So could you shoot video at a constant f/16 or f/22 and get most of the scene in focus? I think so. The downside is that you will need a ton of light in order to pull this off. It will also be easier if you’re shooting with a wide-angle lens rather than a telephoto lens, since you’ll naturally have more depth of field with a wide angle lens. f/11 might be a better choice.

You’ll still need to focus, though. Look up “hyperfocal distance” and learn what it is, then set your lens there.

In general, the focal length of the lens and the aperture of the lens are independent of each other when you’re using them. You can set the lens to f/8 at 18mm, then zoom to 30mm and change it to f/11. However, the one place where this sometimes doesn’t apply is when you’re talking about the maximum aperture of the lens– the widest it can possibly go.

Remember that the f/stop is a ratio– the focal length of the lens divided by the size of the lens opening. At 55mm and f/5.6, your lens has an opening of around 9.8mm. (55/9.8=5.6)

At 18mm, it only takes a 5mm opening to get f/3.5. At 55mm, it would take a 15.7mm opening to get that.

Now think of those numbers in terms of the width of the lens barrel, and the size (and cost) of high-precision pieces of ground glass required to make a lens opening that wide. The kit lens is physically very small, which means it can be made inexpensively. In order to get f/3.5 at 55mm the lens would have to be physically much larger, and cost more to make.

There are lots of lenses out there that have constant max aperture through their entire focal range. That’s because the lens manufacturer has worked pretty hard to change the size of the maximum opening at shorter lengths such that it all remains constant when you use it.

I’m not sure if this helps or not. It’s really a hard thing to explain, but once you get it you’ll have an “aha!” moment.

As explained by you,
at 18mm the max f is 3.5
at 55mm the max f is 5.6

I always thought that the range of the focal length is within 18-55mm and
the range of the f is within 3.5 – 5.6 , and both are independent. So I have been wondering why can’t I have the setting of 55mm f/3.5 ?

Yes, that’s exactly right! Write those two sentences on a card and put them next to you while you go play around with the camera and try different apertures. It won’t take very long at all before you no longer need the card– it will all just become second nature.

small f/ = small area in focus = big hole for light

big f/ = big area in focus = small hole for light

Is that right?

http://www.dofmaster.com/dofjs.html

One thing to know is that one third of your depth of field will be in front of your focus point, and two thirds will be behind.