K5 Lens dilemma for wildlife

Hi guys
Snappy new year to you all.
Can you guys help me with some lens advice.
I am currently using the K5 with Pentax 60-250mm lens mainly for wildlife/birding photography.
The lens is a cracking lens but i am finding that I have to crop quite heavily on a lot of my images and am wondering if i would be better selling this lens and buying something with more reach eg. Sigma 50-500 or 150-500.
My main concerns are image quality and the Sigma lenses slower aperture.
How does the AF speed /accuracy compare?.
Do you think I would regret selling the 60-250mm ( I can only afford to have one long zoom).
Thanks in advance for any guidance you can give me.
Regards
Andy

Have you considered investing in a good quality teleconverter, a 2X would take you to minimum aperture of f8 and give you an effective 120-500 lens, the f8 shouldn't be a problem with the K5's ability to handle noise at high ISO
Perhaps you could get some feedback from anyone who's used the 60-250 with a T/C.

Andy, you've got some answers in other topic you have created here, if you use search tool here you will find plenty of answers to your question, also I see that you are asking the same question on dpreview... why not look through info already posted on the board...

Very difficult question.

I have the 50-500mm (old non OS version) and the 60-250mm .. to be honest I wouldn't get rid of either but on the other hand I have lately been thinking of selling the Sigma just to upgrade to the latest OS version.

Not really sure if it would be worth the extra outlay though, the old version really is very good, especially on the K-5.

Maybe worth looking for a used 50-500mm?

Couple of pics from mine.



Bournemouth Air Festival 2011 by Mike.Pursey, on Flickr



Bournemouth Air Festival 2011 by Mike.Pursey, on Flickr

I had thought about using a T/C with the 60-250 but when i contacted SRS they said that there wasn't a T/C which works successfully with the 60-250mm

You need either a Pentax AF 1.7x TC (around £250) or a Kenko 1.5x/Tamron 1.4x that has the contacts for SDM (around £150). There are a couple more but tbh they are even harder to find.

both would have to be bought used.

Thanks Mike
Do you think the teleconverter option is the best way forward?
Any clues as to where i can get one in the UK?

Anybody know if this teleconverter would work with the 60.250mm.
http://www.srsmicrosystems.co.uk/348/Kenko-Teleplus-DG-1-4X-MC-Teleconverter---P...

No it won't.... as Mike has already said no new converters
are suitable it has to be SH and I haven't noticed anyone
raving about one with a 60-250mm

No, I would try to find a used 50-500mm or 150-500mm and keep the 60-250mm .. otherwise sell the Pentax lens and buy a new OS 50-500mm.

I don't use my teleconverter with the 60-250mm the af speed is bad enough on it's own but add the TC and it takes forever. If you want one though you can post an ad on the forum and keep an eye on Ebay.

No it won't.

There is a 50-500 on ebay link

Cheers
I am already watching it!!

Choices at the long end for Pentax aren't that numerous. This thread was very useful :

link

It's such a pity that it didn't include the SIGMA zooms!

I'm currently tempted to change my 120-400 for a DA*300 - or go mad and get a SIGMA 300mm f2.8!

Great link Martyn, Thanks.
It looks like I need to track down a Tamron Teleconverter.
Let me know if you hear of one.
Cheers
Andy

The Tamron 1.4x converter is pretty good (excellent even!) and you only loose one f stop. I can vouch that they work fine with SDM as I have one. The SRS one won't.

The converter is a much lighter option than adding a Bigma if you're out in the field humping your gear about but optically a lens without a converter should normally be better. But the 60-250mm sounds a pretty good zoom (I haven't had one to try) so may well work just as well if you can find a Tamron/Kenko 1.4x converter to try on it. It takes up little space and weight and can always be used with any tele lens (they are not so good with wide angle lenses.)

John