In any optical system, such as photography, focal length measures and calculates how light is either concentrated or scattered. It’s the inverse of a camera’s optical power. So that positive focal length means the camera is gathering light, and that negative focal length means that the camera is scattering light, or letting it diverge. Cameras with a short focal length bend light rays at a sharper angle, which brings the subject into sharper focus at a shorter distance. Positive focal length relies on parallel light rays. Negative focal length means that the subject of a photograph must be placed further away, in the distance, not close in or as a close up. Put in simple terms, negative focal length is best used for landscape photography, and positive focal length is best for portrait photography.
Professional photographers are extremely well versed and practiced in the uses of negative and positive focal length, but the subject can be studied and mastered pretty easily by anyone who is interested in improving the quality of their own photography.
Don’t get object distance and image distance confused. Object distance is the distance between the object being photographed and the lens of the camera. Image distance refers to the simple distance between the actual lens of the camera and the film (or, more commonly in our digital age, the screen that processes the light photons admitted by the lens.) In all but antique cameras this is always measured in millimeters.
Since most cameras in use today are digital cameras, and since most of the digital cameras on the market today are fully automatic, most of the focal length settings will be handled automatically by the camera itself. This means that in most cases the photographer will not have to make any calculations or measurements to take a good picture. But there are times when a photographer, whether professional or amateur, may want to adjust the focal length manually for a particular photographic effect that the automated device won’t produce by itself.
For instance in landscape photography a photographer may want to increase the focal length by a few millimeters in order to emphasize the murkiness of a rolling cloudbank coming down a mountainside. Conversely, some portrait photographers go for a shortened focal length in order to highlight facial or skin texture — throwing into relief such things as wrinkles, warts, pimples, crows feet, etc. In the hands of an experienced professional this can produce engaging and vivid portraits — but amateur photographers would be well advised to tread lightly with this technique, since not very many spouses, relatives, or friends appreciate having their exterior flaws clearly defined in a photograph.
Focal length is also crucial when background is important in a photograph. Over the years millions of photographs have been spoiled or have had their impact reduced by fuzzy and misty backgrounds, or by backgrounds that stand out more than the subject of the photograph. To avoid this happening, always check the focal length on the automatic setting with the amount of light available. And of course, you can use the retouchme editing app if you want to fix any of those issues suer fast. When the light is so bright it begins to overshadow the subject, it is best to manually reduce the focal length my one or two millimeters.