Lens Basics
Toby asked, ‘That thing sticking out the front of the camera? What is that? Are you shooting laser beams with it - is that why you call it shooting photos?’
Toby didn’t take physics in school. Mostly he learned to sit and come - when he felt like it, that is.
The lens is the thing that is sticking out in front, I told him. There are zoom lenses that go from wider to narrower and prime lenses that are a fixed focal length. Focal length is a term that relates to how wide the lens ‘sees’. An APS-C sensor/lens combination will see a narrower field of view than an equivalent full frame sensor/lens combination. The conversion for Sony is about 1.5x so a 20mm lens on a Sony APS-C body will see about as wide as a 30mm lens on a Sony full frame body. For now, I will stick to discussing full frame focal lengths.
Lenses attach to the body using a mount. Generally, within a camera company’s system mirrorless lenses and bodies are interchangeable. Usually there will be a different mount for DSLR and mirrorless systems and between camera companies. In that case you can buy an adapter if there is a lens that you need to use outside of it’s ‘normal’ system. In many cases the body will not be able to ‘talk’ to an adapted lens and some things like autofocus or autoexposure may not work.
The job of the lens is to collect light and focus it on the sensor. As you spend more money you typically get more light through the lens and better sharpness from corner to corner of the photograph. Typically full frame lenses are larger and more expensive than APS-C lenses due to their size and quality. For example the Sony 70-200mm f/2.8 zoom is twice as expensive as the 70-200mm f/4 zoom as it can let twice the light through to the sensor and has improved overall optical quality. More light through the lens can allow a faster shutter speed to freeze action or lower iso to reduce noise.
Focal length is directly related to field of view within a sensor size. A 35mm full frame lens on a Sony will see the same as a 35mm full frame lens on a Canon or Nikon. The 12-24mm Sony zoom will see half as much at 24mm as it does at 12mm. If you compared it to the 200-600mm zoom at 600mm the camera will be zoomed in by a factor of 50 from 12mm.
To squeeze a wide shot onto the sensor or to spread out a narrow shot sometimes creates distortion, especially if the camera is tilted up or down in relation to the subject. This can be a creative effect or something to be avoided depending on the situation.
Many lenses also have stabilization built into them though often mirrorless camera bodies have stabilization built in to the sensor for lenses that aren’t stabilized. This helps increase the shutter speed and overall light to the sensor for hand-held shots.