An Alternative to the Zone System for Digital Photography

By Simon Forsyth

Recently I have come across an alternative take on Ansel Adams’ Zone system which was formulated way back in the 1940s, long before digital photography was even thought of!

The premise of the Zone system, as many will know, centred around measuring a mid-gray tone and using that to calculate exposure. Mostly, this was for black and white film as there was almost no colour film back then. Meters were then calibrated to read a mid-gray tone. While the system could and was used for colour film that came later, the trick was working out what was mid-gray in terms of brightness or luminosity. Also, nearly all meters were reflective and measured the total scene in front of them. Spot meters did not appear until later.

The problem with reflected meters is that they assume that all tones in the scene that it is pointed at will average mid-gray which is defined as 18% gray. This means that it reflects 18% of the light falling on it! The other option is an incident meter which measures the light falling on a subject as opposed to the light reflected from it. As long as the light measured is the same as the light falling on the scene this can result in a more accurate reading.

Spot meters measure a small spot, typically 1-5° wide, to get the exposure value. Nowadays most cameras have a spot meter facility built in, but a handheld spot meter is a useful option! The issue with inbuilt spot meters is that the spot is not always 1%! Also, beware of zoom lenses with variable apertures as the exposure will change because the aperture changes as you zoom in and out!

All meters are calibrated to an ISO standard which is ± ½ a stop, so technically measuring with two meters could result in the readings being a whole stop out! Also at play here is the accuracy of the sensor, which like film can vary, and the dynamic range comes into play here. While digital sensors have more dynamic range than film ever had they still cannot reveal the range that the human eye can see. This is because the eye can adjust to the brightness of the area it is looking at automatically, which the camera cannot do.

While the Zone System can help in the accuracy of metering, it can take time, unless the photographer can visualise a mid-gray tone in colour! Sometimes it is assumed that grass is a mid-tone, but that depends on the light and how glossy the grass is!

I have been reading The Optimum Digital Exposure by Bob DiNatale who has developed a system called the OneZone and has registered the trademark. He outlines two methods to achieve an exposure that retains as much information in both the dark and the bright areas!

Initially, there is some measuring involved to work out the metering and dynamic range of the sensor, but once this is done things should be reasonably simple. Even getting it right with the Zone System involved initially measuring things and this method is easier than doing that. When I was setting up for the Zone System it involved shooting black and white film at different exposures, processing it and then measuring it with a densitometer to ensure accuracy! I’m glad those days are behind me!

The first step in calibration involves photographing a white card. Preferably use the back side of a Gray Card which is white! The card doesn’t need to be in focus, so I suggest you switch to manual focus. Fill the frame with the card, then set the ISO and Aperture so that the meter reads the shutter speed as 1/250th sec. Use manual mode to prevent the meter from changing any settings. Turn Auto ISO off! Most cameras have a spot metering facility nowadays so preferably use this. Also, be sure to shoot in RAW!

Now take a series of frames from the recommended exposure (which will result in a mid-gray tone as the meter thinks it should be this tone) in 1/3rd stops, all the way down to 1/8th sec. Movement doesn’t matter as you are only looking for a tone. What you will have is a series of 16 exposures over a five stop range from mid-gray to white.

Next, look at the images on the back of the camera with the blinkies turned on, or Zebra Stripes if you use a Sony! The blinkies indicate what the camera thinks is blown out! Note that the image on the back of the camera is a jpeg from the raw file. Also, camera manufacturers are conservative in deciding when the blinkies turn on. Usually this is at 90% so the highlights aren’t blown out at all and there is still detail there, even though the histogram on the camera shows it climbing the right side.

Next, import your images into your raw processing software (I use Lightroom) and evaluate them. Turn on the highlight clipping warning in the software. You are aiming to find your Optimum White Point. Do this as follows:

1 a Place the cursor in the middle of the first exposure and note the %RGB values under the histogram. Do this in the develop module in Lightroom.

1 b Advance through the exposures until the %RGB value reads 99%. This will be the exposure before the highlight clipping warning shows in Lightroom.

Note that the clipping warning in Lightroom is different from the blinkies on the back of your camera. The blinkies usually appear at 90% whereas the Highlight clipping warning occurs at 99%! This is Your Optimum White Point and the exposure after this will show the highlight clipping warning. Ideally, this should be at +4EV! What you have is the +EV value that is your OneZone bias.