Precise Digital Exposure

April 15, 2015 ·

Anders Loof

The Basis Is Per Ansel Adams

ETTR gives a correct exposure for optimum color and tonal densities, and the reason was as explained by Michael Reichmann in the article Expose (to the) Right in 2003. Most of us apply ETTR per that method by applying averaged or matrix metering with exposure compensation. Our departure point is thus an averaged exposure, and we make an exposure compensation for what we know works for a typical scene and tweak it for other scenes (trial and error).

A recent article on Luminous-Landscape titled Optimum Exposure presents essentially nothing new. A publication titled Perfect Exposure on my website dates from 2011 and was reviewed by Michael Reichman at the time. It points out both the simplified concept to upon one single spot meter reading determine exposure for precise location of the latitude of a sensor compared to a scene, and much more. For correctness this all dates back to what Ansel Adams wrote relating to the Zone System and how to determine spot metering for black and white negative film, Polaroid Land Print and more. Many have misunderstood the Zone System and believe it to merely consist of how Adams applied it for black and white negative film. My publication is, as far as I am aware, first to apply the Zone System principles and the spot metering correct for digital, simply because I did so according to the basis Adams wrote of. I thus tested the spectral response (sensitivity to light) for the particular sensor in my digital back and in detail evaluated the RGB values in my RAW converter of choice, Capture One. This is similar to what anyone will need to do for a particular digital camera or digital back because the spectral response cannot be expected to be the same for all digital sensors.

Let us reference Ansel Adams from his book The Negative:

“It has been said at various times that the advent of certain materials obviates the need for such a vehicle of thought and control as the Zone System. Such a statement reflects the misconception that the Zone System is useful only for relating subject luminance with print values as they occur with a specific set of materials. If the materials change, we do not discard the Zone System any more than we discard sensitometry (or our exposure meters, for that matter). A change of materials often does require adaptation in the way we apply the Zone System, but in no way eliminates its principles or usefulness in creative visualization. As long as we must be able to work from a range of subject luminances that are to be represented as we want them to be by a range of grey values (or color values) in print, the Zone System seems certain to provide an extremely useful framework.”

“I eagerly await new concepts and processes. I believe the electronic image will be the next major advance. Such systems will have their own inherent and inescapable structural characteristics, and the artist and functional practitioner will again strive to comprehend and control them.”

What then is the Zone System?

The bare fundamentals of The Zone System can be defined as visualization and a precise control of the process, both of which aim at control of densities in exposure, thorough development and the print process.

Chiefly, metering for the Zone System is by spot metering of minimum one tonal value in a scene.

The Problem At Hand

ETTR by trial and error works well until we wish to place the choice of exposure in our hands by the precise determining of exposure compared to the latitude of our digital sensor and the scene at hand. With analogue photography in such situations we apply spot metering, preferably using a 1-degree spot meter with which we can precisely pin point in a scene. I maintain that nothing has changed in these regards and that for digital photography we can likewise apply a spot meter to precisely determine the exposure. However, the obvious needs to be stated, that the spectral response of a digital sensor does not match that of black and white negative film, slide film or other films. The digital sensor has different exposure latitude that responds with its own uniqueness towards the highlight and shadow ends, and which vary for different digital cameras and digital backs.

Exposure graphic

The same is for the radiographic film in the above figure [reference www.sprawls.org], all films and digital sensors respond to light with a spectral response (sensitivity to light). They therefore have a film specific or sensor specific latitude or response with which they are capable to capture an image. Per our choice of exposure settings at the time for exposure, we can determine where to allocate that latitude compared to the dynamic range of a scene.

Thus the first step is to test our sensor for its spectral response by photographing an 18% grey card, and which I choose to do for my Leaf AFi-II 12 digital back at base ISO 50 by using Capture One to read RGB values at default settings. For this purpose, middle tone at defaults in Capture One was at RGB 127, which matched middle tone exposure in the actual scene. In summary, I was able to establish the following key values to spot meter, where (in the figure below) M indicates middle tone for which we make our camera settings (not necessarily what corresponds medium tone in the scene or final image). Red and blue stop values represent the number of stops from M that the location of extreme highlight and shadow values with RGB values maintained or with recovery (for shadows also maximum negative exposure value in Capture One). It is emphasized that the stop locations for these values is for my particular digital back and are not necessarily same for other digital cameras or backs.

