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Focal Length: A Fine Point

Let's say you have a powerful telephoto zoom.

You zoom from 400 mm to 800 mm.

It's a change of 100%.

Let's say you put a wide-angle zoom on your camera.

You zoom from 12.5 mm to 24 mm.

Even though you didn't move many numbers (12.5 to 25), it's the same percentage change as going from 400 to 800: 100%.

So, the takeaway is:

• Wide-angle focal-length numbers are small numerically—but represent a large change in magnification.

• Telephoto focal-length numbers are large numerically—but represent a small change in magnification.

The chart below is another way to make the above fine point.

Chart

The focal lengths below are for full-frame digital SLR cameras.

50 mm is considered the normal focal length, i.e., the scene is not magnified.

Not magnified means that the 50 mm focal length isn't wide-angle or telephoto.

The power value for the 50 mm focal length is therefore 1x.

You can see below that going from 12.5 mm to 25 mm is a change of 100%.

You get a big change in magnification by moving very few numbers.

Going from 400 mm to 800 mm is the same change, 100%, but its a bigger change in the numbers.

 

Power Change

From 50 mm

% Change

Between Rows

12.5 mm 1/4x 100%
25 mm 1/2x 100%
50 mm 1x 0
100 mm 2x 100%
200 mm 4x 100%
400 mm 8x 100%
800 mm  16x 100%