Understanding Focal Length and Field of View

Focal length and field of view are two terms I feel are sometimes thrown around, interchanged and confused. They are not the same, while focal length is a constant and won’t change; field of view is dependable upon the focal length as well as the size of the sensor or film.

Demonstration of Field of View Changing with Focal Length 24mm - 72mm

Demonstration of Field of View Changing with Focal Length 24mm – 72mm

When discussing focal lengths and field of view we traditionally talk about lenses on a camera with a 35mm film sized frame. A 50mm lens is considered normal, 28mm wide while anything over 85mm being telephoto and over 300mm being a super telephoto lens.

Focal Length

Focal length is the measurement of your lens in mm (50mm 80mm ect). It is measured from the optical centre of the lens to the focal plane, normally a sensor or the film in the camera. This is a physical distance, when a lens is switched from camera to camera the focal length doesn’t change.
A prime lens will only have one focal length while a zoom lens will have multiple focal lengths depending on how far in or out you have moved the lens.

Field Of View

cropped_sensor_viewField of view, sometimes known as angle of view, measures how big of an area you can see with a lens. The field of view is measured in three angles; diagonal, vertical and horizontal. The field of view of a lens is different depending on how big the sensor or film is; for example on a full frame DSLR a 50mm lens field of view will be different  if the lens was put on a cropped sensor. On a cropped sensor a 50mm lens would have the comparative field of view of an 85mm lens. The lens’s focal length has not changed but a cropped sensor is smaller and with a smaller sensor you get a more magnified image and intern a narrower field of view.

Cropped sensors

Digital cameras come in a variety of sensor sizes, the rule of thumb being the bigger the better; the same is true with pixels (size).  With DSLRs there are two main types of sensors Full frame which are the same size as a 35mm film and cropped sensors which are smaller. The Nikon D800, D600,D610,Df,D4s and Canon 1Dx, 5D, 6d to name a few have full frame sensors, while the Nikon’s DX format cameras like the D7100,D3200,D5300 and Canon 7D 60D 700D 1100D all have cropped sensors. Since the sensors of these cameras are smaller we have to use a Digital Multiplier to calculate a lenses relative focal length on a full frame camera, by multiplying the focal length by the digital multiplier. For Canon APS-C sensor the digital multiplier is 1.62 and for Nikon DX format cameras it is 1.52.

crop_sensorMost camera manufactures have created lens for their cropped sensor models. Canons has the EF-S lens that are designed for APS-C sensors, while Nikon DX lenses are designed for their cropped sensor line. Sony has their DT lenses while Pentax has their DA lenses. These lenses don’t fix the difference in field of view but since the lenses are designed for smaller sensors reducing the vignette which can come from using a lens developed for a full frame camera.

The relationship

Understanding the relationship between field of view and focal length may not seem important, yet it is, especially when thinking about buying a new lens or equipment. If you are graduating from cropped sensor to a full frame camera, those lenses you have been so used to using and seeing the world through will give you a totally new angle of view. Also if you are reading photography tutorial or self-help books/blogs then they may talk about 50mm 28mm lenses, these focal lengths are being thought of as if using a for full frame camera. A lens with an equivalent field of view of a 50mm lens on a cropped sensor would in fact be 28mm and an equivalent field of view of a 28mm lens on a cropped sensor would be 12mm.
This relationship between the focal length, media size and field of view transfers naturally to large and medium format cameras. On a large format camera an 80 mm lens is considered a wide angle lens with a wide field of view.

744px-Full-frame_vs_APS-C.svg

Considerations

When renting, borrowing, or purchasing a lens check the format of your lens and also what format the lens is designed for. If you have a cropped sensor and the lens was design for a full frame one you will need to multiply the length of the lens by the Digital multiplier for the sensor to know the equivalent field of view. Also some lenses designed for crop sensors may not correctly fit onto your full frame camera; Canons EF-S Lenses won’t fit onto full frame bodies as they protrude deeper into the camera and will stop the mirror from moving. With this in mind it may not be worth buying a lens for a cropped sensor if you are thinking in the future of buying a full frame camera.

If you have any questions or comments feel free to contact me via the comment box below.

Source; University Notes 
Image Sources; Wikipedia 

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