Part 1: The Physics of Light
Section A: Propagation of Waves
Section B: Color Signature Frequencies
Section C: Combining Energy Signatures for Exposure, Kelvin Temperature Scale
Part 2: Dynamic Range, 18% Grey Card, EV (Exposure Value)
Part 3: Synchronization, The Light Meter
Part 4: Metering for Exposure
Section A: Direct Light and Reflected Light, Ambient Light and Added Light
Section B: Light Intensity affected by Light Modifiers
Part 5: Setting Exposure Variables
Part 6: Exposure Evaluation using Histograms:
Appended as a separate section for more space and thoroughness.
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Life is a series of moments that leaves us with memories, both splintered and whole.
If you are just joining us, the PRELUDE & SYLLABUS section is the logical starting point for the series.
Welcome to Digital Photography #101
by Virtual Studio Photography (VSPHO)
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While developing course material for the upcoming Digital Darkroom sections, I realized that I missed the mark in the original EXPOSURE section. My apologizes, I hope this will make up for it.
I would also include that as I am writing this section, I am truly humbled by the sheer genius of all the inventor/physicists in the past three hundred plus years that have contributed to our understanding of physics.
Photographic Exposure is the manipulation of light. I believe that understanding the basic principles of light makes manipulating the exposure a bit more fun.
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Part 1: The Physics of Light
Section A: Propagation of Waves
We will expand on some of the analogies we used in earlier sections.
Light is ElectroMagnetic Radiation through the Visible Light Spectrum.
Water, sound and light share some common traits as they all travel in waves. Water travels at the slowest wave speed, like ocean waves crashing against the shore. Sound waves travel at approximately 768 MPH. Light waves travel at around 700 million miles an hour, varying only slightly in the medium through which they travel.
Where they start to differ is that sound waves are a cluster of air molecules that form a wave of various frequencies defining tones. In music and speech, there are several sound waves of various frequencies, but the wave itself defines the amplitude or volume, and the frequency or tone we hear. Sound waves are pulsating in a constant forward/backward motion creating a high/low air pressure area that our inner-ear captures as vibrations for our brain to interpret. (Trivia: There is no sound in a vacuum. Or like a scary movie tag line; “In space, no one can hear you scream.”)
Conversely, with light, each individual photon has a unique frequency that is independent within the light wave. So each light wave carries a varying quantity of photons, and each photon carries a unique color frequency signature. This is known as Wave-Particle Duality.
Light travels in waves because light waves are propagated from bursts of high energy heat that catapults photon clusters in sets of waves. As we shall see, the total amount of photons in a wave, combined with averaging each photon’s energy (color signature), defines the overall hue (color) of that light wave.
Trivia: Light waves spread out after their propagation, making them look weaker the farther they travel (the total wave energy is not weaker, just spread out). You may have heard of “The Inverse Square Law.” Simply put, it means there are only 1/4 the photons available for exposure when you move from one position to twice the distance from the light source.
Example: When using a flash, you will only have 1/4 of the light intensity on your subject at 20 feet than at 10 feet (more in lighting).
With light waves that carry photons with a proportionally equal amount of frequencies across the light spectrum, the light appears to be white.
Conversely, there is often an imbalance of several different frequencies of photons within a light wave, the blending of all the frequencies decoded by our brain produces a potential of about ten million colors we can see.
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Part 1: (continued)
Section B: Color Signature Frequencies
Let’s clarify what me mean by frequencies. Sound and light are measured in Hertz (1 Hertz = one cycle per second, as in a speaker surface moving forward, then backward and again returning to center in one second.) Quality sound systems strive to reproduce the sound spectrum of 20 hertz to 20,000 hertz (20-20kHz). Deep bass is mostly felt by our body, and frequencies above 14kHz ~ 16kHz are mostly a presence of clarity outside of our normal hearing range.
With a single photon, imagine a tiny ping-pong ball moving back and forth, from side to side (or up and down), vibrating very quickly in their perpetual cycle, but traveling (as a group) at the speed of light. Therefore, unlike sound, each photon carries its own unique color signature through vibration.
Note: After photons are reflected from a surface, it is common for photons to continue traveling in smaller groups of waves, or even individually. This scattering of photons is caused by diffusion as explained in Part 4: Metering for Exposure.
This does ask the question:
If photons carry their own energy signature through frequency, what determines brightness and how often do these waves propagate?
