Camera Exposure and Light Metering

Camera Exposure and Light Metering

How do we make a photo which is the brightness that we want? There are a number of different methods which I will cover in this tutorial, starting with how do determine this without a light meter, then looking at the incident light meter, the reflective light meter, and the Zone System. Finally we make some other important observations about the range of brightness the camera can record.

[Please note that this post is available as a PDF where the tables and illustrations can be seen at there correct size – click here]

The Sunny 16 Rule

Back when there were only film cameras and very few photographers had light meters, the manufacturers of film supplied a tiny slip of paper with recommended exposure settings for the light conditions. So for example, the guide for a 200 ISO film looked like this…

Lighting conditions and weather Shutter Aperture
Bright sun/  Shadows distinct or hard 1/200      f/16
Slightly overcast / Hazy / Soft shadows 1/200      f/11
Overcast cloudy / Faint shadows 1/200      f/8
Very overcast and cloudy / Open shade from sun / No shadows 1/200      f/5.6
Around sunset time / Deep shade 1/200      f/4
Just after sunset / Bright neon lights 1/200      f/2.8

 

You can see that the shutter speed recommended is 1/ISO. This guide is just as relevant today with modern digital cameras, where we can change the ISO for every photo. It is a reliable and simple starting point to getting a good exposure without using a light meter.

Measuring Light

There a two very different words we use to explain ‘brightness’[1] in photography… illuminance and luminance.[2]

  1. Luminance describes the measurement of the amount of light emitting, passing through or reflected from a particular surface from a solid angle. It also indicates how much luminous power can be perceived by the human eye. This means that luminance indicates the brightness of light emitted or reflected off a surface.
  2. Illuminance is a term that describes the measurement of the amount of light falling onto (illuminating) and spreading over a given surface area. Illuminance also correlates with how humans perceive the brightness of an illuminated area. As a result, most people use the terms illuminance and brightness interchangeably which leads to confusion, as brightness can also be used to describe luminance. To clarify the difference, illuminance refers to a specific kind of light measurement, while brightness refers to the visual perceptions and physiological sensations of light.

Light illuminance is measured in lux. One ‘Lux’ is the illuminance on a one-square metre surface from one candle (1 lumen) at 1 meter from the illuminated surface.

In photography this standard is directly related to an Exposure Value (EV),

-1 EV = 1.25 Lux                                     5 EV = 80 Lux
0 EV = 2.5 Lux                                         6 EV = 160 Lux
1 EV = 5 Lux                                            7 EV = 320 Lux
2 EV = 10 Lux                                          8 EV = 640 Lux
3 EV = 20 Lux                                          9 EV = 1280 lux
4 EV = 40 Lux                                          10 EV = 2560 lux

EV Type of Lighting Situation
-6 Night, away from city lights, subject under starlight only.
-5 Night, away from city lights, subject under crescent moon.
-4 Night, away from city lights, subject under half moon.
-3 Night, away from city lights, subject under full moon.
-2 Night, away from city lights, snowscape under full moon.
-1 Subjects lit by dim ambient artificial light
0 Subjects lit by dim ambient artificial light
1 Distant view of lighted skyline.
2 Lightning (with time exposure). Total eclipse of the moon.
3 Fireworks (with time exposure)
4 Candle lit close-ups, Christmas lights, floodlit buildings, fountains, and monuments. Subjects under bright street lamps.
5 Night home interiors, average light. School or church auditoriums. Subjects lit by campfires.
6 Brightly lit home interiors at night. Fairs, amusement parks.
7 Bottom of rainforest canopy. Brightly lighted night time streets. Indoor sports. Stage shows, circuses.
8 Store Windows. Campfires, bonfires, burning buildings. Interiors with bright florescent lights.
9 Landscapes and city skylines 10 minutes after sunset.
10 Landscapes and skylines immediately after sunset
11 The light on the land at sunset.
12 Heavily overcast cloudy daytime conditions / Areas in open shade on sunny day
13 Subjects in cloudy-bright light. No shadows.
14 Daylight from weak hazy sun. Soft Shadows
15 Daylight from slightly hazy sun. Distinct shadows (Sunny f/16 rule)
16 Bright daylight with distinct shadows.
17-21 Rarely encountered in nature. Some artificial lighting.
22-24 Extremely bright. Rarely encountered in nature.

