Saturday, May 22, 2010

Beginner's Simplified Guide to Exposure with Digital Cameras (Basic)

If you've never ventured outside of your camera's auto mode and you don't know what aperture, shutter speed, "and all that stuff" mean, this guide is for you.

1. Your digital camera can get the exposure wrong, resulting in pictures that are too bright, too dark, or with lost detail. You can fix this if you understand exposure.
2. You can control the appearance of the image to match your vision (e.g., blurry background, motion blur, etc.)

Exposure is the amount of light allowed to fall on a sensor (or film). If what's white looks gray or important details in the shadows are lost, the image is too dark (underexposed). If what's black looks gray or important details in the highlights are lost, it's too bright (overexposed). Sometimes, a scene has extreme dark and bright areas so that preserving the details in one part of the picture will cause the loss of detail in another part. If so, you have to compromise by picking the important area in your picture (e.g., face), making sure that area has the right exposure, then hoping the best for the rest.

To simplify light measurement, photographers think in terms of "stops" of light. Each stop higher is double the light. Each stop lower is half the light. 2 stops higher is therefore 4x the light. 3 stops lower = 1/8th the light.

You control exposure with 4 variables: aperture, shutter speed, ISO, exposure compensation.
1. aperture: the size of the "hole" where light enters. Wider aperture allows more light. Aperture is specified in f-numbers. The *smaller* the f-number, the *wider* the aperture. The standard "f-stops," which are each one stop apart are: 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, etc. Note: Each f-number in the sequence is approximately a factor of 1.4 (square root of 2); an f-number that is 2 stops higher is double the f-number).
2. shutter speed: how long the sensor is exposed to the light. Longer shutter speed allows more light. The standard shutter speeds (in seconds) are 1 sec., 1/2 sec., 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, etc. Each of those speeds are considered 1 stop apart (even though they're not exactly double).
3. ISO: sensor sensitivity setting. Higher sensitivity allows more light.  Standard ISO settings are 100, 200, 400, 800, 1600, 3200, etc. (they are each one stop apart).
4. Exposure compensation: you can adjust the default exposure within +/- 2 or 3 stops, usually in 1/3 or 1/2 stop increments.
Most cameras have these exposure modes:
1. Auto mode (not present in professional cameras): camera guesses the proper exposure, guesses your intent (e.g., portrait shot, sport shot, etc.), and chooses the combination of aperture, shutter speed, and ISO based on its guess of your intent.
2. Program mode (P): like auto mode, the camera guesses the exposure. The camera doesn't guess your intent but instead picks an "average" combination of aperture, shutter speed, and ISO. The camera lets you change more variables than in Auto mode, such as the ISO, EV compensation, and whether flash is used.
3. Aperture priority (Av or A): the camera guesses the exposure. You choose the aperture, then the camera picks the shutter speed (and in some cameras, the ISO as well) to achieve its guess of the exposure.
4. Shutter priority (Tv or S): the camera guesses the exposure. You choose the shutter speed, then the camera picks the combination of aperture (and sometimes, the ISO) to achieve its guess of the exposure.
5. Manual mode (M): you choose the aperture, shutter speed, and ISO (some cameras may pick the ISO automatically) with the help of a light meter (which displays the camera's guess of exposure in real time).
P, A, or S mode are "semi-automatic" modes because it's still the camera that is guessing the exposure.  If the camera guessed wrong (see above re overexposure, or underexposure) use EV compensation to increase or decrease exposure, or switch to manual mode.
There are many combinations of variables that result in equivalent exposure. You can get the same exposure if you adjust the aperture a stop higher (narrower) while adjusting the shutter speed a stop slower, etc. For example,
1/60 f/5.6 ISO 400 EV comp 0 is the same exposure as
1/30 f8.0 ISO 400 EV Comp 0.
However, aperture and shutter speed affect other things too, which gives you a reason to choose between equivalent exposures:
1. Depth of field: some pictures have a blurry background while others are sharp everywhere. The ones with a blurry background have a shallow depth of field, meaning the area that is in focus is narrow (sometimes within a few inches). This is useful for emphasizing the subject. A large depth of field is useful for landscape photos. Depth of field is affected by aperture (as well as the distance between the camera to the subject, the focal length, and the sensor size). Wider aperture = smaller depth of field.