Brightness is determined by combining and averaging, both the quantity and energy level (frequency) of each individual photon in a light wave. The more photons in a wave (with the cumulative total energy of individual photon frequencies), the higher the light intensity, also called luminosity or brightness. Note: Blue light (Ultra Violet is hottest) has more energy than Red light (more below).
If a light wave carries a shipment of mostly Hot Blue Photons and the quantity is relatively large, the intensity/brightness of that wave will be a bright Blue light as in our blue sky example, BLUE SKY.jpg (Once open, hit CNTL + or – to control image size).
Conversely, a relatively small quantity of Blue Photons would produce a faint Blue light.
Wave propagation is determined by the energy source. Each photon having a unique heat signature through its frequency has no affect on the frequentness of wave propagation. The most common example of wave propagation frequentness of manufactured light is the incandescent light bulb. The energy source determines the frequentness of energy (electrical) pulses. The American standard for household electricity is a 60 Hz cycle (60 pulses per second, we’re not talking voltage). Therefore, the common light bulb pulsates light and propagates a light wave 60 times per second.
Trivia: A single light wave can produce a bright flash of light; example, a camera’s strobe. Tests have shown that the human eye can detect as few as five photons (some people even fewer) in a controlled environment (pitch black room). Traditional motion pictures are shot at 24 frames per second. The light from a film projector is from a 60Hz Arc Lamp, so there are only 2.5 light waves generated for every frame (60Hz/24 frames/sec). Even though a light wave can transport a very high number of photons, it only takes a few light waves to form an image that the human mind can comprehend.
The most common source of light is, of course, from the Sun. Sun light is from a process called Nuclear Fusion which produces a very consistent light color. Sun light is also a perpetual flow of light as the Fusion process continuously overlaps wave propagation. (More on Sun light below.)
In contrast, man first harvested light through fire. Fire is the result of a chemical reaction called rapid oxidation. Light propagation from combustion is fairly rhythmic, but not synchronized like a light bulb. When enough heat energy has built up, a spontaneous release of the energy is released creating photons (that’s why candles flicker).
Photons carry a heat/energy signature (Color is the speed of their vibration) measured as a frequency in the visual light spectrum. Our vision starts at 430 trillion Hertz, that is the top end of Infra Red, to the upper visual range of 750 trillion Hertz which is the beginning of Ultra Violet; just past Blue. (Total visible light spectrum: 430 THz ~ 750 THz; Terahertz).
The faster these tiny ping-pong balls (photons) quiver back and forth, the less time they have to complete one cycle. Therefore, faster frequencies travel a shorter distance for each cycle. Note: Photons have no mass that’s detectable, but the faster they quiver/vibrate, the more energy they carry.
Slower color cycles (upper Infra Red, 430 THz) travel approximately 700 Billionths of a Meter (700 nm, nanometers) back and forth per cycle.
Faster color cycles (750 THz, lower UV range) travel a proportionally shorter distance of 400 Billionths of a Meter (400 nm, nanometers) per cycle.
To reiterate, higher frequency (shorter wave length) photons carry more energy, meaning they are hotter. That is why we can easily burn our eyes and our skin with too much exposure to UV rays. (More on heat below.)
This steps us right into the combined energy levels of photon frequency/color signatures.
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Part 1: (continued)
Section C: Combining Energy Signatures for Exposure, Kelvin Temperature Scale
The light source determines both the frequentness of wave propagation, and the total energy the light wave will carry.
When we are metering with our camera’s light sensor, we are evaluating the combined total energy within the light wave (actually, successive light waves) for the Exposure. Once we’ve set the proper Exposure settings for the light intensity level, we can then expose our digital sensor which filters for the individual heat signatures of each individual photon (RGB Color Filters).
We previously covered in ISO Sensitivity that color film only has a quantum efficiency of about 2%, a 98% loss of exposure efficiency because of the color filtering. But digital sensors currently have a quantum efficiency of about 70%, only a 30% loss of information.
Trivia: “Red Hot” is man’s metaphor for “burn the skin off your fingers hot” from as far back as the Iron Age (e.g. red hot iron). “Cool Blue” associated with refraction (the light prism effect) of blue light that ice and our atmosphere (blue sky) give off. The reality is the opposite, Blue photons are hotter and carry much more energy than Red photons.
