Lens spec notation question

[vfilby]

I often see lenses described like this

EF 75-300mm 1:4-5.6

I understand the 'EF 75-300mm' part and I understand '4-5.6' is the max aperture for the focal length. What I don't understand is the '1:'. Can anyone offer an easy explanation?

Thanks,

[cecilc]

You know, I don't know if there's a direct answer to that question in these two web references or not, but there's certainly the beginnings of an answer to your question ....

I at least understood it better after reading them ....

http://www.unc.edu/~jimlee/photolecture.htm
http://www.overtonphoto.com/tech/optics.htm

Good luck and I hope that helps (a little!)....

[cmM]

on all the lenses I've ever seen, it's always a "1"
I've got no clue where it comes from.

[Belmondo]

As you know, the larger the number, the smaller the aperture opening. The '1:' is simply the numerator in a fraction, i.e. 1/4.5.

1/2.8 (or in this case) 1:2.8 is a larger number than 1/4.5, for example. This way, the relationship between the f/stop and the amount of light makes sense. Without expressing aperture as a fraction, we're acually expressing it numerically by its reciprocal, although that's what we all do. Canon is just being tenchically correct by expressing it as a fraction (or ratio).

[vfilby]

I was wondering if that might be the case. Thank you for the responses.

Regards,

[hmhm]

An aperture value is inherently a "ratio", and "m:n" is the notation for expressing a ratio (though expressing ratios as fractions is pretty much equivalent). If you consider the picture of the "simple lens" that you see in physics textbooks (that one convex glass element projecting an image onto a plane behind it), then the aperture is the ratio of the diameter of the lens to the focal length.
-harry