to digital signals and have some margin of error in what is displayed and captured.
Figure 3 - Camera Converts Analog Signals to Digital Signals
Maintaining the solar image just slightly below 255 and above 0 ensures that you have captured the maximum amount of detail that your camera can capture. When photographing the sun you cannot get away from 0 simply because space is black. In this case you want to ensure that you are getting a large amount of data in the very black region of the histogram to ensure that your solar image is not so dark that you have lost detail.
Contrast and Focus
The third observation is the dip in the dark region of the histogram. This region indicates that you have reached the suns limb and space where there contrast is the greatest. This dip can also be used to help focus the sun by watching the rise and fall as you bring the sun into focus. This is illustrated in next series of images. I have placed a red line horizontally just below the dip as reference.
In figure 4 the image the solar disk is not in focus. As I bring the sun into focus the dip lowers and becomes more apparent as shown in figure 5. And finally, adjusting past focus will cause the dip to rise again as shown in figure 6.
Figure 4 - Approaching Focus
Figure 5 - In Focus
Figure 6 -Leaving Focus
I have stacked the 3 histograms in figure 7 to illustrate this focusing function further. Focusing using the histogram is only possible because of the sudden contrast different between the suns limb and space. Under normal photography situations you could not use this "undocumented" feature. Try this for yourself during your next solar session. I should note that if you are zoomed into the sun at a point where space is not visible then this will not necessarily work. It depends on the feature and the surrounding area.
Figure 7 - Stacked histograms comparing focus points
Modern cameras use this type of contrast detection in their auto-focusing features to bring the image into focus. You may have noticed that your camera has a difficult time of focusing when there is little difference between features or when it is too dark or too bright.
The primary use of the histogram is dynamic range control and exposure controls how the histogram shifts left and right. Too much exposure and the histogram will be shifted to the right. Too little exposure and the histogram will be to the left. If you are shooting through your telescope with a DSLR with attached lens then you are using ISO, aperture, and shuttle speed to control your exposure to control dynamic range and what detail will be captured. If you attach your camera directly to the telescope (prime focus) then you only use ISO and shuttle speed to dynamic range of the subject. On the other, if you are using webcams or dedicated astronomy cameras then you primarily use shuttle speed and gain to control dynamic range and these are the two features that I will concentrating on in this next part of the lesson.
Take another look at the histogram (figure 2). In addition to tonality each segments also represent an exposure stop (F/Stop) on the camera. As you change the F/Stop you should notice that the histogram compresses (shifts left) or expands (shifts right) depending on whether you decrease or increase exposure respectfully.
A simple example should help clarify. I have taken two captures of the solar disk. In both cases I have maintained the exact same settings and only changed the exposure settings by a single stop. Figure 8 shows how the histogram has compressed the tonal range of the solar disk as exposure is increased by a single stop.
Figure 8 - Increase Exposure by a Single F/Stop, Compressed Histogram, Reduced Detail
If I continued to increase my shutter speed I would continue to compress my histogram until it was in the very dark range. If I were to decrease my exposure the histogram would shift once again towards the right. Using just exposure control without gain control will decrease your tonal range, and ultimately detail that your camera will capture. Notice that in figure 8 that there is not much "very bright zone" detail. You have a couple of choices here. Either lighten the image in post processing and potentially be missing some detail or use Gain control during the shot to adjust the live histogram and capture the additional detail that is missing.
Gain control provides more granularity in adjusting dynamic range then exposure control. In figure 9 exposures has be maintain between two shots while gain control was adjusted. Gain provides approximately 80 stops between tonal range zones providing very fine control over the dynamic range.
Figure 9- Using Gain Control to Capture More Detail
Using just gain control has its inherent problems as well. Gain is the digital amplification of a signal. Too much gain will amplify noise causing the image to degrade and mask detail. Not enough gain and you can also be losing details that the camera can capture.
I suggest starting with gain between 500 and 600 (middle of the scale) then adjust exposure until the histogram has shifted past the very bright zone and then back on a single stop. This should put you back into the very bright zone of the histogram. At this point examine the histogram and see where it falls in the very bright zone. If the histogram shows that it is right up against 255 then decrease the gain until it is around 250. If the histogram shows that it is barely in the very bright zone then increase gain until it approaches 250. Using 250 on the scale is not an exact science and every camera has slight differences even within the same line so you will need to experiment to find the sweet spot.
Using the histogram when taking pictures can greatly increase the quality of the images you are capturing no matter what type of photography you do. On numerous occasions I have used my solar finder to center the sun in my telescope and histogram to capture a full disk image without ever looking at the image on my computer screen. And finally, understanding and using the live histogram during your solar session not only increases the detail you capture but will also help reduce the amount of post processing time on the computer later. Thanks for reading my tutorial and I hope you have learned something new that will help you become a better photographer.
A PDF version of the tutorial is available by clicking ... HERE