 

The reason why Exposure Values are important to photographers, is because a difference of 1 EV relates to a 1 stop adjustment in camera settings achieved by doubling or halving the duration of time the shutter is open (shutter speed),[3] or doubling or halving the area of the lens opening (aperture),[4] or doubling or halving the sensitivity of the sensor to the light (ISO).[5]

Here is a chart that gives different combinations of camera settings for ISO, shutter speed, and aperture, for light levels measured in Exposure Values (or Light Values[6]):

[this chart is best viewed in the PDF version of this post]

ISO 25 ISO 50 ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200   f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
1 0 -1 -2 -3 -4 -5 -6 15 sec 30 sec 1 min 2 min 4 min 8 min 16 min
2 1 0 -1 -2 -3 -4 -5 8 sec 15 sec 30 sec 1 min 2 min 4 min 8 min
3 2 1 0 -1 -2 -3 -4 4 sec 8 sec 15 sec 30 sec 1 min 2 min 4 min
4 3 2 1 0 -1 -2 -3 2 sec 4 sec 8 sec 15 sec 30 sec 1 min 2 min
5 4 3 2 1 0 -1 -2 1 sec 2 sec 4 sec 8 sec 15 sec 30 sec 1 min
6 5 4 3 2 1 0 -1 1/2 sec 1 sec 2 sec 4 sec 8 sec 15 sec 30 sec
7 6 5 4 3 2 1 0 1/4 sec 1/2 sec 1 sec 2 sec 4 sec 8 sec 15 sec
8 7 6 5 4 3 2 1 1/8 sec 1/4 sec 1/2 sec 1 sec 2 sec 4 sec 8 sec
9 8 7 6 5 4 3 2 1/15 sec 1/8 sec 1/4 sec 1/2 sec 1 sec 2 sec 4 sec
10 9 8 7 6 5 4 3 1/30 sec 1/15 sec 1/8 sec 1/4 sec 1/2 sec 1 sec 2 sec
11 10 9 8 7 6 5 4 1/60 sec 1/30 sec 1/15 sec 1/8 sec 1/4 sec 1/2 sec 1 sec
12 11 10 9 8 7 6 5 1/125 sec 1/60 sec 1/30 sec 1/15 sec 1/8 sec 1/4 sec 1/2 sec
13 12 11 10 9 8 7 6 1/250 sec 1/125 sec 1/60 sec 1/30 sec 1/15 sec 1/8 sec 1/4 sec
14 13 12 11 10 9 8 7 1/500 sec 1/250 sec 1/125 sec 1/60 sec 1/30 sec 1/15 sec 1/8 sec
15 14 13 12 11 10 9 8 1/1000 sec 1/500 sec 1/250 sec 1/125 sec 1/60 sec 1/30 sec 1/15 sec
16 15 14 13 12 11 10 9 1/2000 sec 1/1000 sec 1/500 sec 1/250 sec 1/125 sec 1/60 sec 1/30 sec
17 16 15 14 13 12 11 10 1/4000 sec 1/2000 sec 1/1000 sec 1/500 sec 1/250 sec 1/125 sec 1/60 sec
18 17 16 15 14 13 12 11 1/8000 sec 1/4000 sec 1/2000 sec 1/1000 sec 1/500 sec 1/250 sec 1/125 sec
19 18 17 16 15 14 13 12 1/15000 sec 1/8000 sec 1/4000 sec 1/2000 sec 1/1000 sec 1/500 sec 1/250 sec
20 19 18 17 16 15 14 13 1/30000 sec 1/15000 sec 1/8000 sec 1/4000 sec 1/2000 sec 1/1000 sec 1/500 sec
21 20 19 18 17 16 15 14 1/60000 sec 1/30000 sec 1/15000 sec 1/8000 sec 1/4000 sec 1/2000 sec 1/1000 sec
22 21 20 19 18 17 16 15 1/125000 sec 1/60000 sec 1/30000 sec 1/15000 sec 1/8000 sec 1/4000 sec 1/2000 sec
23 22 21 20 19 18 17 16 1/250000 sec 1/125000 sec 1/60000 sec 1/30000 sec 1/15000sec 1/8000 sec 1/4000 sec

 

Incident Light Meters

In the late 1930s the handheld incident light meter was developed, and was the main light meter used by photographers for the next 30 years.[7] It remains the chosen light meter for measuring the brightness of the light that is falling on the subject.[8] It measures illuminance.

