Focal length determines the angle of view. Lenses are divided into two groups based on focal length – fixed focus and zoom.
Fixed focus lenses are suitable for specific areas of photography, and possess better lens speed and picture quality.
Zoom lenses are more versatile and flexible in their use.
Focal length is important in determining the use of various lens types.
- 8 – 14 mm lenses (fisheye) produce wide-angle pictures. Their main characteristic is perspective distortion.
- 15 – 24 mm lenses (ultra wide-angle) are suitable for landscape, interior, and architecture shots. Perspective distortion is mostly eliminated compared to fisheye lenses.
- 25 – 34 mm lenses (wide-angle) are suitable for photojournalism, as well as interior and landscape photography.
- 50 mm lenses (human eye perspective) are comparable to the human eye in angle of view. These lenses are considered "the base". They are suitable for photojournalism and portraits.
- 35 – 100 mm lenses (medium) are similar to the human eye in perspective. They are used for everyday situations and portraits.
- 100 – 300 mm lenses (telephoto) are suitable for portraits, photojournalism, and landscapes. With quick focus, they work well in sports photography.
- over 300 mm lenses (super telephoto) are used mainly to shoot sports and wild animals.
Lens speed
Determines how much light the lens lets through.
- The higher the lens speed, the more light the lens is able to let through at any given moment. This greatly influences shutter speed. Reasonably low shutter speeds are desirable.
- Insufficient lens speed can lead to longer exposure time, which results in shaking (the photographer cannot hold the camera in the same position during exposure) and movement blur (the photographed object can move during exposure).
- Lens speed is also important for depth of field (the ability of the lens to blur the background behind the photographed object).
Lens speed is a key factor in determining the purpose of a lens.
- F/1.8 and lower is suitable for potraits, where low depth of field is required. Low speeds are attainable mainly with fixed focus lenses.
- Speeds around f/2.8 are attainable with zoom lenses. It is suitable for photojournalism and sports photography.
- High f-numbers (like f/16) find their use in landscape and architecture photography. The entire picture should be sharp, unlike a portrait.
Other
- Stabilisation – certain lenses provide image stabilisation, limiting bluriness caused by shaking hands. Stabilisation is important in photojournalism and portrait photography.
- Focusing – fast and accurate focus is important, especially in sports photography. While focusing, the lens is relying either on a motor inside the DSLR, or on its own ultrasonic motor, which is usually faster.
- Minimum focus distance – a key factor, especially for capturing details (macro photography).
- It is important to differentiate between APS and Full frame cameras.
