This is the second installment of my tutorial on using metering tools to obtain correct exposure. After all that detail, the tutorial on how to obtain correct exposure using a light meter, can now–to some extent–be thrown out the window, thanks to the tools given us by most digital cameras! I want to start out by saying that, like all of my tutorials here, I did not invent the wheel. This is (hopefully) an understandable distillation of what a number of much more knowledgeable gurus have taught me through their writings and seminars.
Photoshop (and similar software) users are already familiar with the histogram, which depicts, in a graphical way, the number of pixels in an image ranging from pure black to pure white. And, we know that if there are spikes at either end of the graph, we have problems with under, or over-exposure (known in digital terminology as “clipping”). The left example shows pixels “stacked” on the left and indicates lost detail in the shadows (underexposure). The right example, conversely, shows pixels stacked on the right and loss of detail in the highlights. The middle example is close to an ideal histogram and is neutral. There are no lost details. The LCD readout on the back of the camera measures the same phenomena.
Using The LCD on the Back of The Camera
Today’s digital cameras give us this tool, built into the camera and we are foolish if we don’t take full advantage of it! Digital capture gives us something we never had before–immediate feed back about our exposure. We can now take the shot and look at the LCD on the camera back to see the result. But be careful! It is easy to be fooled by this great tool, if you do not understand how to set it up and use it.
The actual image you see on the back of the camera, for example, is rarely useful as a tool to determine exposure.
It can perhaps tell you something about composition and maybe about sharpness. But it is not a good measure of exposure. The image on the LCD screen is a jpg which is interpreted by the camera’s internal software and displayed on a screen that is not likely calibrated like your computer screen. So any judgment you make about exposure and rely on is likely to be disappointing.
The tool you need to understand is the histogram to see whether your exposure is technically correct. This is a wonderful, and time-saving tool, because it is a pretty accurate measure, immediately after-the-fact, of your exposure. However, caveats apply. First, notice that I say “technically” correct. Recall from my tutorial on Using The Light Meter, that it is up to you, as the photographer, to determine the aesthetically correct exposure. This remains true even when using the DSLR and the histogram (though the aesthetics can be more easily done as a matter of post processing now). Second, the histogram is based on the camera’s interpretation of a jpeg image and only the “luminousity” channel (measuring mainly the black and whites in the image). It is possible that one of the RGB channels might be overexposed (blown out) and you will not know it. This is probably a minor concern that in practice isn’t a serious issue. And, some of the newer bodies now have an RGB histogram display. Last, but certainly not least,
There is a danger in becoming so “married” to the histogram that we begin to shoot mechanically and technically
missing a crucial moment or artistic approach (this is why I still believe sound exposure knowledge covered in previous tutorials on exposure are necessary as an internalized fundamental understanding. Indeed, this penchant we have developed to shoot, then check the LCD on the back of the camera has been given the name, “chimping,” for its similarity to a chimpanzee looking up and down.
In order to take full advantage of the features offered by your camera, you will need to go into the camera’s menu system and make sure the histogram feature is enabled. While you are in there, you should also enable the camera’s feature (most DSLRs) that shows a flashing display on the LCD image for “blown highlights” (more below).
Reading the Histogram
I think the world of Michael Reichman’s contribution to digital knowledge and his Luminous Landscape is a must-bookmark site for any serious DSLR user. His article on understanding histograms is clear and concise and I recommend reading it. Reading your histogram is essentially similar to the Photoshop Levels histogram discussed above. As a general rule, you want the histogram to show an even dispersion of pixels between the far left and right of the graph. If the graph is “stacked” against either side, it is an indication of an exposure problem. Stacked against the left side means you are likely underexposed. Stacked against the right means you are likely overexposed. As a general rule, particularly if you are shooting jpg images, having the histogram centered is going to give you a good exposure (if you are shooting RAW, however, there is a better way–more below). It is worth mentioning here again, that the LCD image on the back of the camera may even look pretty good. Do not believe it. Believe the histogram! It is also worth noting that the shape of the histogram is not really important. My “neutral” histogram is the theoretical shape of an “ideal” histogram with an even dispersion of tones. Many (if not most) images will not have that characteristic. If your image has a lot of shadow or darker content, the histogram will show more pixels to the left and they will likely reach higher up on the graph (perhaps making it look more steep and spikey). If there is a lot of bright areas in the photo (highlights of water, clouds, and bright skies are a good example), it will likely show more steepness to the right. The critical determination is whether it stacks flat against the right or left. If that happens, you are clipping important digital data and not getting the most from your exposure.
The beauty of the histogram is that it allows us to make that measurement immediately after the shot and adjust exposure until we get it correct. Obviously, there are instances when that is not going to work. Action photos may only give us one chance. If you can anticipate action, however, the histogram allows us to take some “pre” test shots and be ready when the action happens.
