mm vs. degrees

nxthree

So this has really been bothering me lately. Other than a marketing ploy to get you to think you have something great, why are lenses measured in focal length (mm) instead of their angle of view(degrees)?:scratch

e.g.: A Sigma 10-20 EX DC HSM has a 102.4 degree angle of view... while the new Nikon 10-24 ED AF-S DX has a 109 degree angle of view. Both lenses are 10mm on the wide end, but have a very large difference in their angle of view.

So if the angle of view is a better measurement, why not use it?:dunno

Just a rant.

Thanks!

Brian

mercphoto

nxthree wrote:

So this has really been bothering me lately. Other than a marketing ploy to get you to think you have something great, why are lenses measured in focal length (mm) instead of their angle of view(degrees)?

e.g.: A Sigma 10-20 EX DC HSM has a 102.4 degree angle of view... while the new Nikon 10-24 ED AF-S DX has a 109 degree angle of view. Both lenses are 10mm on the wide end, but have a very large difference in their angle of view.

So if the angle of view is a better measurement, why not use it?

Whether angle of view is a "better" measurement depends entirely upon who you are speaking to. The optical engineer thinks in terms of focal length. Also the focal length of a lens does not change based on the size of sensor or film the image is projected onto. In that respect its a constant. Imagine Canon trying to market their 200mm/2.8L lens for example. Do they advertise the field of view based on their full-frame 1Ds and 5D line? Based on their 1.3 crop factor 1D line? On the 1.6 crop factor x0D series? On the other hand that lens is ALWAYS a 200mm focal length lens, no matter which camera it is attached to.

nxthree

Good point... I guess it matters more for the wider-angle lenses. It was mainly the two completely different 10mm crop lenses that caught my eye, because of the extreme variance in the angles of view. I guess it's a buyer beware type thing. If you're going wide, check the angle of view against other lenses before purchasing.

mercphoto wrote:

Whether angle of view is a "better" measurement depends entirely upon who you are speaking to. The optical engineer thinks in terms of focal length. Also the focal length of a lens does not change based on the size of sensor or film the image is projected onto. In that respect its a constant. Imagine Canon trying to market their 200mm/2.8L lens for example. Do they advertise the field of view based on their full-frame 1Ds and 5D line? Based on their 1.3 crop factor 1D line? On the 1.6 crop factor x0D series? On the other hand that lens is ALWAYS a 200mm focal length lens, no matter which camera it is attached to.