Photzy

THE REAL STORY BEHIND YOUR CAMERA METER

AND WHY IT CAN BE FOOLED
Quick Guide by Kent DuFault

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 1
 Understanding exactly what the camera meter is, Here is what we will cover:
and what it does behind the scenes, can indeed be
challenging. ·· What is the camera light meter?

In your quest to produce the very best photographs ·· How does the camera light meter “read” a
that you’re capable of, you must have an situation to determine exposure?
understanding of the camera metering system. ·· How does the camera metering mode change
Granted, it’s a bit easier today with the camera what the camera light meter is going to do?
RAW file format. You can miss the exposure mark by ·· How do we analyze a scene to determine if we
quite a bit, and in many cases you can still recover a need to override the camera light meter?
decent image file.

However, this is fact. Recommended Reading: Want to learn how

to make your photos stand out from everyone
If it’s necessary to recover huge amounts of else’s? Grab a copy of Photzy’s premium guide:
data (from the RAW file) due to gross over or Effective Storytelling.
underexposure, your image will look “off.” It will
never look as good as an appropriately planned and
exposed shot.

Sure, small adjustments are no problem, but accuracy

should be your goal.

Today, we are going to discuss some potential

situations that can throw your camera light metering
system way off track. We will also look at why that
happens.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 2
 WHAT IS THE CAMERA LIGHT METER?

The device that is built into your camera is often referred The incident model measures the intensity of the light as it
to as “the camera meter,” or “the exposure meter,” or “the arrives at the subject.
light meter,” or the “the camera light meter.”
The light meter built into your camera is a reflectance
In actuality, the device described in the previous paragraph light meter. This guide will concentrate only on the
is a “reflectance light meter.” reflectance light meter built into your camera.

In the very old days, before light meters were integrated

into camera bodies, photographers had a separate device
in their equipment bag called a “handheld light meter.”

The handheld device worked similarly to what we have

today, except the photographer took the light reading
with the handheld light meter and then transferred the Illustration by Kent DuFault
information by manually setting the exposure on their Zone System Grey Scale (Zone 5 is 50% Grey).
camera.
The reflectance light meter does not look at the physical
Some purist photographers – especially professional scene displayed in your viewfinder.
commercial photographers – still use handheld light
meters today. It also does not see the colors displayed within your scene.

The handheld light meter comes in two varieties: the What it sees is the amount of light reflecting off of the
reflectance model and the incidence model. objects within your camera frame, otherwise known as
“Luminance,” and then it converts that amount of light
The reflectance model measures the amount of light reflectance into varying shades of grey.
reflecting off of an object.
At this point, everything happening within your camera
light meter is occurring in the background.

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 HOW DOES THE CAMERA LIGHT METER “READ” A
SITUATION TO DETERMINE EXPOSURE?

the “area that it sees,” throwing those readings into a

pot, stirring them around, and outputting an exposure
setting (aperture and shutter speed) based upon the light
sensitivity of the imaging source (digital sensor or film) that
Illustration by Kent DuFault will produce an overall average of 50% grey that reflects
In order to establish the various levels of light reflectance, 18% of the light.
there needs to be a baseline. That baseline is 18% grey.
18% light reflectance is the baseline for the light meter to
This is kind of confusing, so pay attention. The ‘color’ isn’t determine the light reflectance variations from white to
18% grey; the ‘color’ is actually 50% grey – exactly half black.
way between pure black and pure white. However,
However, in order to convert that into an exposure, it
50% grey reflects 18% of the light that hits it. This is
needs to know the light sensitivity of the imaging source.
how it came to be known as the “18% Grey Standard.”
In other words, the light meter needs to be calibrated in
order to take the light reflectance data and convert it into
a real-life exposure setting.

This calibration is what is known as the ISO setting.

The ISO setting on your camera tells the light meter how
Illustration by Kent DuFault sensitive your camera is to light so that it can calculate the
When your camera meter analyzes a scene, essentially collected data into an exposure setting!
what it is doing is taking light reflectance readings from

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 4
 HOW DOES THE CAMERA METERING MODE CHANGE WHAT
THE CAMERA LIGHT METER IS GOING TO DO?

Illustration by Kent DuFault

The camera metering mode tells the reflectance light The center-weighted metering mode places an emphasis
meter where to look when gathering its data. This is on the central part of the frame to gather the light
called the metering pattern. reflectance data.