Example of shallow depth of field (aperture here was f/1.4):

Example of large depth of field (aperture here was f/16):
2. Motion blur: sometimes you want motion blur (e.g., picture of a moving car) while other times you want to freeze the image (e.g. child jumping in the air). Longer shutter speed = more motion blur.

Example of high shutter speed (shutter speed here was 1/1000):
Example of slow shutter speed (shutter speed of 1/15):
3. Camera shake: if you don't use a tripod, your image could look blurry because of your hand's natural tremor. Faster shutter speed minimizes this problem. Rule of thumb for cameras: to reduce visible camera shake, use a shutter speed faster than 1/(focal length in 35mm equivalent). With a lens that's equivalent to a 50mm lens, shoot at 1/50 or faster.  Some cameras and some lenses have image stabilization that mitigates camera shake somewhat, with varying degrees of effectiveness.

4. Noise: The higher the sensitivity (ISO), the more noise there will be (visible as speckles or grain).

Image with low noise (ISO was 200):

A noisy image (ISO: 800 -- actually this was a disposable film camera, so the correct term is "ASA").  See especially the grainy shadow areas:

1. Take photos at varying exposure combinations to practice manipulating the exposure variables.
2. Learn about using the histogram of an image as a guide for exposure.  When you take a picture, a digital camera can usually display the histogram of the image (a graph showing the number of pixels in each shade of gray, from pure black to pure white). If the pixels are at the leftmost edge of the histogram, it means that no detail has been captured and those pixels will simply be pure black.  If the pixels are at the rightmost edge, no detail has been captured either and those pixels will look pure white.  (To alert you that no details have been captured at certain parts of the image, many DSLRs can show you the "blown out" highlights as blinking highlights.)  A good exposure will not allow important details to reach either the leftmost edge or rightmost edge of the histogram.  Many digital cameras can also show a histogram not just for the gray channel, but also for the red, blue and green channels.  If a pixel is within the middle of the gray channel but at the leftmost or rightmost edge of the red, blue or green channel, then there will be detail but the color will not be accurate.
3. Look at photo websites and look at the combinations used for pictures you like to get a general idea of how that photo was taken (long or short shutter speed? Wide or small aperture? etc.). This is usually shown by clicking on the EXIF information, "properties," or "more information." Sample: Practice by guessing what variables you would use to achieve the same effect.
4. Learn about the various metering modes (matrix, average, center, spot).  Choosing the metering mode lets you control how much of the scene the camera will look at when determining exposure.  Matrix (the default setting) will allow the camera to look at the entire scene and use its programming to guess the correct exposure based on patterns.  For example if there is a large amount of bright tones in the lower part of the scene, the camera might guess that you are taking a picture of snow and will try to render the bright tones as white instead of middle gray).  Average mode (present in only some cameras) will also look at the entire scene but simply averages the amount of light rather than try to guess the correct exposure based on patterns.  Center will look at only the middle of the scene.  Spot will look at a very small portion of the scene (in the Nikon D300 for example, it will only look at an image circle that is about 3mm in diameter and represents 2% of what the camera can see).
5. Understanding exposure is only one aspect of good photography (albeit a fundamental one). Learn also about colors and white balance, composition, focal lengths and perspective, flash techniques, post-processing (adjusting the image on your computer), various types of photography (glamour, street, portrait, macro, etc.) and a gazillion other things.

