Focal Length in Photography.
Wikipedia states “The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system’s optical power.”
Alright I guess I have lost you right at the start. Have I? OK, Let me try to simplify it further.
“It’s the measure of the distance in millimeters between the rear nodal point and the focal point of the lens when you focus the lens at infinity.”
I guess the question running through your mind right now is, how does this technical definition help me make a photograph. Well the truth is the technical definition does not but, the concept behind it does.
Do not forget to read about The rule of thirds, and Different elements of composition, which will help you to tell a story through your photographs.
Definition
A simpler way of explaining this is – It is the distance between the optical center of the lens to the capture medium (film, sensor etc) at which parallel rays of light converge to a single point . ( remember this single point, it’s going to come back later to haunt you). Also keep in mind that this is based on a hypothetical situation that the lens has a single optical element.
It is defined in mm and the number is mentioned on the lens body; say; 50 mm that means that the lens has a fixed focal length of 50 mm you cannot change it. These kind of lenses are called prime lenses or block lenses. Another lens might have 24-70 mm written on it. That means the lens has a variable focal length between 24 mm and 70 mm and can be changed to any number in between. A broader term used for these lenses is zoom lens or variable focal length lens. So are we together till now?
So why is it Important for us to know what these numbers mean? The right choice of the focal length is necessary for the subject you are going to photograph.
Rule of thumb to use focal length in photography
Landscapes are photographed using a shorter focal length ( fisheye to 40 mm).
The same rule applies for architecture where you want to showcase the whole structure
For portraits use mid range focal lengths (50 to 135 mm)
Wildlife is easier to photograph with telephoto lenses.(150 to 800 or even further 1200-1900mm which are made to order and probably would require you to sell both your kidneys).
Now before the master trainers and experts come out and shoot me down for the above points, let me clarify something. You make your own rules, shooting portraits at extremely short focal length works really well in some cases so does shooting landscapes at longer focal length. Photography is a science as well as art and do not hesitate to explore beyond the boundaries.
Pirin mountains at 70mm
A portrait of a model at 17 mm
As we change to focal length it Impacts two things directly and one sort of indirectly. What are they?
- Angle of view.
- Magnification – the change of magnification invariably affects;
- The depth of field.
So In the next article let’s visit them.