The incident light meter measures illuminance in Exposure Values and shows the corresponding ISO, Shutter Speed, and Aperture setting combinations that may be used for an exposure under this EV level. This is easier and more accurate than referring to the charts above.  The incident light meter does this regardless of reflectance from the subject, background colours and luminance, or the subject texture, as the following demonstrates. Each of the following three photos are identical exposures (same illuminance) as determined by an incident light meter.

     

Incident light meters work very well when you’re photographing a subject under the same lighting conditions as where you are standing or you can easily walk to the subject and make the reading. But obviously this is just not practical for many photos e.g. you might be standing in the shade and the subject is standing in sunlight, or the subject could be miles away.

Measuring the light with Reflective Light Meters

We have talked above about measuring illuminance (the light falling on a subject) with an incident light meter. Now we shall describe measuring luminance – the light reflected off an object or emitted from an object, with a reflective light meter.

The first SLR cameras that included ‘through the lens metering’ were introduced by camera manufacturers in the mid-1960s. Since then all cameras use this method, measuring the light reflected from the subject, its luminance.

There are significant difficulties for the photographer determining an exposure setting based on the light measurement from a reflected light meter.

  1. There may be many different things reflecting different levels of light and or emitting light as seen through the camera’s viewfinder. Different metering modes and camera technologies attempt to address this problem with limited success.
  2. No reflective light meter knows what it is measuring other than a light level.
  3. A reflective light meter does not recognise what the actual luminance of something is.
  4. No reflective light meter knows what luminance the photographer wishes to render the subject as in the photo being made.
  5. All reflective light meters are calibrated to Middle Grey, as the following explains.

Exposure Meter Calibration – Middle Grey

“In most cases, an incident-light meter will cause a medium tone to be recorded as a medium tone, and a reflected-light meter will cause whatever is metered to be recorded as a medium tone.”[9]

So what is the ‘medium tone’? It is what some call ‘middle grey’… the same as the Kodak Grey Card. It is the calibration constant (K) for reflective light meters.[10]

If you point the camera at something which is ‘middle grey’ luminosity (whatever colour it is) and adjust the camera settings (shutter, aperture, and ISO) so that the meter indicator is on ‘0’, then the resulting photo will be the correct ‘lightness’.[11]

The photos in this column are made using an incident light meter with the camera settings applied as the light meter suggested. The photos in this column are made using the camera’s internal reflected light meter and the camera determining the camera settings without manual EV compensation.
 

ISO 64, 1/20th sec, f/8

ISO 64, 1/125th sec, f/5.6 This is underexposed.

   
ISO 64, 1/160th sec, f/8 ISO 64, 1/320th sec, f/9

In the following photo I have used the spot metering mode on the camera’s meter, and a Kodak Grey Card.

But what if this is not the ‘brightness’[12] of the subject? If you are photographing something mostly bright and you adjust the camera settings so that the meter shows ‘0’ (or if you use any auto exposure mode on the camera), the resulting photo will be under-exposed and grey toned, as is the case with this photo of the same white car (see also the illustration on the previous page)…

The same goes for photographing something mostly dark or black with a ‘0’ meter reading.  In the resulting ‘over-exposed’ photo the black will become grey toned. So the photographer must adjust the camera settings to make an exposure for the same tone / luminosity as that for the subject being metered.

Reflected Light and the Zone System

This is where the Zone System comes in.[13] There are two stages to using the Zone System:

  • Actual Luminance. How bright/dark are the different areas in the scene that you are photographing? To calculate that we use a reflective light meter to measure the brightness and interpret that light reading using the Zone System to determine the actual brightness or luminance of the subject or areas in the viewfinder.
  • Visualisation. How bright/dark do you want these areas to be in the photograph you are making? Having determined the actual brightness, the decision now is what brightness do you want this to be in your photo.

How does this work?

Look at the actual brightness of what is being metered, match it with the Zone Scale (see the following), and adjust the camera settings by the number of stops as indicated here…

That is easier said than done. All reflective light meters, in whatever ‘reading’ mode (centre weighted, multi-zone, spot metering, etc.) calculate the average light level for the area(s) metered and give a reading based on middle grey (Zone V). Some light metering systems also adjust this according to the reflective brightness of different colours present, bright highlights, etc.

