Metering and Modes

Photography Fundamentals

Course 1 · Chapter 6 · Metering and Modes: Letting the Camera Help

Who Sets the Exposure?

Photography Fundamentals 1 showed that aperture, shutter speed and ISO trade against each other. In practice the camera measures the light and can set some or all of them for you. This chapter covers how it measures (metering), how you correct it (exposure compensation) and who chooses what in each shooting mode.

Metering Modes

Wikipedia's Metering mode article describes these ways of measuring a scene:

Mode	What it measures (per the article)
Multi-zone (evaluative, matrix, honeycomb)	Readings at several points combined; zone counts range from several to over a thousand; aims to reduce the need for exposure compensation and biases toward the autofocus point
Centre-weighted average	Concentrates on the central 60 to 80% of the scene and fades the influence of the edges
Partial (selective)	Roughly 10 to 15% of the frame; useful when very bright or dark edges would skew the reading
Spot	A very small area, about 1 to 5% of the viewfinder, by default at the centre; not influenced by other areas

When the meter is fooled

The article says average metering, which weights the whole scene equally, can underexpose a snowy landscape "by 2 f-stops or more". A bright background can lead the meter to underexpose a backlit subject; spot metering on the subject exposes it correctly but overexposes the bright surroundings. These are the article's examples; other situations exist (check).

Exposure Compensation

Wikipedia's Exposure compensation article calls it a technique for adjusting the exposure indicated by an exposure meter, to account for factors that could make that reading less than ideal. Camera compensation is usually in EV, where one EV is one stop. Marks between whole numbers usually represent 1/3 stop, and positive values add exposure while negative values reduce it. The typical range is about two or three steps in either direction, and some cameras offer more (the article's examples are Canon EOS R6 Mark II at plus or minus 3 EV and Nikon Z 50 at plus or minus 5 EV; current specifications may differ, check).

Which setting changes depends on the mode: in aperture priority, compensation changes the shutter time; in shutter priority, it changes the aperture. The article says meters are calibrated for typical scenes, so a predominantly light subject (its example is a white horse) often ends up underexposed and needs positive compensation, while a predominantly dark subject needs negative compensation.

Shooting Modes

Mode	You set	Camera sets
Manual	Aperture and shutter speed	Nothing
Aperture priority (A or Av)	Aperture	Shutter speed
Shutter priority (S, or Tv for "time value")	Shutter speed	Aperture
Program	Neither	Both

According to the Shutter priority article, "S" is common on Nikon, Minolta, Sony and others, while "Tv" is used by Canon, Pentax and Leica. The Aperture priority article gives A or Av. The abbreviations for program and manual (P and M) were not on the pages I read, so they are from my own knowledge (check). I also did not find a page covering these modes together; the Automatic exposure page I tried returned an error.

Seeing Compensation Work

The script below simulates aperture priority: the meter gives an EV, you fix the aperture, and the camera computes the shutter time from EV = log2(N squared / t), rearranged as t = N squared / 2 to the power EV. Positive compensation asks for more light, so the camera uses a lower EV, meaning a longer time. This is my own illustration, and the sign convention is the part I would check against a real camera (check). I ran it and the output is shown.

from math import log2

def shutter_time(n, ev):
    """Time in seconds that gives exposure value ev at f-number n."""
    return n * n / 2 ** ev

def show(label, t):
    print(f"{label:<28} 1/{1 / t:.0f} s")

metered_ev = 15  # a bright scene at ISO 100, as the meter reads it
n = 8
base = shutter_time(n, metered_ev)
show("Meter says (EV 15), f/8", base)
for comp in (+1, +2, -1):
    ev_used = metered_ev - comp  # + compensation = more light = lower EV setting
    show(f"Compensation {comp:+d} EV", shutter_time(n, ev_used))

# Output
Meter says (EV 15), f/8      1/512 s
Compensation +1 EV           1/256 s
Compensation +2 EV           1/128 s
Compensation -1 EV           1/1024 s

Reading the output

Each +1 EV halves the shutter speed number, doubling the time, which is one stop more light, as Chapter 1 predicts. The meter's 1/512 s is the exact result; a real camera would round to a nearby standard speed such as 1/500 s. In the snow example, the article's "2 f-stops or more" underexposure corresponds to the +2 line, where the camera is told to let in four times as much light.

Not Yet Verified, and Sources

Sources. Wikipedia articles Metering mode, Exposure compensation, Shutter priority and Aperture priority, all read through page summaries. The Automatic exposure page returned an error and was not read.
Not on the pages read. The 18% grey assumption behind meters (the Metering mode page does not mention it), the P and M abbreviations, the real sign convention on any given camera, and current camera specifications.
Not tested. I did not test any of this on a physical camera.

Exercises

Exercise 1

Adapt the script for f/4 with a metered EV of 12 and compensation of +1 and -2. Give each shutter time and say which standard speed from Chapter 1 is nearest.

→ Solution

Exercise 2

For each scene, name the metering mode you would pick and say whether you would add positive or negative compensation, with one sentence of reason: (a) a snowy field, (b) a person standing in front of a bright window, (c) a dark-coloured subject filling the frame.

→ Solution

Exercise 3

Pick the mode (manual, aperture priority, shutter priority or program) you would choose for (a) a blurred background in a portrait, (b) freezing a moving subject, (c) a quick snapshot with no time to think, and say what the camera sets in each case.

→ Solution

Quick Reference

Metering: multi-zone, centre-weighted, partial, spot
Compensation: in EV; 1 EV = 1 stop; positive adds exposure
Aperture priority: you set aperture, camera sets shutter
Shutter priority: you set shutter, camera sets aperture
Check: grey assumption, P and M abbreviations, sign convention

Coming next

Photography Fundamentals 7 covers colour and white balance, using the colour temperature figures from Chapter 4.