Geoff, another member mentioned looking up the term 'diffraction'. This was an attempt to explain why a lot of photographers would choose an
aperture of f/16, rather than f/22 to get optimal
sharpness for a landscape image. When you use a really small
aperture, you start getting diffraction occurring. Someone correct me if I'm wrong, but I believe this is where
light is bent when passing through a small opening. What this means for your images is that rather than being nice and
sharp, you'll end up with fuzziness all over the frame.
You may want to check this page out here -
http://www.secondpicture.com/blog/di...apertures.html
It has some really good examples showing the fuzziness from diffraction, compared to using some larger apertures.
After searching a little, I found one person suggesting that diffraction starts occurring with your 18-200
lens at around f/8. Which means if you
stop down to f/11 or further, you will actually end up with images that are less
sharp than at f/8. Diffraction gets worse as you
stop down further, so the diffraction occurring at f/11 won't be as bad as the diffraction occurring at f/22. So it can be a bit of a compromise between how much
depth of field you want, and how much diffraction you're prepared to live with.
I know you didn't ask about this, but judging by your questions it may be of interest to you. If you want to maximize the amount of in focus area within your photos, something you'll be interested in calculating is 'hyper focal distance'. You can calculate it on the net here -
http://www.dofmaster.com/dofjs.html
Just put in the details of your camera, and what focal length you're going to use (ie 18mm) and what
aperture you're going to use (ie f/11 or f/16). Subject distance doesn't really matter at this point, but change the feet to meters if you're more comfortable with those measurements. Then press calculate. You will see Hyperfocal distance listed on the right hand column, near the bottom. With 18mm at f/11, on a
canon 40D, this distance is 1.53m. So when you go out next time, try setting your camera with those settings, focus on something that is about 1.5m away (keeping in mind you'd rather over estimate and focus on something that's further than 1.5 than something that's closer.)
Find the hyperfocal distances for 18mm at f/8, f/11, and f/16, then try using them out in the field, each time focusing at the corresponding hyperfocal distance. You might be surprised by your results as to which
aperture settings produce the sharpest images. Another listing in that
depth of field calculator is the near focus limit. I checked how that changes between f/8, f/11, and f/16, you only gain an extra 40cm each time you make the
aperture smaller. Considering the near focus limit at f/8 is 1.76m, you don't actually gain any extra benefit by making your
aperture smaller, unless you have something you want in focus closer than 1.76m...
Phew - that turned out a bit longer than I expected... hope it's helpful - if anything's not clear, feel free to ask
Luke