Modern Photography/Exposure controls

Photography is the act of writing with light. A light-sensitive surface is exposed to light coming from the scene through the camera lens. As modern methods allow for this to be done in ever-smaller fractions of a second or over multiple days, we must control exposure—how much light enters the camera.

In order to get a perfectly-exposed image (one that has all of the desired levels of gray from pure dark to pure white) a photographer must balance and coordinate three settings—aperture, shutter speed, and sensitivity—against the scene to be shot.

Level, tone, value
These terms are used interchangeably to describe how light or dark a region of a picture may be. For printed images, the lightest tone available is usually that of the bare paper on which the print is made, and less often a specific white pigment used in some printing methods; the darkest tone results from being fully saturated with black ink/pigment/dye/toner. For images viewed on a screen, the lightest tone occurs at full illumination, and the darkest tone with emitted light being completely obscured, or no light being emitted.

We aim to keep all visible details within the range of tones we can reproduce in print or screen—dynamic range. This range is *much* smaller than the range of tones we encounter in the real world—from full sunlight to complete darkness. Our eyes adjust to changing lighting conditions, instantly and involuntarily. Meanwhile, our cameras must measure more carefully, and determine what exposure settings to apply in response. However, cameras can still operate in conditions that humans would find blinding or too dark, and images can be processed in a way that exceeds the dynamic range normally available.

Contrast
Contrast is the overall difference between light and dark areas. Human vision is more sensitive to changes in brightness than changes in color. So it is that contrast in an image provides clarity through high-contrast edges, and impact through contrast between regions of an image. Images with low contrast can appear dull and uninteresting, or may even be difficult to comprehend. Extreme contrast—where most of the tones are pure white or black—is generally avoided for many images, but can prove very striking, and is also the primary aim of written text.

Key
Key is the predominance of tones in a scene, with "high key" referring to bright tones, and "low key" for dim tones. By necessity, a shot taken in broad daylight will often be high-key, while a shot taken in moonlight will be low-key. The subject may yet be properly exposed with a full range of tone, even though the remainder of the scene is crushed or muddied.

High key can lend a feeling of cleanliness (such as for product photos or architecture) or discomforting sterility (plain white walls of an institution). Low key can emphasize intimacy (a figure in a darkened bedroom) or terror (a forest at night, creatures hiding in shadows). Both can also evoke feelings of isolation or loneliness from a lack of visible surroundings.

This is a different concept from a "key light", the primary light used to illuminate a subject. "High key" here instead means a key light placed above the subject, and "low key" for light from below.

Stops
A stop is a difference of double a lower value, and one-half a higher value. Stops are used because illumination and vision are dependent on this geometric scale. The perceived difference in light between one and two lamps appears greater than the difference between eleven and twelve of the same, but would appear the same as the difference between one hundred and two hundred.

Stops are often used when speaking of aperture, as the term originates from the literal stops that secure the aperture ring along its adjustable range. However, stops can also describe the differences between shutter speed and sensitivity values all the same. Camera controls are typically available in thirds of a stop.

Exposure value
Exposure value is an alternate system of quantifying a scene using a logarithmic scale. Instead of large numbers and fractions, exposure values are given in whole numbers that grow or decrease by one with each stop. This can make calculations easier as one needs only add or subtract, instead of the multiplication or division needed for dealing with stops on a linear scale.

Overexposure and underexposure
Overexposure occurs when too much light is captured for the chosen exposure settings, and underexposure when too little. These are often undesirable when contrast is severely reduced.

Minor exposure problems can be corrected during processing, especially with standard negative film as it is exposed a second time to produce a print. Still, the lowered dynamic range from the original shot may result in a lower quality image.

A digital image processor may simply render over- and underexposed areas as pure white or black—clipping to the maximum or minimum available values. Detail cannot be recovered from clipped areas. Because the sensor is often a permanent part of the camera, it may be damaged by extreme overexposure. Care should be taken to avoid shooting direct sunlight or bright sources of light such as encountered with welding or lasers.

Shooter controls
Before the camera even comes into play, the shooter has an important role in deciding what will be shot in the first place, and adjusting how much light is in the scene.

Composition
Natural lighting comes from the sun, which casts very strong light in one direction. Natural lighting also comes from materials illuminated by the sun. This includes the sky, terrain, buildings, and large objects like trees. Light reflected off of these sources is generally diffuse and comes from multiple directions.

Weather and time of day play major roles in using natural lighting. It is not uncommon for shoots to be planned for a specific time of day or year, or to be cancelled due to undesirable conditions.

Artificial lighting is created by humans for visibility and public safety. This is much less intense than natural lighting. It may even appear in the shooting scene without severely disrupting exposure.

