How much does your lens see
It is one of the defining parameters of the lens, but the number does not make so much sense. Technically it determines the field of view in a roundabout way. The larger the number, the smaller window will be projected on your sensor, magnifying it.
What does it all mean
Your lens will project the image onto the sensor/film of the camera. The focal length is the distance of this lens from your sensor. So when you talk about 35mm lens, the focusing lens is 3.5cm away. This also explains why zooms expand and grow as you change the focal length (the Nikon 200-500 for example needs to move 30cm to cover the zoom range.
When taking a picture, the focal length does not tell you too much, after all, you do not really care about the physics of the equipment, you just want a picture. The focal length directly determines the field of view how much of the scene is being projected on your sensor. The higher the focal length the smaller window you will capture.
Do longer focal lengths allow us to see further
Well yes and no. Technically you are just seeing a smaller portion of the scene on your sensor, so you can see this part in more detail. But anything that is between you and the object will still distort and disturb the image. Haze, fog and dust will be working against you. So unless you have very clear air you will not really see further, but magnify a small portion of your view including all its problems..
|Full frame focal length||Field of View||Comment|
|20mm||82°||very wide angle|
|50mm||40°||35mm on an APS-C|
|500mm||4°||way too long|
What happens with APS-C
The focal length to field of view calculation is a pure geometric calculation. In an APS-C camera the sensor is smaller. So a lens with (say) 35mm focal length projects onto a sensor that is 1.5x smaller. Physically the focal length stays the same, but on an APS-C camera, a 35mm focal length results in a field of view that is the same as a lens with a focal length of 1.5x larger (roughly 50mm, or 52.5mm to be exact).
What is very annoying is that lenses made for APS-C cameras usually have the real focal length in APS-C, and lenses that work on both have the focal length expressed for full frame cameras. This is why Nikon 10-24, an APS-C lens, has it written as 10mm - 24mm although its field of view equals to a full frame lens of 15mm - 36mm, while the Nikon 200-500, has it the other way around, on an APS-C camera it has a field of view that would be like a 400mm - 750mm lens.
- For shorter focal lengths, a small change makes a lot of difference. 20mm, 24mm, 28mm are quite different.
- Longer focal lengths, the difference is not that dramatic, 200mm, 300mm are not so different.
- By adjusting your distance to your subject, you can get the image to be the same size using different focal lengths. So you have many possibilities, to have (for example) a portrait. You get a more natural view with longer focal lengths.
- For a portrait, rule of thumb is to be 20x the focal length away. So 50mm lens 1m, 200mm lens 4m.
- Another rule of thumb is that you need at least an exposure time of 1/focal length you are using. So with a 50mm lens you need 1/50s exposure time. This will change form person to person, but the rule is good to keep in mind, especially if you are concerned about sharpness.
These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.