How do I get the 'equivalent' focal length for a DSLR lens for my camera?

Question

On my Nikon D3000, how do I figure the focal length equivalence? In other words, if I want a lens that is just like a 50mm lens on a 35mm film SLR, I need to multiply by a factor. What is the factor, and how is it derived?

Also, when I buy a new lens for it, and the box says 70mm-300mm, I assume that's the true focal length of the camera and not the equivalent, so I have to apply the factor. Is that correct?

Answer

What is the factor, and how is it derived?

The factor (also commonly called a 'crop factor') is a measurement of how much larger a full-frame image sensor is than the sensor in your camera. People say the D3000's crop factor is 1.5 because a full-frame sensor is 1.5x larger than the D3000 sensor.

A full-frame sensor is 36mm x 24 mm. The sensor in the Nikon D3000 is 23.6mm x 15.8mm.

Comparing the sensor sizes along each dimension:

36mm / 23.6mm = 1.52 (approximately)

24mm / 15.8mm = 1.52 (approximately)

You can see how the full-frame sensor is around 1.5x the size of the D3000 sensor, hence the 1.5x crop factor.

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