We measure the color of the heat source with a heat scale called Kelvin. Kelvin is unlike Celsius and Fahrenheit which are calibrated for practical human usage (zero degrees calibrated at where pure-water and salt-water freeze, respectively). Kelvin is calibrated for use in physics with zero starting at absolute zero as black, the temperature and color of space.
So don’t bother trying to correlate any logical reference to temperature for color as we know it. Just know that White Balance is based off the Kelvin Scale.
To clarify, White Balance is evaluating the overall hue (tonal value) of our light source, using the Kelvin Scale. Our visible color spectrum goes from Infra Red, to Orange, to Yellow, to Green, to Cyan, to Blue, to Ultra Violet. The Kelvin Scale goes from 1,700 degrees for Infra Red to 10,000 degrees for Ultra Violet. This is a measurement of color against black, not any actual physical temperatures, just color temperature.
As an example, the color temperature of the sun on a clear day is around 5500 to 6000 degrees Kelvin. But on a cloudy day, the Kelvin temperature of light jumps to almost 10,000 degrees Kelvin because mostly higher light frequencies pass through the clouds.
Our eyes see clearest in middle temperature light. Most camera flash/strobe lights are color balanced to 5500 to 6000 degrees Kelvin for this reason.
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Part 2: DynamicRange, 18% Grey Card, EV (Exposure Value)
Now that we understand the color of light, we will define the actual variations in light intensity called the DynamicRange.
DynamicRange is the measurement of light intensity measured as the disparity in the quantity of photons from the darkest to lightest parts in our image.
In the 1950’s, Germany established a standard for Film Sensitivity correlated to Aperture and Shutter Speed needed for proper Exposure. This standard was called the Exposure Value (EV) table. We will be discussing the EV (Exposure Value) in Part 3: Synchronization and Part 4: Metering for Exposure.
Also in the 1950s, Eastman-Kodak figured out an ingenious system to meter the total light needed for the EV (Exposure Value) that would also distribute the light proportionally (dark to light) for a perfect exposure. As it obviously takes much more light to expose lighter images in a picture, much less light is needed for the darker areas. This system that correlates the EV table to the balanced exposure of film is called the 18% Grey Card.
It is a dark grey card that allows only 18% of light to reflect off of it. Film Photographers used the 18% grey card to calibrate their exposure settings. You simply take a light meter reading from the surface of the card. It shows you the exposure settings of only 18% of the total light reflected from the card. This comes pretty close to the middle of the DynamicRange for film and digital.
Note: When you meter on a spot, you are assigning 1/2 of the total DynamicRange to the lighter half of the exposure, and 1/2 of the total DynamicRange to the darker half from that metered spot.
However, for digital imaging, Digital Sensors seem to work better closer to a darker, 12% grey card. That puts the middle of the DynamicRange a little lower at 79 grey scale bits (there are 256 bits total in the JPG grey scale standard).
The 12% grey scale as the mid-point correlates to approximately 88% of the total light reflected into our camera’s sensor being dedicated to the top half of the total DynamicRange. That also means that only 12% of the light is used to expose the bottom half of the DynamicRange. Mathematically, this works out perfectly for digital data distribution as explained in Part 6: Exposure Evaluation using Histograms.
Just like our sound analogy, sound uses double the vibrations per each higher octave. Likewise, it takes proportionally many more photons to expose brighter areas than darker areas of an image. Logically, it takes zero photons to expose total black.
Although some photographers still utilize the 12% ~ 18% grey card, they are no longer necessary with digital imaging. The end of this section explores the advanced ability for Evaluating Exposure with Histograms. As we will soon see, we can evaluate light distribution immediately after every shot and adjust for extreme lighting conditions, faster and with more precision with the Histogram.
Note: The grey card is used for metering before the shot is taken. It is useful, but more hassle than just shooting a quick test shot and looking at the Histogram. Plus, the Histogram gives you instantaneous results after every shot.
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Part 3: Synchronization, The Light Meter
As we outlined previously in ISO Sensitivity, and above with the EV (Exposure Value), we need a given quantity of light to fully expose our digital sensor. The Exposure depends on synchronizing three variables:
1: Aperture is like the kitchen faucet’s water pressure. The more you open it, the faster it fills the glass of water.
2: Shutter Speed is the equivalent to how long you leave the faucet open to fill the glass of water.
