What does the apperature settins mean for a lens?

[GoingUp]

Please help me make sure I'm understanding this correctly...

With a lens like the Canon 70-200mm f/2.8L USM EF, does that mean it can do a max of 2.8, but through the entire zoom of 70 through 200?

Then with the Canon 28-135mm F/3.5-5.6 IS USM EF Lens, the max app is defined as 3.5-5.6 but its limited depending on where you are in the zoom? Is this correct?

If the above is correct, and the app is a range, how do you know where in the zoom youre limited to a certain app?

 

[fuzzybabybunny]

Yes. The 28-135mm f/3.5-5.6 would be f/3.5 at 28mm and f/5.6 at 135mm. There's really no way to know what the maximum aperture is in between 28mm and 135mm besides looking for a chart with this information or testing the lens out yourself.

Chart:

http://www.photozone.de/8Revie...8135_3556_is/index.htm

Look at the MTF50 graphs. Looks like at 70mm it's max aperture is f/5.