It depends what you want to photograph, really. Any of the current ~100mm Sigma, Tamron and Pentax macros are roughly equally excellent.

If you're doing macro work, then the aperture is often academic as you'll be stopping it down, but you'd want to start around f2.8~f4 to get a nice bright image in the VF.

If you want the ultimate insect lens available new, then I'd recommend the Sigma 180mm hands-down - I have it and it's great for insects and a quick change to a medium telephoto lens too.

Second hand, there's the various incarnations of the Pentax 200mm Macro (arguably the best there's been), the Voigtlander 125mm Macro, and also the various copies of the Vivitar Series 1 105mm macro.

I'd love Pentax to come out with a weather-sealed ~150mm macro - would be ideal for me

For birds, I faffed around with the various Sigma X~500mm zooms and was never really happy as the quality at the long end without very good light wasn't up to what I wanted.

For the money, I'd go with the DA* 300mm and try and get closer. Or, I'd go with Georges idea of an older 300mm f2.8 with a matching TC.

Personally, I went for the Sigma 500mm f4.5. But that's a serious investment.

The last one is an adult Assassin bug, they predate other insects. Lovely colours.

I find it quite difficult getting good photo's with reeds as a background. I don't think the lighting on the head of the mallard helps.

Hope you don't mind, but I had a play with that last JPG you uploaded and this was the best I could do:



I'd prefer to crop it more to the left so that the duck is moving into the image a little more, and tighten the crop - but that would need to be done with the original.

Unless it's one of these
http://kmp.bdimitrov.de/lenses/primes/short-tele/FA85f2.8-Soft.html

I always us SR when using my monopod. Never on a tripod.

They're quite abundant around here (Berkshire), saw four of them together yesterday performing flight acrobatics. Great birds. They glide over the rooftops scaring all the garden birds off and our parrots have a fit

I've not found them easy to photograph. Like you've found here, they have a knack of flying at just the right level to thwart the light and getting good lighting is very difficult indeed. I think you've done very well. Patience and timing is everything

My best effort so far is this one when it banked, but I'm not happy with it and am still waiting for the best light to grab one of these blighters right:
http://www.flickr.com/photos/chirpy_uk/3010847614/sizes/l/

My DFA 100mm Macro isn't particularly sharp wide open. f8 and beyond it is very sharp indeed. This is what I expected from a Macro lens. I always shoot RAW with Macro and use unsharp mask as John recommends to good effect.

Ah! My ailing memory

I used to use B+W filters. Regardless, they did cause perceived issues for me at 1:1 macro work, so that's one of the reasons I now longer bother and have never had need for them.

I don't use them. Most of my photography is out in the countryside, but I still wouldn't consider using them for the reasons Stefan gives.

If I were at the beach or other environment where corrosive elements in the air could attack the lens, then maybe I'd use them for those situations.

Otherwise, I simply use the lens hood. It's a relatively cheap piece of plastic that can be replaced, if damaged it's unlikely to affect the rest of the lens, and it has the benefit of reducing glare.

Dan may relate his experience of a shattered filter severely scratching the front lens element when it broke on dropping the lens.

Also, with macro photography and also long telephoto shots, I believe that I do see noticeable degradation of the image and light when sticking extra glass in front of the lens that the manufacturers didn't design it for. As an example, the Sigma 500mm f4.5 actually comes with its own protective front element built in - but that was designed specifically by Sigma for that lens.