3: ISO Sensitivity is the equivalent to how big the glass is, how much water it takes to fill the glass. A faster ISO is like a smaller glass of water, it’s faster to fill, but allows less light to evaluate for image and color accuracy.
The actual quantity/intensity of light needed for our Exposure is already programmed into our Digital Camera as the EV (Exposure Value) reading of our light meter.
So with respect to Synchronization, we are simply learning to let the camera’s light meter guide us to the proper Exposure through synchronizing all three variables listed above.
Now it’s time to experiment with the built in light meter on your digital camera.
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Part 4: Metering for Exposure
Section A: Direct Light and Reflected Light,
Ambient Light and Added Light
Metering for Exposure simply means following the indicator on our light meter. On any camera, the Light Meter is intuitive to read.
If it indicates a positive amount, it means Over-Exposure; maybe a little, maybe a lot, the meter tells you exactly how much.
If it indicates a negative amount, it means you are Under-Exposing and the image will be dark.
“Metering” means the EV value, evaluating the total amount of light available, not the Dynamic Range.
Here is where the 12% ~ 18% grey card or the Histogram is useful and necessary.
In any metering mode, on a bright image with a very LowDynamicRange, the camera still averages the middle of the image’s DynamicRange. However, if there is only bright light (no dark areas), it will set the exposure based on the EV scale to the middle of the image’s DynamicRange. That’s why either the grey card or the Histogram are necessary.
Example:
This was shot with a fully automatic exposure. The exposure was automatically balanced in respect to the image’s DynamicRange. The irony is that this specific DynamicRange goes from the middle of the grey scale to the top of the grey scale, there are no dark areas for balancing. Therefore, Automatic Exposure Mode adjusted the middle of the DynamicRange, 2.5 Stops darker than reality.
CLOUDS (Once open, hit CNTL + or – to control image size)
Conversely, this example is primarily dark, so it was shot in Manual Mode to avoid the Auto Exposure pitfalls with unusual lighting.
PYRAMID (Once open, hit CNTL + or – to control image size)
Experiment with reading your light meter. (That’s the fun of owning a cool camera.)
If you are using your meter as a spot meter, then the 12% grey card mentality is utilized. That is to say, if you don’t want to carry a grey card all the time, simply use the spot meter on a dark part of the image as it will at least put you in the proper exposure range. (We will be using Histograms to fine tune exposure in Section 6.)
SWIMMER (Once open, hit CNTL + or – to control image size)
Your camera also has other automatic exposure modes. If you use a full image evaluation mode, it will take samples of light intensity from the entire image and set the exposure as the best estimate (usually pretty good). Again, the Histogram is the best evaluation tool after a picture is taken for adjustments to the next picture, if needed.
Likewise, there are often varying considerations when metering. That is, if the DynamicRange is very wide, you either need to offset the difference with flash or decide what is important in the shot. Sometimes the WideDynamicRange actually creates opportunities for silhouetting through posterization.
CITY (Once open, hit CNTL + or – to control image size)
When metering light, we are metering direct light or reflected light.
Reflected light has three basic categories of surfaces:
1. A smooth specular surface such as a mirror or standing water. This allows close to a 100% light reflection efficiency allowing light waves to continue travel in close to their original wave pattern.
2. A matte (not smooth) surface which diffuses reflected light evenly. Photographic paper commonly utilizes a matte surface to give color prints a smoother effect, easier on the eyes. Skin and faces are in this category.
3. A rough surface where light is totally scattered or absorbed (like a tree). Light is diffused rapidly and light intensity decreases in proportion.
Reflected light that comes from a natural source like the sun or moon, or existing inside room lighting is called “Ambient Light.” Ambient light is any light that is already present when we are metering for a picture. If we need to balance the light on our subject, then we would use “Added Light” such as a strobe/flash or equivalent lighting schema.
So to be clear;
There are two types of light:
1: Reflected or
2: Direct
Reflected or Direct light can be either:
1: Ambient or
2: Added
“Ambient light” is existing light, Reflected or Direct.
“Added light” is also Reflected or Direct light that is “Added” to balance the exposure.
Example: Sun Reflectors (like at the beach) are often used as “added light” instead of Direct flash/strobe lighting.
We will discuss offsetting overbearing background ambient light using reflectors or flash/strobe in the lighting section.