Indoor scenes can be challenging to expose properly because the light level is much lower than outdoors, and daylight from outside can easily overpower. Without adjustment, shots taken indoors may require such long shutter speeds or high sensitivity that the resulting image is undesirable.

Staging
The lighting within a scene may be manipulated by the shooter with various tools. Some shots may require use of a studio, a space where the photographer may more easily control shooting conditions.

Devices such as on-camera flash, studio lamps, and strobes provide additional light for the photographer. Lighting can also be redirected from elsewhere by using reflecting screens. Care should be taken to avoid including these devices in the shot.

Light can be removed from a scene by the use of flags and panels to block light, by closing doors and curtains while indoors, and other obstructions. Screens and scrims may be applied to lighting to reduce their effect on the scene, or applied outside windows to allow outdoor objects to be exposed along with the indoors.

Properties of light such as diffusion and color can be altered with various materials.

Camera controls
Three primary exposure controls are located on the camera. Aperture is controlled by the iris diaphragm inside the camera lens. Shutter speed is controlled by the shutter curtain inside the camera body. Sensitivity is a property of the speed of film chosen, or the digital image sensor used.

Aperture
The aperture is a hole in the camera lens through which light is transmitted from outside the camera. Changing the size of the aperture allows more or less light through as needed. The aperture size also has an effect on depth of field—how much of the scene before and behind the focal plane is in focus. There may also be minor side effects depending on the quality of the lens.

<!-- F stop! I am often amazed at how many “professionals” do not really understand what an f stop is or does. Here it comes, plain and simple. Memorize this and you will be way ahead of the game. When you push the shutter release on an SLR a mirror inside your camera body flips up and the shutter exposes the film to whatever is showing through the lens. Behind the lens is an adjustable opening called the aperture, through which the light passes on its way to the film. The F stop is a measure of the size of this opening. The size of the hole through the lens, f stop, does not change what is in the film frame; it only controls how much light gets through. Full f stops in order from LARGEST HOLE to SMALLEST HOLE are (scale): f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. The smaller the number, the bigger the hole. Each successive full f stop allows in half as much light as the next. Example f/4 allows twice as much light through as f/5.6 and f/5.6 allows twice as much light as f/8.

Now, the aperture's diameter is not the same from lens to lens. A lens' aperture is a ratio between an aperture's diameter and that lens' focal length. A 50mm lens' aperture f/2 has a diameter of 25mm. A 100mm lens' f/2 has a diameter of 50mm. But both of these lenses at the same aperture setting allow the same amount of light to reach the film because each aperture's diameter is exactly half of that lens' focal length, 1/2. f/2 is an abbreviation for the formula to find an aperture's diameter where f = focal length of the lens. f/4 for 210mm lens. 210/4=53mm aperture diameter. The real question is: why do we care about f stops? Why not just have a great big hole and let as much light fall onto the film as possible. Because a larger aperture gives a smaller depth of field (explained below). The Aperture Control is a ring located on the circumference of the lens. Some lenses have half-stops as well as the full stops given above.

Example
Uncle Ed is sitting on a bench in the park. Behind him is a yucky old warehouse. We do not want the to capture the detail of the warehouse, so we will use an f 2, big hole, and its corresponding shutter speed for the light conditions. We hit the shutter release button and we have Uncle Ed crisply on film with a warehouse behind him that is soft or very soft and lacking in detail, basically a blur that shows the light and dark areas only and all blurry. Clear as mud? OK, now we are at the beach with a fabulously lovely young woman. Let’s call her Marriah. We want her and the wonderful cliffs a half mile behind her all crisp and in focus. We focus on Marriah, Goddess, and use a “long” f stop like f 22. We then dial in the necessary shutter speed for proper exposure and take the image. Marriah is captured in sharp focus in all her glory as are cliffs way behind her.

Depth of field
Depth of field is the distance, front and back, that is the in focus on the film from whatever we are “hard” focused on. A small f stop (big hole) requires a faster shutter speed so the film does not have as much time to record detail (focus) on object in front of or behind the subject we are focused on. A large f stop (small hole, big number like f 22) allows the light through more slowly so we have to use a longer shutter speed to get the right exposure. Depth of field for each f stop is often shown on as a small bars with numbers on the f stop ring of our lens. This is best learned by looking at a camera lens that has these calibration marks. It will show that if we are focused on something that is 20 feet away for example at f 5.6 then everything between 15 and 25 feet is within the depth of field for that f stop. This is hard to visualize unless you are looking a high quality lens that has the markings.

Now, lets assume that we had enough light for the previous 2 examples that we were able to use a shutter speed of 1/60th of a second or faster (1/125th, 1/250th …) so that we did not have to use a tripod. But how do we know?