AndersLoof---Graphic2

In principle, with my particular sensor at ISO 50 and Capture One defaults, we could apply spot metering by simply using one single spot meter at 2⅓ from medium tone as exposure compensation in camera. However, we need to be aware that the spot meter in the camera is rather large compared to a 1-degree spot meter with which we could pinpoint a scene.

In order to clarify, we can chose our spot metered “none recovered highlight end” to be the value immediate at clipping of all channels (RGB 255), or at start of the shoulder at defaults in Capture One. The latter was my personal choice and is basis for 2⅓ in above.

Additionally, and to note, the highlight end for our digital sensor tapers off rather abrupt compared to film, and that we at capture (pending on the scene at hand) should consider applying slightly more for a highlight transition than the difference between +2⅓ and +3½ (again these values apply for my digital back). By doing so we will enable adequate transition zone towards the highlight end similar to film, and will be able to create a more pleasing gradual shoulder per adjustment using curves. Thereby, we can note that for certain scenes we should expose slightly towards the left compared to the advocate for max ETTR.

Precise Spot Meter

For spot metering landscapes and similar scenes, my own preference is to use the same spot meter as Ansel Adams used, the discontinued Pentax Digital Spot Meter. This meter displays EV (Exposure Values) in a readout display in the viewfinder, that are then transferred manually to the inner wheel, which features EV values and shutter speeds, while an outer wheel features ISO and aperture, please refer to the below. Reading of shutter and aperture combinations are thus at a quick glance since all such are available before the eye. For my AFi-II 12 I have modified a Pentax Digital spot meter with my own personal Digital Zone Scale, which is similar to how large format photographers frequent modify such meter affixing a Zone Scale:

AndersLoof---Graphic3

AndersLoof---Graphic4

Of course, I could have made this personal zone scale instead upon stops compared to medium tone:

AndersLoof---Graphic5 Additionally, I should mention that for at least my particular sensor, the optimum RGB densities appear from -3 ~ -2 stops and to RGB 255 in Capture One.

The Take-Away

What is the point with the above? In the end it comes down to the choice for a tool, and the choice for a methodology for how to work and pre-visualize exposure. The important is thereby not mere to spot meter with purpose to precise allocate the latitude of our particular digital sensor compared to the dynamic range of a scene, but it also brings us a tool to precisely pre-visualize the contrast values of a scene prior to pressing the shutter or even touching our camera. This brings us a tool to precisely pre-determine exposure per our aesthetic minds eye before the making of a picture. For digital, this brings back the principles of the Zone System by Ansel Adams, and how he taught us to visualize the image in our minds eye at the time of capture. In bare essence, the principles briefly outlined in this article bring a tool for precisely determining the ends of the latitude of our sensor per spot metering in a scene, including allowing an extended highlight shoulder in post, and for if we will be able to capture the shadow end of a high dynamic scene.

For a more extended explanation, please refer to the publication for download with same name.

All photos in the article are captured in or next to Yue Yuan Garden, Shanghai, ISO 50, hand held, Pentax Digital Spot Meter, Leaf AFi-II 12 on Rolleiflex Hy6 and Schneider AFD Xenotar 80mm f/2.8 PQS.

Updated: April 10, 2015

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Anders Loof

See all articles by Anders Loof

Anders Loof is an advanced photographer with a non-photographic full time job as expat overseas. He currently resides in Seoul, South Korea. The advancing or “cradle of his photography” was a travel to Kenya in year 2000 which ignited his passion into serious photography, together with travels in 2003 round the world and to remote areas in Guizou province in China. Since these he has further advanced in photography with primary focus on landscapes and people living traditional lives during travels and when living overseas, and along with some beauty portraiture.