A little about metering

Digital Photography School has an introductory article about metering modes. It’s a good start, but I thought I’d point out some issues with it.

Evaluative Metering

In the first section called “Overall Metering (Multi-Segment/Zone Metering).” (This is also called “Matrix metering” (Nikon trade name), “Evaluative metering” (common name), and sometimes honeycomb metering. This first appeared in the Nikon FA camera. The author writes:

“It assesses overall lighting from all these zones and takes a best guess by averaging them to decide on how to expose the shot.”

Wrong, wrong, a thousand times wrong!

The way an evaluative metering works is to take a sample of each of the zones separately (35 monochromatic zones in the case of her Canon 5D, and 1005 RGB zones in the case of my Nikon D3) then compare that to a pre-programmed expert system of metering modes built from a proprietary database of professional photographs to make a “best guess” as to the metering mode.

In other words, the closer you use frame your subject and setup your lighting to the way a professional would, the more accurate the evaluative meter. What the author describes is known as “average metering” and is closer to (3) center weighted meter—it also isn’t common in any camera anymore.

You may be wondering why I made a distinction between the Canon and Nikon metering systems. Truth be told the difference is not very much except in a few rare instances. Here are some examples I’ve run into:

  • When shooting on a beach on a very sunny day, monochromatic evaluative meters tend to underexpose slightly. A color evaluative meter can detect the yellow reflectivity off the sand and realize it’s a beach scene and overexpose slightly, which is the preferred shot
  • When shooting at a bright winter day (e.g. skiing in the West), monochromatic evaluative meters tend to underexpose a lot. A color evaluative meter can detect similar brightness blue coming from the top of the scene and white coming from the bottom of the scene and realize it’s a snow scene and compensate.

I’m sure there are others, but differences in the database between vendors just make for different quirks. For instance, on sunny days when shooting directly to JPEGs (travel photography), I tend to feel my Nikon is a little underexposed compared to my preferred style and I overexpose vs. the meter valuation by 1/3 of a stop—I do 2/3 of a stop on my D3 trusting my Active D-Lighting to recover some of the lost dynamic range.

Similarly, if you are shooting a Canon. There’s nobody holding a gun to your head saying you can’t dial it up 1/3 of a stop on the beach, up 2/3 to a full stop on the ski slopes, or down 1/3 of a stop when shooting a red sunset (unless you like the oversaturated fire-engine red look).

You learn to adjust.

Another difference about the color metering and the huge number of points is that on new Nikons the color metering acts as an assist to the focusing system when in continuous focusing mode to allow object tracking even if the object leaves the focusing points (or even the frame of the camera briefly). That’s nice and handy, though this doesn’t work if you are using any of the live view modes. This is a relatively recent feature so I can’t comment yet on its utility.

Spot Metering

I would add that the location and size of the spot can be adjusted, and can be configured to follow a primary focus point. When the spot is very large (>10% of entire frame area), it’s actually called partial metering.

Center-Weighted Metering

The explanation in this section is sound but it leads me to a general observation…

When to use what metering

…that the advice of when to use which metering in this article is either non-existent or unsound!

So here is my advice.

Your primary mode of operation should be evaluative metering almost all the time. Periodically review a shot’s histogram, adjusting exposure compensation to the right. After a while you’ll get used to how your particular brand’s meter “thinks” about scenes you commonly shoot.

In particularly difficult metering scenarios that break a lot of evaluative meters, opt for partial metering or spot metering. What are these scenarios?

Well let’s say you have a backlit outdoor scene, then spot meter off a persons face and adjust if necessary.

Let’s say the contrast on the edges is way different from the subject (as is the case of almost all macro photographs and other creative exposures), then partial metering the subject would be a good idea.

Another example is if you happen to have an 18% gray card. Just spot on that puppy to meter off it.

Another good example? Shooting the moon

Moonshine through redwood branches
Moonshine through redwood branches
Riverstone Townhomes, Mountain View, California

Nikon D70, Nikkor 70-200mm f/2.8G VR, Gitzo 1228LVL tripod, RRS BH-55 ballhead
3 exposures @ f/9, iso 200, 200mm (300mm)

While this is a composite of three exposures, the moonshot was taken by spot metering off the moon and then overexposing by a stop and a half, or I could have just applied the Sunny f/16 rule if I knew how to do arithmetic (I don’t).

I overlooked a form of metering

This last part makes me mention a very common form of metering I do… none at all. In particular, because evaluative meters are based on images taken years ago, in some shooting situations which sit at the cutting edge of a dSLRs ability to shoot there is a massive breakdown.

For instance, Color ISO19200 never existed in the era of film. Flashes didn’t have fancy electronics in them back then either. So an indoor club scene with flash, the evaluative meter doesn’t pull in enough ambient light to show the environment. In this case, I “fix” one metering dimension by using into manual exposure mode and a fixed (non-auto) ISO set extremely high. The I let the flash fill in the subject where the evaluative metering is used to assist the flash level (iTTL in Nikon-speak).

Wrong Kevin!
Wrong Kevin!
111 Minna, South of Market, San Francisco, California

Nikon D3, Nikkor 24-70mm f/2.8G, SB-800
1/30sec @ f/2.8, iso1250, 24mm

Without a manual exposure and ISO fixed at 1250, There wouldn’t have been enough light to see the crowded bar behind Kevin and Melody. This photo was taken exactly one year ago, on my birthday, when I was severely shitfaced so having the right camera setting was very important.

Another case is when you are going to be taking a panorama, you want all the photos at the same exposure so they stitch together cleanly without enblending.

What about center-metering?

The biggest advantage of center-metering is consistency. No matter what the brand of camera, it will center meter the same. So when you are using a camera brand you aren’t familiar with, you can’t trust the evaluative meter to be like your camera. The problem with this is with the advent of digital photography, you can just review your damn shot and figure it out. 🙂

Well, whatever, the only metering on my Leica M8 is a warped strip-style reflective meter closest to center-metering so maybe I should have learned center-metering after all. 😀

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