On this page
What decides how much a lens can see?
The amount of a scene a camera takes in is its field of view, or angle of view. A University of California, Berkeley lecture on cameras and lenses sums it up simply: field of view depends on focal length and sensor size.
For a fixed sensor size, a shorter focal length gives a wider field of view. The lecture gives these examples for the 35mm format, with a 36 x 24mm frame:
| Focal length on 35mm | Type | Angle of view |
|---|---|---|
| 17mm | Wide angle | 104° |
| 50mm | “Normal” lens | 47° |
| 200mm | Telephoto | 12° |
The sensor matters just as much. A smaller sensor takes in a smaller part of the image the lens forms, so the same lens gives a tighter view. To keep the same field of view on a smaller sensor, the Berkeley lecture says you shorten the focal length in proportion to the sensor’s width or height.
What is crop factor?
Cambridge in Colour, which describes itself as a learning community for photographers, defines crop factor as a sensor’s diagonal size compared with a full frame 35mm sensor. It works as a focal length multiplier: a 50mm lens on a sensor with a 1.6x crop factor gives the same field of view as an 80mm lens on full frame, because 1.6 x 50 = 80.
Cambridge in Colour adds an important warning: the lens’s focal length does not change on a different sensor, only its angle of view. A 50mm lens is always a 50mm lens. That is why a viewfinder app asks for both the sensor and the lens, instead of just the focal length.
Phones are a good example. The Berkeley lecture notes that when a phone camera is described as roughly 28mm “equivalent”, that uses the 35mm convention. The physical focal length is often 5 to 6 times shorter, because the sensor is correspondingly smaller.
How does aspect ratio change the frame?
Aspect ratio is the shape of the frame: its width compared with its height. The same lens and sensor can be cropped to different shapes. A 16:9 frame is wider than it is tall. A 2.39 frame is much wider again, a long letterbox shape. A 4:5 frame is slightly taller than it is wide.
Choosing the ratio before you scout tells you what will fall at the top and bottom of the shot, which matters as much as what is at the sides.
How do you use a viewfinder app on a scout?
- Set the sensor and lens first. Match them to the camera and lenses you plan to use, so the frame lines mean something.
- Pick the aspect ratio you will deliver in.
- Stand where the camera will stand. The Berkeley lecture notes that composition depends on camera position as well as focal length, so move around until the frame works.
- Try the next lens up and down. It is much quicker to compare on a phone than to change real lenses.
- Record what you chose. A still with the sensor, lens and ratio written on it saves notes later.
Questions people ask
Why does the same lens look different on two cameras?
Because the sensors are different sizes. The focal length stays the same, but the angle of view changes.
What does a 1.6x crop factor mean?
A lens on that sensor gives the same field of view as a lens 1.6 times longer on full frame, so 50mm behaves like 80mm.
Is my phone’s “28mm” lens really 28mm?
No. According to the Berkeley lecture, that figure is a 35mm equivalent. The actual focal length is often 5 to 6 times shorter.
Where Viewfinder helps, and where it does not
Viewfinder is our Android director’s viewfinder for your phone. It works out the frame from the sensor size and focal length you choose and draws it over the live camera. It has aspect ratio guides including 2.39, 16:9 and 4:5. You can take a still with a strip showing the sensor, lens, aspect ratio and field of view. It works offline, needs no account and keeps your stills on your phone.
The free version includes 3 sensors, 3 aspect ratios, the full 8 to 300 mm lens range and watermarked stills. A one-time Pro unlock adds every camera in the library and your own custom sensor size, lens compare, frame lines, projects that keep each shoot's stills together, contact sheets, custom aspect ratios, extra strip styles and stills without the watermark. There is no subscription.
What it is good for: quick location scouting and planning shots without the camera.
What it will not do: its frame lines are a practical guide worked out from sensor size and focal length. They are not an exact match for every camera body, and the app does not claim to show how a particular lens renders the image. For a critical shot, check on the actual camera.
Questions or bugs: Email us.
Sources
- Berkeley, CS184/284A, Cameras and LensesUniversity of California cs184.eecs.berkeley.edu
- Digital camera sensor sizesCambridge in Colour cambridgeincolour.com