Sunday, January 23, 2022

AYWMC: Part 1 Technical, Lesson 6: Metering

 This post is part of a series entitled A Year With My Camera.
Lesson 1.6 in my own words:
  • Review
    • The camera has 3 settings that control light: aperture, shutter speed, and ISO.
    • Each has a creative effect or consequence.
  • Practice: Throughout the year, repeat the exercises of the first 5 lessons
    • Shoot on aperture and shutter priority modes.
    • Look at shots taken on auto, and think how they could be better
    • At least once a month, practice on manual mode
  • Histogram: A bar chart showing light values.
    • For each photograph taken, the camera counts the pixels, calculates how many of each tone there are, then plots them on a bar chart (histogram).
    • Black tones are on the left.
    • White tones are on the right
  • Metering: When the camera measures how much light is falling on the scene.
    • The camera is programmed for a choice of three kinds of metering.
      • Matrix, or evaluative metering (usually default) - The camera takes a broad reading from the entire frame. Can be skewed if, for example, the background is very light or dark compared to the subject.
      • Center-weighted metering - Priority is given to whatever is in the middle of the frame. A good all-purpose setting.
      • Spot metering - The photographer chooses one small area of the frame for a reading, usually one of the small rectangles seen in the viewfinder indicating where the camera is focused. This is the most accurate way of getting correct exposure.
    • Metering will vary depending on the camera's mode.
    • The histogram represents the metering for each potential picture.
  • Exposure compensation
    • An alternate way to correct exposure
    • Quicker than adjusting shutter speed, aperture, or ISO.
    • The camera's exposure compensation function lets you make changes without having to fiddle with the other settings.
    • Represented as a scale from -3 to +3, with increments in full and third stops.
    • Can be used visually (eyeball it) or with the histogram.
      • If the histogram shows underexposed, use exposure compensation to choose more light.
      • If the histogram shows overexposed, choose less light.

This Week's Project:
  • Select one of the auto modes and photograph a piece of white paper.
  • Check the histogram. The camera will display it for mid-gray.
  • Using manual mode or exposure compensation, adjust the histogram to display the paper as white, with the bars all the way to the right.
  • Try other subjects, indoors and out, and observe the histogram.

Before tackling the homework, I first needed to find the exposure compensation function on my camera and experiment with it (because I'm not ready to try this on manual mode yet!)

Exposure compensation 0

Exposure compensation +1

Exposure compensation +2

Exposure compensation +3

Exposure compensation -1

Exposure compensation -2

Exposure compensation -3

Then it was on to the actual homework assignment, which was a repeat of lesson 1. We were to photograph white poster board so that it filled the entire camera frame. But this time, I used the camera's built-in histogram and corrected exposure so that my photo looked like what I saw.

There is no way to download the camera's histograms to illustrate the lesson, so I used my photo editor, Gimp. I took three photos; the first with the settings the camera chose, the second and third are my adjustments with exposure compensation. Then, I opened the photos in Gimp took screenshots of the histograms.

This first photo is as interpreted by the camera, which assumes a total exposure of mid-tone grey.

White poster board, as interpreted by the camera (see lesson 1 for explanation)

Here's the histogram for the above photo.

The histogram shows that the camera interpreted it in middle tone greys.

Second photo, trying to adjust the histogram with exposure compensation.

White poster board with exposure compensation +2.

Here's the histogram for that photo.

The histogram is moved considerably toward white.

One more try.

White poster board with exposure compensation +3.

Here's how it looked on the histogram.

Basically, a thick white vertical line on the right of the graph.

I'll experiment with this more over the next couple of days.

What I learned:

  • Let's start with what I didn't learn. Even though the lesson was entitled "metering," I did nothing with the camera's built-in meter!
  • I did learn how to find and use another of my camera's buttons (exposure compensation).
  • I learned the basics of how to read a histogram.
  • One of the points of the lesson is to compare what your eye sees with what the camera sees (via the view screen), and to adjust exposure so that the camera matches the eye. For example, if the subject is too light or too dark compared to the background, I now know how to correct it.
  • If I change ISO manually, then I need to reset it to AUTO to give control back to the camera.


Toirdhealbheach Beucail said...

Huh. The visual appearance of the poster board and the corresponding histogram are very interesting - the difference between how something looks and how it is broken down.

Leigh said...

TB, it is interesting, but at this point I am clueless as to what to do with it. On the other hand, I realize that this first set of lessons is an introduction to the technical aspects of the camera, so just knowing about it and how to access it feels like an accomplishment!