Field
One lens per focal length
An iPhone Pro has three lenses on the back: ultra wide, main and telephoto. The iPhone’s Camera app treats them as one zoom camera. When you pick the telephoto, it decides from moment to moment which lens actually takes the photo.
What the Camera app does
If the subject is closer than the telephoto can focus, or the light is low, the Camera app takes the photo with the main lens and crops it to look like the telephoto. The main lens focuses closer and gathers more light, so the photo is sharper and cleaner than the telephoto would manage. You don’t see the switch, and the photo says telephoto.
That is a sensible choice for a phone. It is not how a camera works.
What Field does
Field opens each lens directly. When you pick the telephoto, the photo comes from the telephoto, every time. The depth of field and the rendering of the lens are always the same for that focal length. The photos from one lens belong together.
The trade
The telephoto has a longer minimum focus distance. Close to a subject, Field won’t focus where the Camera app would have quietly switched lenses. Step back, or pick a shorter focal length.
The telephoto also has a smaller sensor and a slower aperture. In low light it needs a higher ISO or a longer shutter, so the photo is noisier. At night, the main lens is the better choice, and Field lets you make it.
Crops
On iPhones with 48 MP sensors, some focal lengths are crops from the center of a sensor: 35 mm and 48 mm from the main lens, and twice the telephoto when the telephoto has 48 MP. A crop is the same lens with fewer pixels, so it keeps that lens’s look. Field only offers crops where the sensor has the pixels for it.
The numbers
The main lens is called 24 mm, the convention for iPhone. Field measures every other lens’s field of view against it and rounds to the focal length photographers know. The labels describe your iPhone, not a spec sheet, so different models show different lineups.
You can hide focal lengths you don’t use in Settings under Lenses. The front camera is hidden there by default.