In the zone/evaluative metering mode, the reflectance The spot metering mode measures the data from a small
light meter takes its readings from the entire frame. It then area within the frame. Some cameras allow that small area
does a calculation, with all of that light reflectance data, of measurement to be moved around inside the frame.
to come up with what it believes is the best exposure
combination of aperture and shutter speed (based upon Canon has an additional metering mode called Partial. It is
the ISO baseline) to capture as much detail as possible a larger measuring area than the spot metering mode, but
from white to black with 18% grey as the center baseline. has a more defined line of demarcation than the center-
weighted metering mode.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 5
 Key Lesson: The light meter built into your Why are my camera’s exposures off at times?
camera is a reflectance light meter. It requires a
It’s simple really.
baseline ISO setting so that it can calculate exposure
settings. The baseline for a reflectance light meter is The camera reflectance light meter does not have a brain.
18%, which is actually a 50% grey color reflecting It simply gathers data and then pushes that data along to
18% of the light that hits it. The camera metering the next step in the camera function process.
mode tells the reflectance light meter where to look
when gathering its data. The zone/evaluative metering mode is the closest that
camera manufacturers have gotten to giving the camera
You’ll notice that so far I have said nothing about manual a brain to think through exposure. However, it’s far from
mode, or aperture priority mode, or shutter priority mode, foolproof.
or even auto mode.
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It is because these functions of the camera have absolutely Storytelling.
nothing to do with determining exposure! These functions
take the data supplied by the reflectance light meter
and generate how the camera will function for the
exposure based on that data.

Now that we know what the camera meter is and how it

functions, we must ask…

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 6
 HOW DO WE ANALYZE A
SCENE TO DETERMINE IF
WE NEED TO OVERRIDE THE
CAMERA LIGHT METER?

The image on the left depicts a full range

of light and tone. However, the camera
meter does not see color.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 7
 This is what the reflectance camera light
meter sees. It sees values of grey tone
based on light reflectance and using a
baseline of 18% reflectance.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 8
 This scene is a great scenario for your
camera reflectance light meter. It has a
full range of tone from black to white and
everything in between.

In this scenario, your camera light meter is

not likely to be fooled. The best metering
mode would be the zone/evaluative
setting.

Given that previous statement, would

your camera capture full detail from the
deepest black to the brightest white?
Maybe not; that becomes a function of
the camera’s dynamic range, which is a
completely different subject.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 9
 This is one type of scene that is going to
fool the pants off of your reflectance light
meter.

Remember, the camera light meter

sees in reflectance of tone. It uses 18%
reflectance as the baseline to determine
exposure. The meter wants to create
18% grey reflectance.

Since this scene is predominantly white

in tone, the reflectance light meter is
going to pass along exposure data to the
camera that will result in underexposure.

Why?

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 It’s looking to turn the majority of the
white tone into 18% grey reflectance.
Changing the metering mode would do
little to help in this situation.

Key Lesson: When you are

photographing a scene that is
predominantly white, or light, in
tone, you will need to override the
recommended exposure setting by
increasing exposure 1 to 5 stops
depending on the lighting and
tonal range. This is true no matter if
you’re using manual mode,
aperture priority mode, shutter
priority mode, or program mode. In
“auto” mode, you cannot override
the camera meter, so that would be
a terrible choice for this situation.

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 This is another situation that will wreak
havoc with the reflectance light metering
process. A spot of very bright light, or
strong backlighting, anywhere within
the frame can skew the light meter data
significantly.

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 Key Lesson: In order to correct
for backlighting, which will skew
the camera meter’s ability to
provide a correct exposure
reading, you must turn to your
camera metering modes! That’s
right; this is where the camera
metering modes come into play.
For this situation, you would want
to use the “spot” or “partial” light
meter mode settings. Reposition
the camera so that the area where
the data is going to be collected is
outside the area of bright light
(see the image on the left).

Note: You can try the zone/

evaluative mode. It may function
properly, depending on how bright
and intense the backlighting source is.

With a strong backlight, use the spot metering mode and take your light meter reading
from an area in the scene that is close to a mid-tone and excludes the bright spot.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 13
 If you follow that key lesson, what happens when you You’re going to have to override this using the Exposure
reposition your camera to the correct composition? It’s Compensation setting.
going to change the meter reading, and thus also
change the exposure data that the light meter wants to How much do you override the setting? It depends on how
pass along to the next step in the camera’s operation. much it changes.

