DA*300 + AFA x1.7

Algi, this and your subsequent post are rather missing the point. Nowhere did I say that you can't use TCs with zooms - just that they work much better on primes as there is no issue with in-camera SR and often very little compromise on quality. Just out of thin air you could say : minus 10% with a prime and minus 20 to 30% with a zoom (depending on the zoom & the TC) . Sigma of course just want to sell more TCs / lenses !

As far as the Canon you quoted is concerned - isn't that one of the world's premier zooms of any FL ? In-lense IS along with their own TC ? Don't know if those three factors make a difference - I would guess they do.

Fact is you could keep bringing up links to some great shots or a report showing X or Y combination works - but the fact is that the variable FL is an issue that's not easy to overcome and so the better the lenses (esp. primes) then the greater the likelihood of getting good shots out of the combination - the 55-300 is hardly a premium lense, such as the Canon 70-200/2.8 though, and as I said earlier, even just a x1.4 is going to mean @300 it's now over f8, use a x.1.7 and it's f10 and x2 it's f11.6 !! None of though are conducive to great wildlife / bird photography.

John, maybe that's something to do with the K20D ? I don't have that problem with the K5 or K7 (see attached) -although the camera no longer recognises the lense as a DA*300 it retains the FL I've input and adjusts SR accordingly.

[IMG]http://i53.tinypic.com/2m6289e.jpg[/IMG]

Not really strange and absolutely true ... you need to guess and then input via the menu (or switch your camera off and on in between every change of focal length) the FL with a zoom if you want your SR to work accurately and not affect (blur) your shots ! Naturally this isn't an issue with primes, which generally (and that's a helluva generalisation) have better IQ anyway so the slight loss in IQ is less apparent.

Most Pros use TCs with long primes - Pros don't often use long zooms at sporting events (and definitely not with TCs which would make the lenses too slow - in two senses of the word), just look at the plethora of white at any football match !

Can't argue with that shot Algi, lovely sparrows you have up there, but that's hardly the typical use for a TC !

Normally people want the extra length to capture birds or wildlife that would otherwise represent a little dot in the shot, being easily mistaken for a speck of dust ! The 55-300 is slow enough without a TC - add even just the x1.4 and you're looking at f8 minimum and it's an absolute dog to achieve focus in anything less than great light. Add in the variable focal length - and that's why TCs are not recommended on the 55-300 (especially since the 55-300 achieves it's best sharpness at between 200 & 260).

As someone else said earlier - TCs are primarily made for primes.

Anyone who has seen the work of Lord V. (and all the other top macro shooters mentioned in a thread recently) knows this is an inane statement *pentaxexpert sets the hook and bends into the fish*

This (one of my better macros) of course was all due to the lense and shallow DoF and had sod all to do with stalking / angles / light / patience / imagination and execution :

Is that from personal experience ? It's not my experience at all - the zoom doesn't like TCs (this is well documented) and the IQ is awful.

That's a little misleading considering Pentax don't currently make 400 or 500mm lenses and I don't know anyone who has a 500/2.8 - it would be absolutely massive (about the size of a small car) !

As I said earlier the AFA x1.7 works superbly on the DA*300 = 510mm for around £1,200

I have the DA*300 & both the Tamron x1.4 (PZ version) and AFA x1.7 - I use the x1.7 quite often, it's a great combination most of the time.

This is mostly true - but not always possible. A longer lense is sometimes an absolute necessity.