Focal Length

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus". The camera lens projects part of the scene onto the film or sensor. The field of view (FOV) is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Larger sensors or films have wider FOVs and can capture more of the scene. The FOV associated with a focal length is usually based on the 35mm film photography, given the popularity of this format over other formats.

In 35mm photography, lenses with a focal length of 50mm are called "normal" because they work without reduction or magnification and create images the way we see the scene with our naked eyes (same picture angle of 46°). 

Wide angle lenses (short focal length) capture more because they have a wider picture angle, while tele lenses (long focal length) have a narrower picture angle. Below are some typical focal lengths:

Typical focal lengths and their 35mm format designations 

<>
24mm - 35mm Wide Angle 
50mm Normal Lens 
80mm - 300mm Tele
> 300mm Super Tele 


A change in focal length allows you to come closer to the subject or to move away from it and has therefore an indirect effect on perspective. Some digital cameras suffer from barrel distortion at the wide angle end and from pincushion distortion at the tele end of their zoom ranges. 
35mm Equivalent Focal Length

Focal lengths of digital cameras with a sensor smaller than the surface of a 35mm film can be converted to their 35mm equivalent using the focal length multiplier. 
Optical Zoom (X times zoom) and Digital Zoom 

Optical zoom = maximum focal lenght / minimum focal length
For instance, the optical zoom of a 28-280mm zoom lens is 280mm/28mm or 10X. This means that the size of a subject projected on the film or sensor surface will be ten times larger at maximum tele (280mm) than at maximum wide angle (28mm). Optical zoom should not be confused with digital zoom.

Depth of Field

As you can see, at a large aperture of f/2.4 only the first card is in focus,

while at f/8 the middle card is sharp and the distant card is almost sharp.

Depth of field (DOF) is a term which refers to the areas of the photograph both in front and behind the main focus point which remain "sharp" (in focus). Depth of field is affected by the aperture, subject distance, focal length, and film or sensor format.

A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anything behind or in front of the main focus point will appear blurred. A smaller aperture (larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range behind or in front of the main focus point will also appear sharp.

Coming closer to the subject (reducing subject distance) will reduce depth of field, while moving away from the subject will increase depth of field. 

Lenses with shorter focal lengths produce images with larger DOF. For instance, a 28mm lens at f/5.6 produces images with a greater depth of field than a 70mm lens at the same aperture.

Distortion Display

                                         Distortion Display

Aperture

   Aperture refers to the size of the opening in the lens that determines the amount of light falling onto the film or sensor. The size of the opening is controlled by an adjustable diaphragm of overlapping blades similar to the pupils of our eyes. Aperture affects exposure and depth of field. 
 

   Just like successive shutterspeeds, successive apertures halve the amount of incoming light. To achieve this, the diaphragm reduces the aperture diameter by a factor 1.4 (square root of 2) so that the aperture surface is halved each successive step as shown on this diagram. 
 

   Because of basic optical principles, the absolute aperture sizes and diameters depend on the focal length. For instance, a 25mm aperture diameter on a 100mm lens has the same effect as a 50mm aperture diameter on a 200mm lens. If you divide the aperture diameter by the focal length, you will arrive at 1/4 in both cases, independent of the focal length. Expressing apertures as fractions of the focal length is more practical for photographers than using absolute aperture sizes. These "relative apertures" are called f-numbers or f-stops. On the lens barrel, the above 1/4 is written as f/4 or F4 or 1:4.
 
   We just learned that the next aperture will have a diameter which is 1.4 times smaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. "Stopping down" the lens from f/4 to f/5.6 will halve the amount of incoming light, regardless of the focal length. You now understand the meaning of the f/numbers found on lenses: 

   Because f-numbers are fractions of the focal length, "higher" f-numbers represent smaller apertures. 
 

   Maximum Aperture or Lens Speed
The "maximum aperture" of a lens is also called its "lens speed". Aperture and shutterspeed are interrelated via exposure. A lens with a large maximum aperture (e.g. f/2) is called a "fast" lens because the large aperture allows you to use high (fast) shutterspeeds and still receive sufficient exposure. Such lenses are ideal to shoot moving subjects in low light conditions.
 
   Zoom lenses specify the maximum aperture at both the wide angle and tele ends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies that the maximum aperture is f/2.8 throughout the zoom range. Such zoom lenses are more expensive and heavy.

Shutterspeed

   The shutterspeed determines how long the film or sensor is exposed to light. Normally this is achieved by a mechanical shutter between the lens and the film or sensor which opens and closes for a time period determined by the shutterspeed. For instance, a shutter speed of 1/125s will expose the sensor for 1/125th of a second. Electronic shutters act in a similar way by switching on the light sensitive photodiodes of the sensor for as long as is required by the shutterspeed. Some digital cameras feature both electronic and mechanical shutters. 