Professional nature photographer John Shaw has been an inspiration to me for 25 years. I have most of his books. In the film days, John (as did Bryan Peterson — mentioned in my earlier tutorial in his Understanding Exposure–and many other professional photographer-writers) spent a fair amount of time and text addressing proper exposure techniques. Last winter, I had the pleasure of attending one of John’s weekend seminars. I was taken aback by something he said (and a little abashed that such a simple concept had escaped me until I heard him say it). After acknowledging how much careful time and study he had put into proper exposure and metering techniques (using spot-metering and never trusting the camera’s automatic features) in the old days, he unashamedly acknowledged that he pays no attention to it these days! He takes a shot at the camera’s suggested exposure using the matrix meter–as a test shot– and then uses the histogram to adjust for proper exposure. Wow! How simple is that?
In two full days of sessions, that was the most important and useful piece of information I brought home with me
There are caveats, again, however. First, I shoot in the camera’s RAW format 99.99% of the time. This is a technique that really works best when you are using the most digital information the camera can capture. To me, this means shooting in the camera’s native 12 bit RAW format, and making post-processing conversions in a RAW converter. While using the histogram will work in any format, it shines brightest when shooting RAW. Second, when shooting RAW, the “rules” of reading the histogram change, as we will see next.
Expose To The Right in RAW
What follows only applies if you shoot in the camera’s native RAW format. Jpg shooters (which, unfortunately, includes most Point & Shoot users), disregard this information, center your histogram as much as possible and take a break.
The idea here, contrary to the thought that you want to “center” your histogram, is that you will get the maximum potential from the capture image data if you expose so that your histogram is as far right as possible, without blowing out any highlights. This means that for the most part, that we want to shift that centered histogram to the right as far as we can. Keep in mind that the advantage to this technique will only be realized in the adjustments you make in the image RAW converter prior to opening the image in Photo Shop or other Post Processing software.
I t has been suggested that digital sensors are capable of capturing up to 6 “stops” of range. For the most part, in my experience, 5 stops is more realistic (besides, most other commentators use 5 stops to demonstrate, and the math is a little easier to grasp). It is worth mentioning here that the “Levels” histogram in Photoshop uses 0-255 for its measurement from pure black to pure white. This is based on an 8-bit jpeg model. Most of the images I work on in Photoshop are 12 bit (everything should double and it should be 16 bit, which is–again–what Photoshop says it is, but current DSLR cameras are only able to capture a maximum of 12 bits, so we work with 12 bits in a 16 bit space). I know, too technical for me too! Not to beat it to death, however, but consider how much more range of pixels we have to work with (and hence, margin for error) in a 12 bit image which gives us a 0-2048 pixel range instead of 0-255.
Now, here is why to expose to the right. Consider the graphical 5-stop diagram below. Note that as we progress from 128 to 2048, the area under consideration continues to double. What this means in very simple terms is that the last step contains 50% of all the pixels captured! Note that that is also the highlight. So we want to capture as much of that as possible. We accomplish that by shifting our histogram as far right as we can without blowing any highlights. Another way to say this is that if you do not fill the right side of the histogram you are effectively potentially wasting up to 50% of the available information that your camera is capable of capturing.
There are two excellent tools on the camera-back LCD to determine this. The first is the histogram we have been discussing. Check it to make sure you have shifted it as far right at you can without a spike on the right side, or stacking. The second tool was alluded to above. If you turned on your camera’s flashing highlight display (affectionately called “blinkies” by many of us), the LCD will show blown highlights as blinking, or flashing. From there, simply back off the exposure until we eliminate them, or push it to the right until you get them.
Again, do not be concerned with the look of the image on the LCD display. And, when you open the RAW file in the RAW converter, do not be surprised to see your image look light and overexposed. Just use the image adjustment tools in the RAW converter to bring your image to the look you desire. And, trust me, your images will look better. A couple week after my John Shaw epiphany, I went out and shot, using my newfound approach.
Set the camera meter to matrix, shoot your test shot and check the histogram. Adust the exposure until you have it as far right at possible with no blown highlights. And trust it.
It will be right. Other than that, I did nothing different than I always have. I sent some shots to a fellow photographer in a “what I captured yesterday” kind of email and he responded that my work just keeps getting better. Nice compliment (deserved or not), but the point is that I was able to get all I could out of the capture, with the confidence that my base exposure was dead-on.
Some weeks later, I was experimenting with an old Tilt and Shift lens which did not have metering capability on my camera. I had no real concern about how to properly meter. I did not have my hand-held meter with me. No matter. I took my best guess (based on my fundamental knowledge of light, exposure and metering), took a test shot, and adjusted until I got the histogram I desired.
The histogram measurement tool has freed us to concentrate on the aesthetic aspects of photography.
Filed under: PHOTOGRAPHY, TUTORIALS | Tagged: 18% grey, bracketing, DSLR, exposure, f-stop, histogram, ISO, light meter, PHOTOGRAPHY, RAW, sensor, SLR |