How to compare the focal length of a 50mm prime lens with the default lens of a point and shoot camera?

To imagine the 50mm focal length of a prime lens 1.4F, can I simply zoom the lens of my point and shoot camera up to 50mm?

Will both the focal lengths be same?

Any other way to imagine the focal length 50mm of prime lens 1.4F when what you only have is a point and shoot camera with 70mm max zoom?

Answer

If your point-and-shoot has the typical 1/2.3" format sensor and you are trying to compare it to a 50mm lens on a cropped-sensor DSLR (in your case, a Nikon, if I recall correctly), then there's a little bit of math involved.

The compact's sensor has a 3:4 aspect ratio. It measures 6.16mm by 4.62mm, with a diagonal of 7.70mm.

The Nikon DX sensor has an aspect ratio of 2:3 and measures 23.6mm by 15.8mm. That would give a diagonal of 28.4mm.

It's normal to compare lenses based on the diagonal of the negative/sensor. I don't like that approach, though, since the aspect ratio is different. It's best to compare images with the same aspect ratio, and in the case of both of these sensor formats, the full length of the short side of the sensor will be used when an image is printed in a 4:5 aspect ratio. That is, an 8x10 picture made with either camera would involve cropping the longer dimension. So it's safe to compare just the shorter dimension of each camera/lens combination.

The 50mm lens on the Nikon is about 3.165 times the length of the shorter side (the height when the camera is held horizontally). That means that in order to get the "same" 8X10 from your compact camera, the lens would have to be set to 3.165 times the shorter side of its sensor, or about 14.6mm.

If your compact has a different-sized sensor, the math will still hold. Find out what format of sensor it uses, then multiply the shorter side of the sensor by 3.165 to find out how far out the lens needs to be zoomed to approximate the 50mm lens. Do note, though, that the field of view for our hypothetical 8X10 print is the only thing the two cameras will have in common.

Or, if you want to do it without the math, my rather standard 8-1/2" (about 21.5cm) tall head will completely, but just, fill the frame of a Nikon DX in "landscape" orientation from the top of my bald pate to the bottom of my chin using a 50mm lens from almost exactly three feet away. So a volunteer or a ruler three feet (90cm) away or a mirror 18 inches (45cm) away will be enough to show you how far to zoom.