Previous example:
GROUP (Once open, hit CNTL + or – to control image size)
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Part 4: (Continued)
Section B: Light Intensity affected by Light Modifiers
We know from ISO Sensitivity that the available amount of ambient/reflected light from direct sun light is abundant. But clouds, shade, etc… act as a light modifier and reduces the amount of ambient/reflected light for exposure. This is often welcome and gives a smoother exposure with portraits.
So the light may be bright or dim to the human eye, but may vary in intensity for the camera’s sensor chip.
We may also vary light intensity by using a polarization filter or even a neutral density filter that modifies light and limits the intensity to the sensor (filters in a future section). We can use our ISO setting to contribute or offset these factors if necessary.
But again, higher ISO sensitivity and less light means less information for image accuracy. (Demonstrated in ISO CALIBRATION)
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Part 5: Setting Exposure Variables
Point-and-shoot to DSLR cameras can evaluate and automatically set your exposure. But to shoot truly amazing photographs, you need to understand how your camera actually works to manipulate the metering for the desired exposure.
Let’s recap the three variables for Exposure:
APERTURE regulates the diameter of the shutter’s opening. This impacts the Depth of Field.
Previous example:
INNOCENCE (Once open, hit CNTL + or – to control image size)
SHUTTER SPEED regulates the duration the shutter is open. This controls the sense of motion with blurring, or freezes motion.
Motion Panning:
PANNING (Once open, hit CNTL + or – to control image size)
Freeze Motion:
FREEZE (Once open, hit CNTL + or – to control image size)
This image is the epitome of a disastrously slow Shutter Speed (but funny).
Previous example:
ISO Sensitivity controls how fast the sensor can absorb light. That is to say, it controls the amount of light it will use for evaluation. Faster ISO means less light to evaluate, therefore, more noise and less tonal accuracy. Previous example: ISO CALIBRATION
Let’s start with ISO Sensitivity when Setting Exposure Variables as film had only one ISO once you loaded the film.
We will use our knowledge of ISO CALIBRATION to determine our ISO setting.
Example #1: Shutter Priority Mode
Let’s say we are going to a sporting event where we need a fast Shutter Speed AND a good Depth of Field. We have determined that we can go to ISO 800 and still have very clean images for normal sized prints. Hey, if your camera is clean at 1600, 3200 or 6400+ ISO, take advantage of it.
Likewise, the ISO Setting can be adjusted between every shot if desired, or some DSLR cameras allow for automatic ISO changes (see your manual).
At our hypothetical sporting event, it would be best to use the Shutter Priority Mode. Shutter Priority Mode will automatically change the Aperture. Depending on the lens focal length, make sure to keep the Shutter Speed at least Twice the Reciprocal of the Focal Length (previously outlined ) to control or stop motion blur. That is why we set the ISO as high as cleanly possible, to keep the Depth of Field (Aperture) deep enough to be useful. Note: If you want a shallower Depth of Field, (while in Shutter Priority Mode) simply slow the ISO in proportion. Each ISO slower equals ONE aperture larger (smaller number).
Example #2: Aperture Priority Mode
This time, let’s say we are shooting at a friend’s wedding, nothing is moving too fast (we hope).
Aperture Priority Mode will allow you to set the Aperture, then the camera will set the Shutter Speed accordingly. I will say that f-stop 8 (f-stop 11 is better if you have the lighting) will keep most of the action in focus. However, unless you are in an environment with ample light, be careful your Shutter Speed does not go too slow (Reciprocal of the Focal Length rule). We will also hope that you are allowed to use a flash to add lighting.
Example #3: Fully Automatic Mode
Automatic Mode is great if you are not sure what to concentrate on (speed or depth of field). However, it averages the Shutter Speed and the Aperture with the amount of light available. Therefore, you may get a Shutter Speed too slow with more Depth of Field than needed (or vice versa).
Example #4: Manual Mode
Manual Mode allows for the photographer to adjust the Aperture and Shutter Speed based off of the Light Meter and the desired light manipulation. To practice with Manual Mode, adjust your Shutter Speed against your Aperture while watching your light meter go to center. Change one parameter (Shutter or Aperture), then change the other parameter until the meter returns to center.
Note: While practicing with manually setting your exposure variables, start with your EV, Light Meter Mode set to a full image Matrix Mode (Nikon) or Evaluative Metering (Canon) until you get the hang of the 12% ~ 18% grey card mentality with the Spot Meter Mode.
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Please join us in the next section:
Thank you again,
Virtual Studio Photography (VSPHO)