   Shutterspeeds are expressed in fractions of seconds, typically as (approximate) multiples of 1/2, so that each higher shutterspeed halves the exposure by halving the exposure time: 1/2s, 1/4s, 1/8s, 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s, 1/8000s, etc. Long exposure shutterspeeds are expressed in seconds, e.g. 8s, 4s, 2s, 1s.

   The optimal shutterspeed depends on the situation. A useful rule of thumb is to shoot with a shutterspeed above 1/(focal length) to avoid blurring due to camera shake. Below that speed a tripod or image stabilization is needed. If you want to "freeze" action, e.g. in sports photography, you will typically need shutterspeeds of 1/250s or more. But not all action shots need high shutterspeeds. For instance, keeping a moving car in the center of the viewfinder by panning your camera at the same speed of the car allows for lower shutterspeeds and has the benefit of creating a background with a motion blur.

Exposure Compensation

   The camera's metering system will sometimes determine the wrong exposure value needed to correctly expose the image. This can be corrected by the "EV Compensation" feature found in prosumer and professional cameras. Typically the EV compensation ranges from -2.0 EV to +2.0 EV with adjustments in steps of 0.5 or 0.3 EV. Some digital SLRs have wider EV compensation ranges, e.g. from -5.0 EV to +5.0 EV.

   It is important to understand that increasing the EV compensation by 1 is equivalent to reducing EV by 1 and will therefore double the amount of light. For instance if the camera's automatic mode determined you should be using an aperture of f/8 and a shutterspeed of 1/125s at ISO 100 (13 EV) and the resulting image appears underexposed (e.g. by looking at the histogram), applying a +1.0 EV exposure compensation will cause the camera to use a shutterspeed of 1/60s or an aperture of f/5.6 to allow for more light (12 EV). 

   Of course, as you become more familiar with your camera's metering system, you can already apply an EV compensation before the shooting. For instance if your camera tends to clip highlights and you are shooting a scene with bright clouds, you may want to set the EV compensation to -0.3 or -0.7 EV.

TIFF

TIFF (Tagged Image File Format) is a universal image format that is compatible with most image editing and viewing programs. It can be compressed in a lossless way, internally with LZW or Zip compression, or externally with programs like WinZip. While JPEG only supports 8 bits/channel single layer RGB images, TIFF also supports 16 bits/channel multi-layer CMYK images in PC and Macintosh format. TIFF is therefore widely used as a final format in the printing and publishing industry.

Many digital cameras offer TIFF output as an uncompressed alternative to compressed JPEG. Due to space and processing constraints only the 8 bits/channel version is used in digital cameras. Higher-end scanners offer a 16 bits/channel TIFF option. If available, RAW is a much better alternative for digital cameras than TIFF.

RAW

Unlike JPEG and TIFF, RAW is not an abbreviation but literally means "raw" as in "unprocessed". A RAW file contains the original image information as it comes off the sensor before in-camera processing so you can do that processing afterwards on your PC with special software.

The RAW Storage and Information Advantages

Even though the TIFF file only retains 8 bits/channel of information, it will take up twice the storage space because it has three 8 bit color channels versus one 12 bit RAW channel. JPEG addresses this issue by compression, at the cost of image quality. So RAW offers the best of both worlds as it preserves the original color bit depth and image quality and saves storage space compared to TIFF. Some cameras offer nearly lossless compressed RAW.
 
The Flexibility of RAW 
In addition, many of the camera settings which were applied to the raw data can be undone when using the RAW processing software. For instance, sharpening, white balance, levels and color adjustments can be undone and recalculated based on the raw data. Also, because RAW has 12 bits of available data, you are able to extract shadow and highlight detail which would have been lost in the 8 bits/channel JPEG or TIFF format. 
 
Disadvantages of RAW 
The only drawback is that RAW formats differ between camera manufacturers, and even between cameras, so dedicated software provided by the manufacturer has to be used. Furthermore, opening and processing RAW files is much slower than JPEG or TIFF files. To address this issue, some cameras are offering the option to shoot in RAW and JPEG at the same time. As cameras become faster and memory cards cheaper, this option has no longer performance or storage issues. It allows you to organize and edit your images in a faster way with regular software using the JPEGs. But you retain the option to process in RAW those critical images or images with problems (e.g. white balance or lost shadow and highlight detail). Another trend is that third party image editing and viewing software packages are becoming RAW compatible with most popular camera brands and models. An example is Adobe Photoshop CS. However, as stated in my Photoshop CS review, the way Photoshop processes RAW files can be different from the way the camera manufacturer's software does it and not all settings may be recognized.

White Balance

Color Temperature

   Most light sources are not 100% pure white but have a certain "color temperature", expressed in Kelvin. For instance, the midday sunlight will be much closer to white than the more yellow early morning or late afternoon sunlight. This diagram gives rough averages of some typical light sources.