To work the Zone System you need to measure the tones (levels of luminance in the scene) that are important to you in the final photograph, and determine how bright you want those tones to be in that photo, and adjust your camera exposure accordingly.

To do this accurately you need a spot meter and time. This is not necessary for most of us who use the camera’s inbuilt reflective light meter to give us a reflective light reading. With a little knowledge of how this light meter works (including the different metering modes), and the basics of the Zone System, the photographer looks at the luminance levels before them, interprets the light meter’s reading accordingly, and then manually sets the camera to make the photo at the ‘brightness’ they want.

Bright landscape +1
Subject with a bright background / strong backlighting. Bright subject  in most of the photo. +2 to +4
Dark object and/or dark background -1 to -2

 

   

 

The Dynamic Range[14] of the Camera’s Sensor

Changing the ISO on a digital camera changes the amount of noise. But it also has another significant effect. At a low ISO the sensor records a wider range of luminance than at high ISOs. This can be 6 or more extra stops of luminance range! So if you want to record the greatest range of tones and luminance levels in a high contrast scene, you have to shoot at a low ISO and factor this into your exposure settings. The following graph indicates that at its lowest ISO the Nikon D810 can record a range of almost 15 stops and at a high ISO only 8 stops or less.

This is one of the main advancements in digital camera technology… the ability to now capture a wide dynamic range at low ISO, matching and surpassing the 13 stop dynamic range of film.

The above graphs and information about dynamic range are true of the camera sensor itself, and what can be ‘extracted’ from the digital RAW file made by the camera when processing the image. It is another matter when it comes to seeing this on a computer screen (without processing the image to brighten shadows and darken highlights). The dynamic range of a computer screen and the LCD screen on a camera is less than 10 stops. The range for prints is less than 6 stops.

Recording a wide dynamic range of luminance is also only possible if you are shooting RAW and not JPEG. An 8 bit JPEG file can only reproduce a luminance range of 8 stops, but a 14 bit RAW file has 14 stops of luminance range.[15] So to get the maximum range of luminance recorded in the camera it is necessary to use a low ISO and to shoot RAW.

Dynamic range in acoustics (sound) is measured in decibels, and in photography it is measured in stops. The human ear can hear anything from a quiet murmur to the sound of the loudest heavy metal concert. That’s a difference of over 100 decibels. The human eye can see objects in starlight and in bright sunlight. That is a dynamic range of 30 stops in photography, the equivalent of a 90 decibel range in sound.  The actual dynamic range of luminance in a scene can be much greater again. What if the sun or moon is in the photo? The sun has a luminance of about 1,600,000,000 cd/m2, and the moon is 2,500 cd/m2 (EV 14).  So if shooting a moonlight scene, there is at least a dynamic range of luminance of about 17 stops… from detail in the landscape to seeing detail on the surface of the moon. The dynamic range of luminance for a daytime scene with the sun in the picture or sunlight reflected off a shiny surface is ‘astronomical’. You are just cannot record the sunspots on the surface of the sun and the darkest object on the earth under the shade of the same sun!  Typically the dynamic range of light reflected and emitted in a sunny daytime scene (that does not include the sun in the photo), that we can also see with the human eye, is about 20 stops.

There is an article ‘Why Dynamic Range is NOT Tonal Range’ by Peter Tellone,[16] which helps a lot in understanding the above. Firstly, let’s summarise the dynamic range of luminance that can be recorded and reproduced…

EVs / Stops
The average daytime sunny scene 20
High End Camera at low ISO 14
High Quality Computer Monitor / Most DSLR cameras 10
High End Camera at High ISO (6400) 9
All Cameras shooting JPEG and not RAW 8
Typical Photo Print 6.5

 

I quote from Peter Tellone’s article and use his illustrations in the following. “What happens when we shoot a High Dynamic Range Scene (20EV) with a typical camera. [see the following illustration] We can see that the camera still captures Black to White however it has truncated what was actually in the scene and naturally compresses the scene but not in a way that the full range of tones was captured in the camera or better put, it did not capture them as they actually appeared. If we look in this scenario, a highlight of 207 was recorded or “Mapped” as 255 white, anything above 207 was actually cut off or clipped. A shadow tone of 41 was recorded as 0 Black and anything below 41 was clipped. So yes the image has from black to white but did not record what actually was black and white in the scene and tones above or below a certain point.”

