Confusion About Fixed F/stop Lens

A friend emailed: Hi Kathy. I have a question and I can’t find an answer on the Internet. I am looking at a lens with a fixed aperture of f4. What I’m wondering is how do you get more depth of field with a fixed aperture? People are singing praise for this lens and report it is on their camera all the time. But I’m wondering how it would do for landscape where you would want everything in focus. Or if you were focusing on a closer object and had mountains in the background. Any thoughts? Thanks

The Canon 16-35mm f/4 lens has f/stops from f/4 to f/22

My Answer: A fixed f/4 lens might have an aperture range from f/4 to f/22.  You’d use the f/4 to blur backgrounds and the f/22 for landscapes.  The lens has lots of f/stops and not just one.

A “fixed” lens doesn’t change the f/stop as you zoom the lens.  In the Canon 16-35mm lens, zoom back to 16mm and you can use f/4.  Zoom out to 35mm and you can still use f/4.

A “variable f/stop lens” would change the f/stop as you zoom.  In the Canon 18-55mm lens, zoom back to 18mm and you can use f/3.5.  Zoom out to 55mm and you can only go to f/5.6.  The f/3.5 is no longer available. 

Traditionally, “fixed” f/stop lenses give crisper and clearer photos.  Fixed f/stop lenses are usually more expensive and better made.  I think that’s the reason they give a better photo.