A Nikon P900 has an effective focal length of 2000mm. What's the farthest away that I could get an object in focus with it?
With only a very few exceptions, every lens on every camera can focus to infinity - i.e. you can focus on an object arbitrarily far away. Another way of putting this is "focal length is not what you're thinking it is" - for that, see What is focal length and how does it affect my photographs?.
The Nikon P900 allows you to get really close because it has a “micro” mode. You can focus on objects 10mm before the lens (about ¼ inch) -- that’s super close. Of course the camera sharp focuses on objects as far as the eye can see, like the moon or stars or planets. We call this far distance “infinity” and we often write it using the symbol ∞.
More important, this camera sports a super zoom lens that covers a magnification range from wide-angle thru “normal”, to telephoto. The telephoto range of this camera is extraordinary. The actual focal length range is 4.3mm thru 357mm. Because this camera has a subminiature sensor, the actual focal length values fall on deaf ears. That being true, the industry makes a comparison. The most familiar camera format is the venerable 35mm film camera which has been in continuous use for about 100 years. Using this format as the comparison, it is industry practice to restate this range as 24mm thru to 2000mm. That’s a huge range; it extends from wide-angle to a high magnification telephoto.
How high is high? The “normal” focal length on this equivalent ruler is 50mm. Now your camera zooms in to 2000mm. That’s 2000÷ 50 = 40 written as 40X. In other words, at maximum zoom, your camera magnifies objects 40 times. A bird 1 mile (5280 feet) away appears as if it were only 30 feet away. In metric terms, a bird 1 kilometer away, images as if it were only 25 meters distant.
What can you see using a 40X telephoto? The mountains and craters of the moon, the rings of Saturn, a bumble bee on a flower across the meadow. In other words, the zoom range is super-duper.
Focal length is the distance between focus (substantivum) and the optical centre of the lens. It is F1-C and F2-C distance in the figure. The shorter focal length the more the light beams bend when passing the lens.
On the other hand, picture in focus (adjectivum) and to focus (verb) are related to status of scene (part of the scene; detail) projected on the plane (film, CCD chip, CMOS chip, etc.) When something is projeceted sharp on the chip (Case of DSL) we say it is in focus. AF mode means that the camera will focus on selected point, ie. it will try to make the point projected sharp and the chip.
It's easy to see that the closer the object is, the lager image is projected and the further the centre must be from the chip. The further the object is, the smaler is its projection and the closer it is projected to the focal point F2.
For camera and lens design it means:
- It is easy to make distant object be in focus.
- There is mechanical limit for object too close to be in focus.
- Macro lenses are of special design - their optical centre is further from the mount or they allow moving the centre further form the chip.
- One can easily reduce the limit by adding special ring between camera body and the lens sacrificing the ability of focusing on further objects. Such rings ale called Macro rings.
- Typically, the optical centre is in different position than the geometrical centre. Usually the optical centre is closer to the chip than the geometrical centre is. Therefore turning the lens in opposite direction allows us to focus super-close. Sadly, usually to one distance only.
If there are no modifications to the lens, the furthest point you can focus on lies in infinity regardless on focal length (ZOOM number). The closest possible point in focus is challenging there.