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 

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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.