[Ben_Edict]

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The brand of this lens is Opteka, Japanese Apprently. Any good? They're cheap as, prob terrible quality but I could be wrong perhaps?
Here's the Link By the way: 650-1300mm Lens for Pentax K10D K20D K100D K110D K200D - eBay Digital Camera Lenses, Lenses, Photographic Accessories, Cameras. (end time 20-Dec-09 23:06:26 AEDST)

O.k. this is a very simple refracting zoom lens, just as most other lenses, except for the fact, that it is unuseably slow at the long end, does not have a diaphragm (no change of the aperture, then), and just look at the zoom "mechanism", which is bound to flex etc etc.

Even 20 USD would be asked too much for this.

The joke is, that they even recommend (on the Opteka website) a teleconverter for that lens....

Ben

[Ben_Edict]

Hmm so the brand itself is bad? Would you advise me to buy a Teleconverter or a Mirror lens if I wanted an extra reach? I have a Sigma AF 100-300, a Tamron 70-210, and a Sigma 70-200. All 3 are quality lens and work very well for me so far.
But the shots I saw in that mirror lens thread certainly makes it a bit more interesting. So down to it, what would be better a Teleconverter 2x or a 500mm Mirror lens? They cost about the same from what I've seen...

Whether these lenses are useable with a tc depends. Which Sigma 100-300? Is it the famous 100-300/4, than a 1.4x Sogma Apo tc will give you a 420/5.6, which is o.k. and provides good IQ. The same is true for the Sigma 70-200/2.8, which can used with both Sigma Apo tcs, the 1.4x and 2x, with great results. This is soemthing I could recommend. But if you have slower zooms, I would not add a tc to those for a number of reasons.

A good mirror lens (Sigma600/8, Tamron 500/8 or the old tiny Tokina 500/8) would be better alternatives in that case, especially if you can live with the ring bokeh.

Ben

[Marc Sabatella]

Hmm so the brand itself is bad?

Not necessarily; they do't actually make lenses, but rather stick their name on lenses imported from various other sources. The problem isn't the brand, it's the very nature of this type of "simple hollow tube" lens. As I mentioned before, they provide almost all of the disadvantages of mirror lenses, but with more CA and other IQ problems, and are much larger. I really can't imagine any reason why one would choose such a thing over a mirror lens if one is looking for an inexpensive lightweight lens of long focal length.

So down to it, what would be better a Teleconverter 2x or a 500mm Mirror lens?

A 2X teleconverter causes you too lose 2 stops of light, turning an f/4 lens into f/8, or an f/5.6 into f/11. That plus they usually degrade IQ terribly. Between the two, the mirror lens will generally do better. Not necessarily all *that* much better than simply cropping the results from your 300mm lens, though. there is no free lunch here - if you want really high quality results at longer focal lengths than you currently have, you are going to have to pay for them one way or another. There are quite a few others threads on this forum regarding the various options - usually with the word "wildlife" in them; you might want to check some of those out. The various 400/5.6 lenses are potentially worth considering too.