A very simple way to set depth of field is to focus on your subject, then set your "f-stop" to the number that best represents how much of the foreground and background you would like in focus... i.e.: Say you are shooting a photo of a child and you want very little of the background in focus, you would firstly focus on the child's eyes or at least his/her face. Next, choose an "f-stop" of the lowest available to the lens you are using. Think of the "f-stop" number as the number of meters in front of and behind the subject you would like in focus. The higher the number, the more in focus.... f2.8 would then relate to about 2.8 meters in front of and behind the subject in focus, f22 would mean 22 meters in front of and behind the subject in focus.

Obviously these meterages are not accurate, not even close but it gives you the train of thought for using f-stops accurately....I have used this method to teach people to think of f-stops with great results. -->

Shutter speed
Shutter speed is the length of time for which the shutter is released, permitting light from the lens to reach the sensor. It is measured in seconds, or fractions thereof. Shutter speed has a direct effect on motion blur captured in the image, whether through the motion of subjects or of the camera itself.

Though normally controlled by a timer, long shutter speeds may be achieved through a "bulb" setting. With long shutter speeds, it may be necessary to use neutral density filters to prevent overexposure during the day. The camera should also be stabilized with a tripod or setting it on a sturdy surface. Very long shots, such as with astrophotography, may even require a tracking mount that counteracts the rotation of the Earth.

Sensitivity
Sensitivity or speed—often given as an "ISO number"—is the rate at which the film or sensor is able to capture or react to light. By changing the sensitivity, the same amount of light can produce different tones in the captured image. Each successive grade (double the number) requires half as much light to be captured for the same resulting tone.

Film
Film manufacturers achieve varying degrees of sensitivity by altering the grain size of the light-sensitive crystals embedded in the film. Slower films will have smaller grain than faster films. The grain size affects how the film captures fine detail, and how smoothly it can reproduce gradual changes of color.

Film speeds are generally obtainable from ISO 100 (slow) to ISO 3200 (really fast). In order for the camera to properly calculate exposure, the matching film speed should be set on the camera body's sensitivity control.

Digital
For digital sensors, sensitivity has an indirect relation with image noise—unwanted variations in color. Higher sensitivity (corresponding with a higher ISO number) allows for quicker exposure but risks increased noise; lower sensitivity requires more time but reduces noise captured. Digital cameras may provide a wider range of sensitivity, from ISO 50 to ISO 12800.

Changing the camera body's sensitivity control changes how the image processor translates the sensor readings, simulating a change in sensitivity. Depending on the quality of the image processor, there may be subtle changes in contrast and color rendition as sensitivity is adjusted.

Metering
The proper exposure settings for the scene to be shot can be determined by 'metering—using a light meter that instantly reports the level of light. The readings may simply inform the shooter while using manual exposure, or they may be used by the camera's autoexposure system to automatically set exposure controls.

Manual exposure
With manual exposure, the exposure controls are set by hand. These cannot be changed by the camera, even if the shot is sure to result in over- or underexposure.

Through-the-lens
The camera body often contains a built-in reflected light meter that measures the amount of light entering the camera through the lens (TTL). The suggested exposure settings are usually indicated in the viewfinder.

To better ensure a proper reading, a calibration card may be positioned in view. An 18% gray card provides a neutral, middle gray tone.

External metering
An incident light meter may be used to measure the actual amount of light present in a scene. The user holds the meter where the primary subject is positioned. These often have a white hemisphere that allows light to be collected from all directions. The meter reports what exposure settings should be used, and allow for locking of certain controls. Some can measure flash lighting by reporting the peak value detected.

There are devices that allow a smartphone to be used as a light meter, with an accompanying app to report settings.

Rules of thumb
Approximate settings can be figured by easy-to-memorize rules:


 * Sunny 16—On a sunny day, set aperture to f/16, and shutter speed to the reciprocal of sensitivity. If sensitivity is ISO 100, then shutter speed is 1/100. The aperture can be adjusted for different conditions, such as overcast clouds or shooting in full shade.


 * Looney 11—When shooting the surface of the moon at night, set aperture to f/11, and shutter speed to the reciprocal of sensitivity.

Automatic exposure
The camera body can use automatic exposure controls using the TTL meter and a programmed shooting mode.

Because scenes vary, the camera offers some basic metering modes:


 * Spot metering: Measures a small field, typically centered with the lens. Spot metering is recommended for use with a calibration card.


 * Center-weighted metering: Measures a larger field to account for areas with high contrast. The reading will split the difference between the brightest and darkest areas.


 * Matrix/evaluative metering: The entire scene is analyzed in sections, with priority given to focused areas. Some camera models may compare readings against a database in order to pick settings.


 * Average metering: All available light in a scene is simply averaged, yielding a crude but quick result.

