TS wrote:
>
> I am trying to calculate the focal length of the lens in my digital
> camera.
>
> I understand that a smaller number such as 18mm is wide angle and a
> larger number such as 200mm is a telefoto lens.
>
> If I calculate the angle of view of the camera, can I calculate the
> focal length?
> If so, how  and do I use the horizontal angle or the vertical plane
> (assuming the camera is used in landscape).
I cannot determine from your post exactly what you want to do, but I
am going to assume you want to determine what focal length you are
using (or need to use) in a given situation.
To do this, you will need three measurements, which are as follows:
1. The horizontal (or vertical) size of an object you're going to take
a picture of.
2. The distance from that object to your camera when said object
completely fills the image horizontally (or vertically if you used
vertical in #1 above).
3. The horizontal (or vertical, if you used vertical in #1 above)
size of the sensor in your camera.
Items #1 and #2 must be measured using the same units (i.e. both of
them are measured in feet, or both of them are measured in meters,
or furlongs, or whatever you want, as long as the units are the same).
Item #3 should be measured in millimeters. Most manuals for cameras
list the sensor dimension in the specifications section.
Take the distance from the object to your camera, divide it by the
object's size (#2 / #1). Multiply the result by the size of the
sensor, and you have a pretty good approximation of your current
focal length. For wide angle lenses, the calculation is less
accurate, but it should be good enough for most purposes.
For example, suppose I want to find the focal length that lets me
just fit a one foot ruler in the frame while standing ten feet away.
#1 = 1 foot
#2 = 10 feet
The camera I'm using is a Canon 300D. Its sensor measures about
22.7 millimeters horizontally.
#3 = 22.7
#2 / #1 = 10 feet / 1 foot = 10
That result multiplied by #3 gives us 10 * 22.7 = 227 millimeters.
At ten feet, I would need a 227 millimeter focal length to fill the
frame of my 300D with a ruler measuring one foot across. I would
probably use a 200mm lens.
> Is this calculation an absolute or does it differ for 35mm cameras
> and digital cameras?
As long as you completely fill the frame with the object you've
measured, the calculation above should be independent of the type of
film or size of sensor. Crop factor is irrelevant for the calculation
above.
BJJB
