Explanation of lens info

Discussion in 'Canon EOS' started by fusionrx, May 29, 2003.

1. fusionrx

On the lenses of cameras I see that they have a set of numbers
regarding the f-stop. In this case I see the following:

Fish-eye lens
1:3.5-4.5 <-- This is what I have questions about, see below

17-28 mm

Now, the F-stop has adjustments from 3.5-22.

My question is what does the 3.5-4.5 represent? Is that an optimal
setting for the lens?

Thanks for the help.

Dan Magnusson

fusionrx@hotmail.com

2. nicol_verheem|1

The f3.5-f4.5 is the maximum aperture at the extreme focal lenghts. In your example's case it is not constant, i.e. the maximum aperture at 17 mm will be 3.5, and at 28 it will be 4.5. So the range at 17mm will be 3.5-22, and at 28 it will be 4.5-22 (or maybe 27).

If you get amore expensive fixed maximum aperture lens (e.g. the 17-28 F2.8L) the maximum aperture is a constant 2.8 throughout the zoom range.

nlv

3. joe_chott

I am by NO means an expert, but from what I know, it works like this:

1:2.8

That would mean that the lens has a minimum F stop of 2.8 at all focal lengths (or it could be a prime)

1:3.5-4

That would mean that the lens has a minimum F stop of 3.5 at its minimum focal length and a minimum F stop of 4 at its maximum focal length.

In a more concrete example:

17-40mm f/4L

This lens has a minimum F stop of 4 at ANY focal length - the minimum is the same for all focal lengths.

28-135mm f/3.5-5.6 IS USM

This lens has a minimum F stop of 3.5 at a focal length of 28mm, and a minimum F stop of 5.6 at 135mm. I currently have NO idea how to know (other than trying the lens) what the F stops are for other focal lengths in between the two extremes.

Hopefully this helps you.

-Joe Chott

P.S. If I am wrong on some of this, please, some pro, help me out - I don't wanna steer this guy wrong, but this is the way I understand it.

5. fusionrx

Thanks for all the answers!

Dan