Exposure compensation
Light meters assume that the subject will be exposed to a middle tone. This may not be the case if you are shooting in excessively bright scenes (snow, sand), or if your subject is strongly colored (dark skin or paint). The meter reading can nonetheless be used by setting exposure compensation that adds or subtracts stops.

On a film camera, the sensitivity control may be used for exposure compensation, by setting it a certain number of stops away from the actual film speed. Setting to a lower speed will increase exposure; setting to a higher speed will reduce exposure. This allows for using the shutter speed and aperture controls as intended.

On a digital camera, there is a dedicated exposure compensation control (+/–). The camera will adjust exposure controls depending on what shooting mode is chosen. Camera customization may allow more than one control to be adjusted to ensure a shot can be taken. In other cases, the camera may refuse to shoot and alert the user that its limits have been reached.

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Example
There, again, is Ed’s sitting on the bench. We have decided that we want a shallow depth of field so the warehouse is not in focus. We need a fast f stop, large hole, shallow depth of field, of f2. So we dial in f 2 on our lens by twisting the f stop ring on our lens. We look through the camera at Ed. We focus on him by using the focus ring on our camera lens or tap the shutter release on auto focus cameras. Our meter says we have too much light! We are staying with f 2 hole size so that we have a shallow depth of field and we cannot change the film speed so there is only one thing to do; we must lessen the shutter speed, how long the shutter is open, down to less and less time until the meter is happy and we know the image will be properly exposed.

Review
The perfectly exposed image is obtained by metering available light onto the light sensor with a set speed of whatever film is loaded. We can then adjust one of two things to make the meter happy and get a proper exposure; either f stop (hole through lens size), or shutter speed (how long the shutter is open). If we want a deep (or long) depth of field (Marriah with cliffs) then we want a long or big f stop (big number, small hole) and a slow shutter speed. If we want a shallow depth of field (Ed but no warehouse) then we want a short or small f stop (small number, big hole) and a faster shutter speed. We adjust and coordinate these two things (f stop and shutter speed) until we have the correct exposure (the meter is happy) and the depth of field we want (deep or shallow or somewhere in between). Remember to avoid hand holding for shots that are longer than 1/60th of a second.


 * Twice the light; half the speed. : Since we know that each full f stop allows in twice or half as much light through the lens as the full f stop next to it and that each shutter speed is twice or half as long as its neighbor we can, and will, coordinate them. Warning! Many camera lenses have 1/s stops. This means that they can and will have a click that is halfway between each full stop. It can be handy to get the proper exposure to use half stops but be aware of what f stop you are actually using. I will insist that you keep notes of film, f stop and shutter speed of EVERY shot for a while.


 * (Full f stops again (scale) in order f 1.4, f 2, f 2.8, f 4, f 5.6, f 8, f 11, f 16, f 22, f 32) : Example: We meter Bob the squirrel and come to the conclusion by playing with our f stop and shutter speed that we can properly expose him on the 400 ASA film we have loaded (and told the camera we have loaded) by using an f stop of f 4 and a shutter speed of 1/125th of a second. We take the shot. So Bob is still sitting there playing with his nuts and we realize that the carving on the other end of the bench of Bob would look cooler in the shot if it were also in focus! Scotty! I need more depth of field! So, we dial in the next larger full f stop of f 5.6 (HALF THE LIGHT!) and we double the amount of time we leave the shutter open to 1/60th of a second and get the shot with the end of the bench also in focus. See they work hand in hand! An f stop of f 5.6 requires twice as much time to capture the same image as f 4 and has MORE depth of field. An f stop of f 4 requires half the time as f 5.6 to capture the same image but f 4 has less depth of field.

Exercises
With some practice this will make sense. I will give you exercises and hands on time that will illustrate these principals in an artistic and understandable way. Not to worry. Remember that when you really understand these principles and how film speed, shutter speed and f stop work in unison then you already know WAY more than most. Once you get the basics down it is all art! Yes it takes a long time to learn it all but you will likely be taking good images and making nice prints of them very soon. Apply yourself. It will be worth it.

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 * If an image is properly exposed using 100 ASA film, f 4, 1/125th of a second; what is the correct shutter speed for the same shot using f 5.6? F 8? F 2.8? Try using the shutter speed scale above.
 * If an image is properly exposed using 100 ASA film, f 8, 1/30th of a second; what is the correct f stop if want to use a shutter speed (no tripod) of 1/60th of a second? 1/125th? 1/250th? Try using the full f stop scale above.
 * I am standing on a train going 427 MPH headed South towards Mexico and am wearing a blue shirt. My last shot on a roll was correctly exposed using 100 ASA film, f 8, 1/250th of a second. The only film I have left is 200 ASA film. I load it. What is the proper shutter speed with this film if I want to stay with f 8? What if I only have 400 ASA film and want to stay with f 8?