Cheers mongoose.

The pictures should link to the flickr pictures, which also have the full sized ones available. Im not sure what aperture they were, but think they were all mostly around thr 5.6 mark.

Thats wide open for this lens at 300mm so you wont be getting its best work. F8-F10 is the sweet spot for most lenses, but obviously youll have to ballance this against slower shutter speeds or higher ISO. Close ups of living things are tricky! which of course is what makes them fun, and adds to the satisfaction when you manage to get a good one.

Another good reason for stopping down when close in is that the depth of field is tiny at such close distances. It takes some doing to get the focus and angle just right so that enough of the critter is in focus that it doesnt notice. Particularly with something the size of a dragonfly, getting the whole thing in focus is often at odds with an interesting composition.

yes, the f/ numbers are focal ratios, focal length/aperture diameter.

A 50mm f/2 lens has a maximum aperture of 25mm (50/25=2). Stop it down to F8 and youre only using the central 6.25mm (50/6.25=8). This helps with various lens problems because:

1. its easier to polish a lens to be perfect over a small area than a large one

2. the approximations used in calculating lens formulea are more valid at the centre than at the edges

so by only using the centre of the lens you get a better result.

The apparently counter intuitive f/ value scale allows easy comparison between lenses of different focal lengths. Its much easier to conform to the way light works and have to learn the slightly funny numbers than to try and force a "sensible" scale on it and then have to re-learn each lens separately because a 25mm aperture on a 50mm lens (f/2) is a lot faster than a 25mm aperture on a 200mm lens (f/4).

This is also why your zoom lens is 70-300 f/3.5-5.6, because the focal length is changing, so does the focal ratio. Obviously in reality its a bit more complex, and the aperture of the lens actually changes slightly as you zoom. Some (usually more expensive) zooms have "constant" apperture which actually means the aperture changes as you zoom to fully compensate for the change in focal length. The more you look into this the more complex it gets!

for ease though, smaller numbers = more light, one "stop" = twice as much light.

yes, its all a bit of a trade off.

I when shooting this sort of thing I tend to end up at about ISO 400-800 with the lens closed down to F8. In the case of my Tamron macro, its actually pretty good right from wide open (F2.5), but at that setting the depth of field is essentially zero.

You do end up with more noise this way, but you also get higher shutter speeds and therefore a better hit rate as far as motion blur goes. This is where the shake reduction on my Pentax really comes into its own so you may have to push a bit harder with your Canon. Still the Canon sensor is supposed to be pretty good for noise so you may find that ISO 1600 produces acceptable results.

try Neat Image

its pretty good for noise reduction. The demo version is free for non-commercial use and isnt crippled particularly badly.