So the photographer always needs to make a choice when photographing a scene where the range of luminance exceeds the dynamic range that can be captured by the camera. This choice is to (1) expose for the light highlights and lose detail in the shadows (underexpose), or (2) expose for the dark shadows and lose detail on the highlights (overexpose).

 

The only method to capture the dynamic range of a 20 EV scene is to make three or more camera exposures (to ‘bracket’ the exposures) and then merge them using high dynamic range (HDR) software (e.g. Adobe Lightroom or Photoshop).

The result is a single digital file (Adobe Lightroom makes a RAW DNG file to retain bit depth) which has included all of the tones and detail in the full luminance range of 20 EV/Stops. This is compressed even further, without losing these tones and detail, when viewed on a computer monitor or in a print made from this digital file.

The Histogram

The histogram in digital photography is a graph showing how much information is recorded for all of the light tones between ‘0’ solid black (the left edge of the graph) and ‘256’ pure white (on the right edge of the graph) in a processed JPEG preview file.[17] As said above… the range of luminance recorded by the camera sensor (at low ISO), and the limitations of JPEG processing (only 8 stops of dynamic range), means that the histogram cannot show the full dynamic range of luminance recorded on the camera’s sensor in RAW.[18] Furthermore, the JPEG preview (like any JPEG picture file) has also been processed to adjust the tones, and so is only a rough guide as to what you have actually captured. This is why the image which shows on the LCD screen of the camera always looks much different than the RAW file imported onto a computer prior to processing.

 

What is the best Method for Determining Camera Exposure?

In this paper we have explained that there are two methods:

  1. Measuring the illuminance of light on the scene using either the Sunny 16 rule or using an incident light meter.
  2. Measuring the luminance of the light coming from the scene, using a reflective light meter.

Both methods give an EV reading which is converted into camera settings for that EV… shutter speed, aperture, and ISO. One is not better than the other per se, but one can be better than the other in different lighting situations. The important difference between the two is that… “In most cases, an incident-light meter will cause a medium tone to be recorded as a medium tone, and a reflected-light meter will cause whatever is metered to be recorded as a medium tone.”[19]

This means that if you use a reflective light meter (the meter in the camera), then you must understand the Zone System and the need to:

  1. interpret the light meter reading against the actual luminance of the subject metered
  2. determine the luminance that you want for the subject metered in the photo you are making.

This is very simple in most cases if you’re using the camera in its manual exposure mode, so that…

  1. the camera meter indicates how many stops over or under middle grey the area being metered is for the camera settings chosen.
  2. You can easily adjust one or more of the camera settings (shutter speed, aperture, ISO) to expose that area to the light level that you want in the photo being made.

If using the camera in any automatic exposure mode…

  1. The camera does not indicate over or under exposure for the area metered, because it automatically renders the area metered as middle grey (whatever averaging metering mode is used).
  2. Over-riding the automatic mode with a ‘manual’ EV adjustment is more inconvenient than making the adjustment with a shutter speed or aperture dial. Besides that, the manual EV adjustment in an auto exposure mode stays the same for subsequent shots until changed.

There are other significant reasons for shooting in manual exposure mode, using any of the metering methods described above:

  1. If the light level on the subject does not change, then all of the photos can be made without changing camera settings (ISO, shutter speed, aperture), and all of the photos will be the same ‘brightness’ as you have determined. When using auto exposure, even if the light level on the subject does not change, most of the photos that the camera makes will be made with different camera settings, so most will be a different brightness from each other (depending on what is included in the viewfinder).
  2. When shooting in manual exposure mode you have full control over each exposure, making each photo the brightness that you want.
  3. The exposure settings on the camera (ISO, Shutter Speed, Aperture) are not just about brightness. Each of these settings has a creative effect which is very important in photography: shutter speed for movement, aperture for depth of field, and ISO for dynamic range of brightness and noise control. The photographer loses control of one or more of these settings if shooting in an automatic exposure mode.

The serious photographer who with skill and creative vision ‘makes’ a photograph, uses the camera as a tool… in the same way the paint brush is to a painter, an oven is to a chef, and a piano is to a pianist. My purpose in this paper is to understand the camera as a tool, and to help liberate our creative vision by fully utilising what this tool can do in the hands of a photographer/artist.

