Originally posted by RockvilleBob Exactly - I can keep my excellent K-5 system for all photography except wildlife where I'd like a 500 to 600 lens. For wildlife I'd probably go with an older used Canon Pro body, one with demonstrated fast focus, and a used Canon lens. My reason for Canon over Nikon is there are more used Canon lenses available.
So I am really looking forward to the Forum test of the Sigma 500 and Pentax 560. If these lenses turn out to be high performers than I'd probably add the Sigma just because of price and a stop faster (probably equates to faster focus also.) The Pentax 560 would be an option if its performance is top notch, focus is fast and the price dropped significantly. If both the Sigma and Pentax turn out good but not top notch, then I would lean to buying a used Canon system - probably a top pro body two generations back from their newest model - more than enough pixels to make great prints with a great lens. To me being able to buy a high grade long lens is the driver. A well cared for, used Canon or Nikon will perform terrific.
I use a K-5, Sigma 500mm f4.5 combo and often get results I'm very happy with. There are two things that bother me, one is undoubtedly real and the other may or may not be. The first is continuous autofocus -- forget it -- which means that catching birds in flight or any moving subject is spray and pray. The second is that the manufacturer-provided MTF charts for the Canon and Nikon 500mm offerings look at lot better than for the Sigma. This makes me think I could get shots with even more resolution and contrast if I switched to Canon or Nikon. So, like RockvilleBob, I've been looking, comparing, and thinking.
Last week I was ready to pull the trigger on a new Nikon 500mm and a pre-ordered Nikon d7100, thinking this would give me an ideal combination of reach, resolution, autofocus, and pixel density/quality. And this combo is in the same ballpark in terms of price as the new Pentax 560, of presently unknown quality, and the K-3, whatever it turns out to be and whenever it appears. So I talked to the salesperson, struck the best deal I could, and told him I would do some price comparisons and get back to him in the morning. But by the next morning I had decided to stand pat. What is keeping me with Pentax/Sigma? Weight. The Nikon combo weighs nearly 2 lbs. more. That's going to be the difference between my going out for a long walk with my long lens, which I do all the time with the Pentax/Sigma, and thinking hard about whether I want to lug the weight.
The previous generation Canon 500mm with a pro body is even heavier than the Nikon combo. Canon has nailed the weight issue with the newest 500mm but the lens sells for over $10k. Even if price weren't an issue Canon's sensors don't cut it compared to the Sony or Toshiba sensors used by Nikon and Pentax. I don't want to pay mega bucks for the world's best long lens and then have to strap it onto an inferior body.
What does this have to do with this thread? First, Pentax beats Nikon on weight, either with the Sigma 500 or the Pentax 560. This is important to a lot of customers in the long lens market. Second Pentax beats Nikon and Canon on price with the Sigma 500. Of course you can buy the Sigma for Canikon mounts but you don't get shake reduction/image stabilization, which is a deal killer for most. Pentax doesn't beat Nikon on price by much with the 560, and that in my view is a serious and unnecessary marketing mistake. Third, the forthcoming K-3 will have to a least maintain parity with the new Nikon cropped bodies in pixel density and autofocus if Pentax is to have any chance of appealing to shoppers for a long lens who aren't irrationally committed to the Pentax brand. Fourth, resolution and contrast are really important for people who shoot birds (we want that fine feather detail), so if the Pentax 560 beats the Sigma and is the neighborhood of the Nikon and Canon 500mm on these variables, then maybe Pentax can take some market share, even at a $7000 price point.
Looking forward to the forum review of the lens.
Russ