Not too much, Tony. (Which is just as well, as I'm by no means a post-processing wizard.) I ran Neat Image over it, using the same settings that I use for, oh, about 80% of all shots, but that's really only part of the story so far as noise reduction goes: the other part of it is to get as little noise into the picture in the first place.
As you can see from the EXIF info, I was at 1/200th (way too slow for 400mm hand-held), ISO 1600 and already wide-open at 400mm. There was nothing left by way of camera adjustments to make (ISO 3200 isn't really very helpful - on the 40D it's just an under-exposed ISO 1600 pushed a stop, which makes it very noisy), except for one thing: that is to remember that visible noise is inversely proportional to signal. If you under-expose, you get a lot more noise. If you over-expose, you get noticably less noise,
but this only works if you have enough headroom to play with.
Looking at the histogram after the first few shots, I could see that there was a little room at the right-hand end, so I added 1/3rd of a stop of positive exposure compensation. Any more would have blown the highlights, but even one-third of a stop worth of extra signal makes noticably less noise and is worth having. (Google "expose to the right" for lots of detail on this. People tend to go on and on about it as if it were holy writ, but when it comes to coping with insufficient light as best you can, ETTR really does help.)
That gave me as noise-free an image as was practicable under the circumstances, a little over-exposed but no blown highlights, so easy to correct in PP.
Normally, I'd have done a lot better in the first place by using the right equipment: the 1D III insead of the 40D because it has fantastic low-light performance (ISO 1600 on the 1D III is close to the noise level of ISO 800 on the 40D); the 500/4 instead of the 100-400 (an extra stop would have been really useful, and if I'd had something like a 300/2.8 or a 400/2.8 that would have been better again); and an off-camera flash bracket with Better Beamer (actually at this distance you don't need the Better Beamer, the 580EX II is plenty powerful enough anyway). But, for complicated reasons I won't go into now, all I had with me was the 40D and the 100-400, so it was a case of make do with the tools at hand.
In desperation, I did try the little pop-up flash on the 40D, both in normal flash mode (manual exposure, letting the flash decide how much to apply) and fill-flash mode (aperture priority, which on Canon cameras kicks the camera into fill-flash mode). There is not a lot of point to using the pop-up flash at 400mm: it's too weak to do much and anything it does do risks giving you unnatural reflections and highlights, but I had time to try a few different things with this bird, so why not? Obviously, I had to remove the lens hood as otherwise the onboard flash would not get past the shadow the hood casts.
I
should have dialed in a stop of negative flash exposure compensation, but in the heat of the moment I couldn't remember how to do this on a 40D (I mostly use flash with the 1D III which has a different control layout.)
In the end, there wasn't much difference between the shots with fill flash and the shots without. This particular one is with flash but I selected it more because I liked the pose than because of the extra light, which really only made the bird's white breast feathers look too bright in proportion to everything else. So I used the Photoshop shadow/highlight tool to roll back the highlights a bit, then went a bit further with a very light application of the burn tool. (I should probably do a little bit more again, it's still a touch hot.)
Finally, I decided that I wasn't happy with the white balance. You can tweak WB a bit with JPGs but it's much easier to do with a raw file, so I threw away my PP work so far and started again but with the raw this time, repeating my earlier steps only with better WB. I think I reduced the over-exposure a little bit in three different places rather than trying to do it all at once: in the raw converter, in Photoshop with shadow/highlight, and finally in PMView, which I use to make the final images (cropping, rotating, output sharpening, and sometimes a few other things - in this case, gamma correction).
But despite all the messing about I've detailed above, there are really only three important things I did to get a clean-looking image:
- Neat Image (something you should use for nearly all images, even at ISO 200)
- Don't underexpose, even go a little over if you can.
- Most important of all, get close to the bird! If you have to crop a high ISO image, you are always going to battle to get any detail. This picture has been cropped, but only a little, and for composition rather than to compensate for the bird being too small in the frame. I was close enough to have the bird fill the viewfinder at 400mm, and that's the real secret to low-light, I think.
I hope all this helps!