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Real World Digital Metering and Histograms
January 1, 2004

This article is predicated on the basis that the reader already knows about metering and what a digital histogram is and what kind of information it offers to the photographer. For those wanting a backgrounder on metering, I would point you to my e-Book chapter on the topic. A histogram is a graphical representation of the light values captured by a digital camera. The dark or shadow portions are on the left side of a histogram graph whereas the light or highlight portion are on the right. The histogram provides valuable information to ensure that the photographer keeps all the light values within the histogram graph and to avoid loss of detail in the shadows or highlights, as indicated by extreme left or right placement of the graph outline.

It snowed in Vancouver on new year's eve and given the relative rarity of snow in the Vancouver area, the kids were eager to head out and play and build a snowman before the eventual winter rains would come and wash away their masterpiece. As I was out to document their hard work it occurred to me that it would be a good opportunity to write a new piece about how to relate a digital histogram to a correct exposure and the snow provided a perfect backdrop for this since its reflective qualities is a classic example of when in-camera meters tend to fail spectacularly in obtaining a correct exposure.

Most of us know that in-camera meters are calibrated for a middle-tone grey (which could be either 18% a la Ansel Adam's Zone metering and Kodak grey cards, or it could 12 or 13% per ANSI standards, but this is a debate for another time). What this means is that if your subject matter is predominantly of a middle grey value, your in-camera meter will provide perfect exposure. However, if your subject matter is not predominantly of a middle grey tone, your in-camera meter will err and it will attempt to still render the scene or subject as middle grey.

As good as Nikon's (and other brand's) Matrix meter is, it is not foolproof in complex exposure situations and despite the two-decades of development for these evaluative meters, nothing is going to beat the meter in-between your ears, based upon actual photographic experience. The mistakes made by the in-camera meter are based upon scene of subject reflectance of light because that is what the in-camera meter measures.

Snow and beach scenes are the classic examples of high-key subject matter that will fool the in-camera meter because the snow and sand reflect much more light than middle grey, causing the meter to underexpose the subject. Whereas the black cat or dog in a dark room is a classic example of low-key subjects that will fool the meter because they absorb so much light with little reflected back that the meter will overexpose the subject.

When using film cameras and unforgiving slide films, the photographer had to compensate accordingly and a healthy amount of bracketing would be in order to ensure that the correct exposure would be obtained. Digital cameras provide instant feedback via the LCD screen and while an excellent aid for composition, not necessarily the best for providing a true depiction of the exposure value in the scene. Following a thread on one forum about Nikon D-SLRs, had one D2H user put a neutral density gel in-between the LCD and the clear protective cover in order for the photographer to obtain an accurate reflection of the exposure value captured. Very ingenious!

Most other photographers have also realized similar frustrations about their LCDs not providing an accurate depiction of the exposure value and now rely on the Histogram display available for most of the D-SLRs on the market. Some D-SLRs also provide highlight blowout warnings too, but even these have been found to be less than accurate, leaving the Histogram as the primary tool for ensuring optimal exposure.

Uwe Steinmeuller of the Outback Photo web site states that he and his wife always work in Manual mode for their digital photos and merely use the in-camera meter as a guide while utilizing the Histogram as the true reflection of what should be the optimal exposure. Michael Reichmann of the Luminous Landscape web site is another proponent of the Histogram display and his articles about the Histogram have been enlightening, particularly the one in which he passes on a tip from Thomas Knoll, original developer of Photoshop.

Thomas Knoll revealed that digital captures should be as close to highlight blowout as possible without actually being blown out. A digital file only has so much information that can be contained (relative to the resolution) and by pushing the exposure as close to the right side of the Histogram as possible without blowout, this will capture much of the detail as possible for the digital file. And due to the logarithmic nature of digital capture, by not pushing the exposure to the right side of the Histogram, you could potentially lose a lot more information than thought.

What does this mean for real world usage of D-SLRs and their Histogram displays. Well let's take a look at the sample shots below with their companion Histogram display from my Nikon D100.

#1 Matrix Meter Default f8 & 1/160	We can clearly see that relying on the Matrix meter results in one very dark and grungy looking photo. The Histogram is nowhere close to being blown out
#2 Matrix Meter Plus 1/3 Stop	An incremental 1/3 plus exposure compensation knudges the histogram towards the right direction
#3 Matrix Meter Plus 2/3 Stop	Getting better with a 2/3 plus compensation and now the snow is starting to look actually white instead of grey
#4 Matrix Meter Plus 1 Stop	Even at plus 1 stop compensation, we're still not maximizing the room available to the right of the Histogram
#5 Matrix Meter Plus 1-1/3 Stop	Based upon using the D100's Histogram, this is the ideal exposure. Snow is white and there is still a little bit of headroom to the right of the Histogram, which will ensure that details can still be seen in the highlights
#6 Matrix Meter Plus 1-2/3 Stop	This exposure is borderline acceptable. The main spikes of the histogram are still within the range of seeing detail in the highlights, but this is as close as you can get. As can be seen in the Histogram's extreme right, there is already some lost of highlight detail
#7 Matrix Meter Plus 2 Stops	At plus two stops the exposure has blown out the highlights and there is signifcant detail lost in the file. Although readers cannot deduce much from the medium-resolution JPEG sample at left, large tracts of snow are completely white with nary a detail seen
#8 Incident Meter Exposure with Sekonic 508 f8.7 (f10) & 1/30	This is a shot based upon using a handheld meter to provide the exposure information. Incident meters measure the amount of light falling on the subject and not of the light being reflected back, as in-camera meters do. This is the best exposure of the lot as the Histogram is as close to the highlight side of the range as possible without blowout. Note though that I had to adjust the aperture to be f8 and 2/3 stop, or f10 as the Sekonic meter can provide aperture readings in 1/10 stop increments when used in Aperture Priority mode.
#5 Matrix Meter Plus 1-1/3 Stop	This sample is the same as the "ideal" shot #5 above, but repeated here to provide easier comparison to the Sekonic meter shot.

What does this all mean? Use an incident meter whenever possible with digital capture and barring that, pay attention to your Histogram to ensure optimal exposure without blowing out the highlights. There may be certain limitations to most Histogram displays in that only the green channel information is provided since that is the one that holds the most information after the Bayer Interpolation takes place from the in-camera processing. Ideally, we want to be able to see all three channels, as the Fuji S2 provides, but in a better implementation, so that we can fully review how we are exposing our images.

For critical digital photography, you must use a camera capable of providing a Histogram display and also of compensating the exposure to optimize the capture. For serious photographers looking for a compact digicam to carry with them all the time, I would suggest looking for a model that provides a Histogram display unless you're just looking to do snapshots.

I suppose I should note that exposure is quite subjective and what I consider good may be bad for someone else. The comments above refer to obtaining a "technically" correct exposure based on the tools that a photographer may have available. After that, the photographer can still adjust and season the image to personal taste. Bon appetite!

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