1. Low light photos - You may hear, "You need a fast lens." That means a lens with a wide maximum aperture (generally f/2.8 or wider). It's "fast" because you can take photos at higher shutter speeds (assuming you use the wider apertures). Some may say, "Get a DSLR." DSLRs have larger sensors than point and shoots, and are therefore more sensitive to light (you can use higher ISOs with less noise). Thus you can use faster shutter speeds at higher ISOs. Usually, there's at least a 2 stop difference between SLRs and a point and shoot camera (ISO 1600 on an SLR looks like ISO 400 on a point and shoot - so you could use a shutter speed 2 stops faster at the same aperture with the same amount of noise). However, don't assume that an SLR with a fast lens will automatically enable you to take sharp low light photos -- at wide apertures, the depth of field is very narrow so it's hard to focus on the right area especially when the subject is moving. There are other techniques for low light photos including natural-looking flash techniques.
2. Megapixels vs. Sensor size - All things being equal, a camera with more megapixels will probably have more noise than a similar camera with less megapixels if they have the same sensor size because the higher megapixel camera has to squeeze out more "information" from the same amount of light. A typical point and shoot has a sensor size that is 1/2.5" (some as large as 1/1.6"). However, entry-level SLRs have a sensor that is around 1.8", which is 10x larger in area than a 1/2.5" sensor. See here That's why even a 6mp DSLR will produce images with less noise than a 10mp point and shoot at the same ISO.
3. Newer DSLRs generally have lower noise than older DSLRs. 
4. DSLR vs. point and shoot.
A single lens reflex camera is one that uses a mirror between the lens and the sensor (or film) to show you what the sensor will see, then flips that mirror away when you take the shot to record the image on the sensor.
Practically, the difference is that DSLR cameras have larger sensors compared to point-and-shoots and thus have better image quality especially at higher ISOs. Thus you can often take photos at higher shutter speeds (thus usually sharper photos).
DSLRs also have interchangeable lenses (you can buy lenses that are fast, ultrawide or very long telephoto). Many lenses for DSLRs are also sharper and have higher contrast than typical lenses used in point and shoots (to be fair, there are also poor DSLR lenses).
DSLRs usually also make it easier to manipulate exposure and other variables. "Serious" DSLRs will often let you change aperture, shutter speed, ISO, white balance, focus, flash sync, flash exposure, and other controls without going through a single menu and without removing your eyes from the viewfinder.
DSLRs are larger, more expensive and sometimes have a smaller zoom range for each lens. The latest superzooms have an 18x zoom range while the highest zoom range in an SLR lens is currently 15x. Some DSLR lenses don't even have a zoom (called prime lenses) but they are often fast and have excellent sharpness and contrast.
Until recently, most DSLRs couldn't display the image you're about to capture on the LCD screen (instead you use the viewfinder).  Some DSLRs that do have "live view" can't focus as quickly when live view is activated.
Many DSLRs can't take videos (unlike virtually all point-and-shoot cameras)
4. Why does a camera guess the wrong exposure? Because it doesn't have the intelligence to know which objects are truly white or truly black. A camera doesn't know whether it's looking at a gray wall, a white wall under the shade, or a black wall with a lot of light shining on it. A camera guesses exposure by assuming a scene has the "average" amount of light, measuring the amount of light in a scene with a built-in light meter, then adjusting exposure so the scene reflects the average amount of light in an average scene (18% or 12.5% reflected light depending on whom you ask).
However, if a scene has a lot of white areas, it's supposed to reflect more light than the average scene (e.g., 36%). But a camera usually doesn't know that, so it adjusts the exposure to reflect the average amount of light (18% or 12.5%), which results in underexposure. Vice-versa for overexposed dark scenes. Some cameras have "scene modes" to compensate for unusual situations (e.g., for snow), and some may even choose the scene mode automatically but sometimes the camera guesses wrong.
5. Metering/exposing for highlights means adjusting exposure so that details in bright areas remain visible and not "blown out." Metering for shadows/lowlights aims to preserve details in shadows. With digital cameras, you can usually recover some detail in shadows but not in blown out areas therefore the usual advice is to meter for highlights. A "conservative" camera such as the Pentax K10D will tend to underexpose to preserve details in hopefully the entire image (with some post-processing for details in shadows).
6. Zone System.  This is an exposure technique invented by Ansel Adams and Fred Archer.  In this technique, tones of light are categorized in increasing brightness from Zone 0 to Zone X, each one stop apart, with Zone V being middle gray.  Each scene will have areas that belong to different zones.  For example, if you take a picture of a chessboard, the white squares will belong to Zone VII or VIII while the black squares will belong to Zone II or III.  In the zone system, you would spot meter the area that you know the correct zone for and adjust exposure to render that area in the zone that you want.  In the chessboard example, you could spot meter a white square then adjust exposure so that the white square is 2 or 3 stops above middle gray.  Assuming you expose your target correctly, the rest of the scene will be rendered correctly.
7. Some point and shoot cameras don't have a variable aperture. Instead they use digital neutral density filters to darken the image digitally to simulate smaller apertures. In these cameras, changing aperture doesn't change the depth of field.

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