This may be offensive to the ‘point and shoot’ photographer using automatic exposure modes, because the insinuation could be that you are lesser photographer. But that is not so. Photographers make great photos even in automatic exposure modes and with a bit of luck thrown in. We must avoid snobbery in regard to the cameras we use, and the way we use them. At the end of the day everything comes down to the photo itself, not how you got there! You have nevertheless compromised your vision and picture making abilities by giving control or partial control to the camera which does not know what it is looking at and does not share your creative vision when it ‘takes’ a photo.

Even if you choose to shoot in automatic exposure modes, I hope this paper explains a bit more about how cameras work, and why sometimes your photos are well exposed (as you want) and why sometimes they are not.

 


End Notes

[1] Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed. Brightness refers to an absolute term and should not be confused with ‘Lightness’. See https://en.wikipedia.org/wiki/Brightness

[2] The following is from the Konica Minolta website: http://sensing.konicaminolta.us/2015/08/luminance-vs-illuminance/

[3] There is a one stop difference between these shutter opening durations: 1/500 sec, 1/250 sec, 1/125 sec, 1/60 sec, 1/30 sec, 1/15 sec, 1/8 sec, ¼ sec, ½ sec, 1 second.

[4] There is a one stop difference between these ‘f’ numbers for aperture: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.

[5] There is a one stop difference between these ISO numbers: 50, 100, 200, 400, 800, 1600, 3200, 6400.

[6] There is often confusion about what is thought to be a difference been Exposure Values (EV) and Light Values (LV) when in fact both terms are synonymous and refer to the ‘light level’. The LV and EV numbers are the same for identical lighting situations. The confusion arises from many charts which are based on an ISO of 100 only (unlike the following chart), when ISO along with shutter speed and aperture are all exposure variables. Defining zero EV as f/1.0 at one second makes no sense at all if you don’t also say that this is for ISO 100. So in the chart the settings for f/2.8 (three stops ‘darker than f/1) and 1 second at ISO 800 (three stops ‘brighter than ISO 100) results in the same exposure brightness for 0 EV when the settings are f/1, 1 second, and ISO 100.

[7] http://www.jollinger.com/photo/meters/other/invention.html

[8] A modern handheld incident light meter, like the Sekonic L-858, can measure light from as low as -5 EV to high as 24 EV. This is beyond what camera light meters can measure.

[9] https://en.wikipedia.org/wiki/Light_meter

[10] ISO 2720:1974 recommends a range for K of 10.6 to 13.4 with luminance in cd/m². Two values for K are in common use: 12.5 (Canon, Nikon, and Sekonic) and 14 (Minolta, Kenko, and Pentax); the difference between the two values is approximately 1/6 EV.

[11] This assumes that you are shooting in manual exposure mode, where the camera meter shows this scale.

[12] I’m going to sometimes use the word ‘brightness’ instead of luminosity. The differences are explained earlier in this paper.

[13] The Zone System was developed by Ansel Adams and Fred Archer in the late 1930s. To this day it remains fundamental to understanding exposure throughout the whole photographic process from the camera to the final print or processed image. The primary idea behind the Zone system is to take control of mapping the dynamic range of the scene onto the dynamic range of the sensor, and then mapping the dynamic range of that onto the print. The secondary idea of the Zone system is to place various tonal ranges into various portions of the sensor’s range, with the intention of placing them in various portions of the print’s range. So you might take something that appears grey in life, and make it white in the print — because that’s what you want it to look like. It’s not about reproducing a scene accurately, but rather about control of the process.

[14] https://en.wikipedia.org/wiki/Dynamic_range

[15] http://www.cambridgeincolour.com/tutorials/dynamic-range.htm

[16] The website ‘The HDR Image: HDR and Advanced Photography Learning’ http://thehdrimage.com/why-dynamic-range-is-not-tonal-range/  The article is published 2 August 2016.

[17] The Magic Lantern firmware for Canon cameras does however allow you to see a histogram for the RAW file.

[18]  With many high end DSLR cameras you can ‘recover’ from the RAW file up to 2 stops of blown out highlights which are clipped according to JEPG histogram view.

[19] https://en.wikipedia.org/wiki/Light_meter

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