Resolution

Sensor Resolution

   The number of effective non-interpolated pixels on a sensor is discussed in the topic about pixels. 
 
Image Resolution

   The resolution of a digital image is defined as the number of pixels it contains. A 5 megapixel image is typically 2,560 pixels wide and 1,920 pixels high and has a resolution of 4,915,200 pixels, rounded off to 5 million pixels. It is recommended to shoot at a resolution which corresponds with the camera's effective pixel count. As explained in the pixels topic, shooting at higher (interpolated) resolutions (if available as an option) creates only marginal benefits but takes up more card space. Shooting at lower resolutions only makes sense if you are running out of card space and/or image quality is not important.

JPEG

   The most commonly used digital image format is JPEG (Joint Photographic Experts Group). Universally compatible with browsers, viewers, and image editing software, it allows photographic images to be compressed by a factor 10 to 20 compared to the uncompressed original with very little visible loss in image quality. 
 
The Theory in a Nutshell
   In a nutshell, JPEG rearranges the image information into color and detail information, compressing color more than detail because our eyes are more sensitive to detail than to color, making the compression less visible to the naked eye. Secondly, it sorts the detail information into fine and coarse detail and discards the fine detail first because our eyes are more sensitive to coarse detail than to fine detail. This is achieved by combining several mathematical and compression methods which are beyond the scope of this glossary but explained in detail in 123di. 
 
A Practical Example 

   JPEG allows you to make a trade-off between image file size and image quality. JPEG compression divides the image in squares of 8 x 8 pixels which are compressed independently. Initially these squares manifest themselves through "hair" artifacts around the edges. Then, as you increase the compression, the squares themselves will become visible, as shown in the examples below, which are magnified by a factor 2.

Noise

The standard deviation measured in a uniform area of an image (in the above examples measured in the red channel) is a good way to quantify image noise as it is an indication of how much the pixels in that area differ from the average pixel value in that area. The standard deviation in the noisy examples C and D is much larger than A, B, and E. Crop E shows that noise reduction can go a long way.

The Cause: Sensor Noise 

Each pixel in a camera sensor contains one or more light sensitive photodiodes which convert the incoming light (photons) into an electrical signal which is processed into the color value of the pixel in the final image. If the same pixel would be exposed several times by the same amount of light, the resulting color values would not be identical but have small statistical variations, called "noise". Even without incoming light, the electrical activity of the sensor itself will generate some signal, the equivalent of the background hiss of audio equipment which is switched on without playing any music. This additional signal is "noisy" because it varies per pixel (and over time) and increases with the temperature, and will add to the overall image noise. It is called the "noise floor". The output of a pixel has to be larger than the noise floor in order to be significant (i.e. to be distinguishable from noise). 
 
The Effect: Image Noise 

Noise in digital images is most visible in uniform surfaces (such as blue skies and shadows) as monochromatic grain, similar to film grain (luminance noise) and/or as colored waves (color noise). As mentioned earlier, noise increases with temperature. It also increases with sensitivity, especially the color noise in digital compact cameras (example D below). Noise also increases as pixel size decreases, which is why digital compact cameras generate much noisier images than digital SLRs. Professional grade cameras with higher quality components and more powerful processors that allow for more advanced noise removal algorithms display virtually no noise, especially at lower sensitivities. Noise is typically more visible in the red and blue channels than in the green channel. This is why the unmagnified red channel crops in the examples below are better at illustrating the differences in noise levels.

Sensitivity (ISO)

          ISO 100                               ISO 800

    Conventional film comes in different sensitivities (ASAs) for different

purposes.The lower the sensitivity, the finer the grain, but more light is

needed.This is excellent for outdoor photography, but for low-light conditions

or action photography (where fast shutterspeeds are needed), more sensitive

or "fast" film is used which is more "grainy". Likewise, digital cameras have an

ISO rating indicating their level of sensitivity to light. ISO 100 is the "normal"

setting for most cameras, although some go as low as ISO 50. The sensitivities

can be increased to 200, 400, 800, or even 3,200 on high-end digital SLRs.

When increasing the sensitivity, the output of the sensor is amplified, so less

light is needed. Unfortunately that also amplifies the undesired noise.

Incidentally, this creates more grainy pictures, just like in conventional

photography, but because of different reasons. It is similar to turning up the

volume of a radio with poor reception. Doing so will not only amplify the (desired)music but also the (undesired) hiss and crackle or "noise". Improvements in sensor technology are steadily reducing the noise levels at higher ISOs, especially on higher-end cameras. And unlike conventional film cameras which require a change of film roll or the use of multiple bodies, digital cameras allow you to instantly and conveniently change the sensitivity depending on the circumstances.