Focal Length

Focal Length

• A camera lens may be designated as prime or zoom.
• A prime lens has a fixed focal length that does not change.
• A zoom lens is one that allows the user to alter the focal length.
• Prime lenses are generally higher quality while zoom lenses offer more flexibility.
• The focal length of a lens is defined as the distance in mm from the optical centre of the lens to the focal point, which is located on the 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.
• In 35mm photography, lenses with a focal length of 50mm are called 'normal' because they work without reducing or magnifying and creates images the way we see the scene with our naked eyes (same picture angle 46 degrees).
• Wide angle lenses (short focal length) capture more because they have a wider picture angle.
• Telephoto lenses (long focal length) have a narrower picture angle.
• A longer focal length is associated with larger magnification of distant objects, and a narrower angle of view.
• A shorter focal length is associated with a reduction of objects and a wider angle of view. 

• <20mm = Super wide angle

• 24 - 35mm = Wide angle

• 50mm = Normal lens

• 80mm - 300mm = Tele

• >300mm = Super tele

Wide Angle

• Wide FOV 
• Some Distortion 

Normal (50mm)

• FOV equivalent to the human eye
• Used for variety of purposes.

Telephoto

• Narrow FOV
• Low distortion
• Relatively shallower depth of field

Focal Length Experiments

  

  

In this sequence of images we can see the head and shoulders of this girl present in each image. To capture each of these images I used an increasing focal length, I started at 18mm and ended at 55mm. When increasing the focal length I had to reposition myself to position her head and shoulders in the shot. As I was constantly moving I decided not to use a tripod because it would've taken time to keep readjusting the height and position of the tripod. As we can see from the series of images that I have produced the first few that I took appeared distorted slightly because I was quite close to her face which prevented the lens from capturing a wider angle view. As the focal length increases the distortion decreases significantly, the las picture shows a normal shot of the girl with the highest focal length showing a very wide angle. Everything has stayed in shot throughout this sequence but the normality of it only comes clear in the last few shots. This focal length experiment was carried out to show how the lower the focal length the more distorted and the lower the wider angle of the image. This experiment has worked significantly well because when we compare the last and the first image the difference in distortion is high. 

  

  

This sequence of images was conducted to show how the different focal lengths change the images. Unlike the first experiment I have conducted this one isn't as clear because the images are not technically good images. When increasing the focal length in this experiment we can see how much closer we get to the girl and how much background is cut off. As the sequence of images progress the closer we get to the girls, also the progression of these photos also increases the amount of detail we can see in each image, for example the details of the plants become bolder as the focal length increases. The only downfall with this experiment is as we increase the focal length her head gets slightly cut off which could've been improved if I had position the camera in a different way. Unlike the first experiment I conducted I used a tripod in this experiment because I needed the camera to be steady to show how changing the focal length of a stationary camera decreases the extent of background we can see.