Dumb Question

[nufan]

Ok, so when a lens is, lets say, 50mm; what is that a measurement of?

simple.

[Woolburr]

The distance between the sensor plane (image capture medium) and the focal point (also known as the optical center of the lens) when the lens is focused at infinity.

[BillsBayou]

It's also known as the "focal length" or more specifically, the "effective focal length." It's the distance from the center of the lens to the focal point (the sensor or film in photography).

I'm not into the refractive measurements involved, it's too much math for me, but if you were to actually measure the distance from the middle of the front element of your 50mm lens to the focal plane of your camera, you may notice that the distance is a bit more than 50mm (just shy of 2 inches). Zoom lenses get you even more confused when they have fixed length lenses. 70mm to 200mm zooms do NOT go from 2-3/4 inches to nearly 8 inches in length. Something more is going on here when you're dealing with the multi-element lenses we use.

Someone will come along and give up the better explanation.

50mm is also used to describe the field of view. That is, how much of the scene are we pulling in. On a 35mm FF sensor, 50mm appears to me to be nearly a zero zoom (I'm sure someone will say I'm wrong and offer up the correct number somewhere between 30mm and 70mm). Look through the viewfinder and you'll see things with no loss or increase of magnification. Increasing the measurement increases the magnification of the scene, decreasing the measurement will decrease the magnification of the scene.

Compact digital cameras will offer up 35mm equivalent measurements for their lenses. This is typically done to give you a feel for their field of view range. The Canon Powershot 2 IS gives us the following: "6.0 - 72.0mm f/2.7-3.5 (35mm film equivalent: 36-432mm)" That 6.0mm at the bottom end would be a FANTASTIC fish-eye lens on my SLR! But, oops, that's not what it says. It says the equivalent bottom end for a 35mm camera is 36mm. That's wider than zero zoom, but indoors, you'll have trouble getting a large family to fit in the frame.

If you want a good example of focal length comparisons, Canon has one at their site:
http://www.usa.canon.com/app/html/EFLenses101/focal_length.html

[Jon]

It's also known as the "focal length" or more specifically, the "effective focal length." It's the distance from the center of the lens to the focal point (the sensor or film in photography).
No. Just "focal length". "Effective focal length" is a marketing term designed to fool people into thinking they're getting something more than they really are. It's like the marketing MB, where a megabyte is rounded to be 1,000,000 bytes, not the real 1,048,576. So they short you by 5%; and GB short-change you even more.

[BillsBayou]

No. Just "focal length". "Effective focal length" is a marketing term designed to fool people into thinking they're getting something more than they really are. It's like the marketing MB, where a megabyte is rounded to be 1,000,000 bytes, not the real 1,048,576. So they short you by 5%; and GB short-change you even more.

Thanks, Jon. I'm hoping others will come forward to correct points in my post.