This is where the shooting mode, or exposure mode as This is why in this particular situation manual mode is
some refer to it, comes into play. far easier to use than the auto modes. There are no
concerns for additional calculations.
This situation would work best in manual mode. When
you take your camera meter reading off to the side In an auto mode, you would have to take your light meter
(as explained in the Key Lesson and the image on the reading from the correct spot in the scene, take note of
previous page), you would manually set the aperture and the exposure settings recommended by the camera light
shutter speed based on that light meter reading. meter, reposition the camera, take note of how much the
recommended settings have changed, and then adjust for
When you reposition the camera, it doesn’t matter if the that bias using the Exposure Compensation setting. See!
meter suddenly indicates that you have way too much Manual mode is easier.
light reflectance, because you’ve already set the exposure
based on the light meter reading from the correct area. In
manual mode, the camera cannot override this setting.

If you’re using aperture priority, shutter priority, or program

mode, what happens when you reposition the camera? It’s
going to automatically change the exposure settings back
to being incorrectly skewed due to the backlighting.

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 This is another situation that will send
your camera’s reflectance light meter into
a tailspin. Any shot that you attempt that
contains predominantly black or very dark
tones requires special consideration, just
like a scene that is predominantly white in
tone (image on page 10).

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 When your camera’s reflectance light
meter attempts to evaluate a very dark
scene, it wants to create a baseline of
18% grey reflectance. It’s going to take
those dark areas and turn them into a
grey area – not so nice. The result is an
overexposed image that looks washed
out, with blown highlights and weak
shadows.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 16
 Key Lesson: In a situation, such
as this, where the vast majority of
the tones (dark or light) will skew
the data collected by the light
meter, you need to tell the meter
where to go to get the data. That’s
the first step. Typically, you would
set your meter mode to spot,
partial, or center-weighted
depending on the situation. In this
situation, with so much dark area,
we would recommend using the
spot mode. Place the “spot” in an
area where you have some range of
tone. Take a reading, use manual
mode, set the exposure, take a test
shot, and you may have to adjust
depending on the situation.

Recommended Reading: Want

to learn how to make your photos
stand out from everyone else’s? Grab
a copy of Photzy’s premium guide:
Effective Storytelling.

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 17
 CONCLUSION

There are three situations that are likely to make your Three Important Concepts
camera light meter go bonkers:
1) The ISO setting is the calibration that tells the camera
1) All white or light tones light meter where to set exposure, based on its
gathered data.
2) All black or dark tones
2) 18% grey reflectance is the baseline that the camera
3) Strong backlighting light meter attempts to establish as a median point,
There are three steps to correct for these situations: with the dynamic range then radiating equally toward
the white and black ends of the scale.
1) Your choice of metering mode
3) The shooting mode (exposure mode) has nothing to
2) Where within the frame you tell the camera to collect do with determining exposure. It takes the exposure
the light metering data data, supplied by the camera metering system, and
then sets the aperture and/or shutter speed based on
3) Setting the exposure manually or using exposure the shooting mode parameters.
compensation

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 18
 Self-Check Quiz: Shooting Exercise:

1) When shooting in camera RAW, why is it still ·· Take a day for a photo shoot.
important to get the exposure right?
·· Select a subject dominant in white tones and
2) The light meter built into your camera is another subject dominant in black tones.
what type of light meter?
·· Using what you have learned in this guide,
3) In the zone system, which zone is 50% Grey? take your metering readings and use the
appropriate metering mode and the correct
4) Describe the “18% Grey Standard” and exposure mode.
how it is relevant to your camera light meter
system. ·· Create another shot with strong backlighting
at the end of the day. Use the metering
5) How does the ISO setting fit into the camera method for backlighting described in this
meter system? Why is it important? guide. Also try using the evaluative metering
6) Name three of the four meter modes mode. Practice overriding the meter
discussed in this guide. using manual mode and using exposure
compensation in the auto modes.
7) Is there a difference between a meter mode
and a meter pattern?

8) How much light does the color black reflect?

9) In a scene that is predominantly white in

tone, what should you do to the exposure?

10) What metering mode would you use for

strong backlighting?

THE REAL STORY BEHIND YOUR CAMERA METER AND WHY IT CAN BE FOOLED // © PHOTZY.COM 19
 ABOUT THE AUTHOR Congratulations! You’ve completed
this Photzy guide!

If you liked this photography tutorial from

Kent, you may be interested in his other books.
Check out Kent’s premium guide on how to
effectively create the most memorable images
that you can be proud of: Effective Storytelling.

IF YOU’D LIKE TO CONTINUE

Kent DuFault is a professional photographer and author. LEARNING AND IMPROVING
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