I've done quite a bit of reading lately and I realise with a TC I'm going to lose at least 1 stop of light, fine, but I'm also concerned that I retain autofocus function and don't lose too much image quality.

I've read recently that I should really consider the sigma 1.4 EX DG if I'm using a sigma lens as they are designed to be matched together.

alternatively, the Tamron PZ-AF MC 1.4 and the Kenko PZ-AF 1.5 are talked about quite highly, however, the Tamron appears to be double the price of the Kenko but I have no idea if it justifies that price difference?

I did read one review that raved about the Tamron and said it lost no quality at all which I just don't believe. I realise there are lots of Teleconverters and you really do get what you pay for, but retention of autofocus is a big plus for me and I don't want to go beyond 1.4-1.5 as far increase in mm goes.

can people comment on the Kenko and tamron at all? is the tamron worth £130 notes more in price?

having talked big lenses for airshows I'm just going to have to face that I can't afford a sigma for a while, cry into my cuppa and leave it at that.

so casting round I still also have to face the fact that the Pentax DA 55-300 lens while delivering a nice picture in good light, hunts like a dog when pointed at the sky.

so, my view with my limited pounds is this.

get a new tele 300 lens which is faster focus. here I am at a total loss. I like the fact that the Pentax starts at 55 so at a show I can still get stuff passing by relatively close.

sigma do a 100-300 but seems to get slagged for plastic construction, but then the Da55-300 is as well so that doesn't bother me. but is the sigma fast?

alternatively, can anyone suggest a suitable 300 for airshows.

Out of interest, what 1.4 TC are you using?

Unless the 150-500 or 120-400 is significantly sharper than the 50-500, then I think I have my answer. At Duxford and other places I've found I need the 55mm on my 300 for when stuff taxis past. It saves me from having to swap lenslenses rry a second camera.

At Duxford and shuttleworth, formations I tend to shoot at 180-230mm. And on the high speed passes I only dial back from 300 if they come close to the flightline.

I've also found the 55-300 hunts a lot wide open unless it's a very bright day.

At Duxford, when they come in from stage right over the tank museum, I'm wide open and only have to scale back when the lens hunts to focus and I have to reacquire the target.

Alternatively, what is there in the 300 zoom range of lenses that's nice and fast and doesn't hunt like the Pentax 55-300? I know sigma do a 70-300.

Another thing I have flirted with is keep the 55-300 and mate it with a 1.4 or 1.5 tamron or kenko pz-af tc but considering the 55-300 hunts already I'm not sure that's a smart move.

I'm a 3 year pentax user and have progressed from a K-X to a K30 in that time.

I have the kit 18-55 WR and the 55-300 lens and am now looking to expand my lens collection with a focus towards long lenses for airshows.

I find the 300 okay but would like longer reach and seem to have settled on Sigma as the lens manufacturer of choice.

Sigma 120-400
Sigma 50-500
Sigma 150-500

I have read that the Tokina 80-400 is pretty good but that's quite an old lens now and also like hens teeth in trying to find a good example. And when it does appear it's a big price. My thinking is if I'm going to pay big money, then I may as well pay big money and get a new lens with warranty etc.

I like the idea of the 50-500 as this gives me good reach but also allows me to shoot reasonably close subjects on the flight line if I need to without having to faff and swap lenses.

at Duxford Flying Legends two weeks ago I bumped into two other pentax users.

one had the 150-500 and the other the 120-400.

both agreed the 120-400 had slightly better image stabilisation built into the lens.

now obviously the 400 lens is a lot cheaper but I also read on this forum that of the 150-500 and the 50-500 the 150 is a lot sharper.

My main requirements are sharpness and reach.

I use a hama shoulder mount so I can keep the camera nice and steady.

Of the 3, does any edge it over the other? Alternatively, older Sigma 400 primes are available, what's the view on one of these.