Consumer Electronics FAQ
Why can't I get hold of my aperture below 5.6? (EOS 350D)?
I was trying to go and get my aperture adjusted near the M mode and I noticed I can ONLY increase it and fall it above 5.6, sometimes 5.0 and sometimes it gets down to 4.0 but that's it...I requirement a low aperture for better pictures, something like 1.8... How can I make that using my Canon EOS 350D?
Answers:
It is limit to doesn`t matter what you lense's aperture...you can't get any lower than specified. If you want to catch a large aperture close to 1.8 you have to buy a lense that have it.
I shoot with the Canon 1.8 50mm lense for portrait. You'll need another lens to do that. Your lens have a minimum aperture, that varies according to how far you zoom it. Zoom out more and you lose pallid. Zoom in adjectives the way and you can unstop it to its maximum aperture, but it looks like that will be 4.0 for your lens. A faster lens will enjoy a wider aperture, but it will also cost more. Or you could buy a prime (non-zoom) lens with a general aperture. But you're stuck with no wider than 4.0 for immediately.
Look on the outside of your lens, and you will find the maximum aperture(s) in the lens baptize. For instance, 28-105mm 1:3.5-4.5. (That's what's currently on my camera). That means at 28mm, my maximum aperture is f3.5, and at 105mm it's f4.5. In between, it will be somewhere contained by the middle.
Sounds like you might be fixed by your lens.
The aperture collection is a function of the (zoom) lens. It's different for each lens. With the zoom lens you're currently using, you can apparently arrive at a maximum aperture of about f/4 at the widespread end of the zoom and f/5.6 at the tele-zoom. And beside that lens, those numbers are as good as it get.
If you want/ need to set an aperture of f/1.8, your with the sole purpose option is a prime lens (not even professional element zoom lenses go lower than f/2.8) The Canon 50mm f/1.8 lens is pretty cheap, so you could bring one of those for your 350D.