This probably fits better under the 'Lens discussion' thread, but what do you guys shoot with? I just bought a Pentax DA* 300mm f/4 ($1,017 used). I am between that and sending it back for the sigma 150-500. Opinions?

I have both a Pentax 300 f4 and an original Bigma 50-500. Images cropped and enlarged from the 300 are easily a match for the Bigma @ 500mm (actually about 450mm) without cropping. The Pentax is also a bit smaller/lighter. I would stick with the Pentax.

Alright. Thanks.

I shoot with a number of lenses.

On the long side, Pentax 600mm f5.6 A, Sigma 150-500, Sigma 400mm f5.6 APO version 2, Pentax 300mm f4.5 F.
I shoot a number of macros and other lenses as well.

A DA*300 and the Sigma zoom are not really in the same ball park. If you don't like the DA*300 then maybe it is faulty because I would not give up mine no matter what other lenses I owned. For hand held nature photography it is spot-on.

For wildlife I mostly use Pentax FA*80-200/2,8 and the Sigma 300/2,8 and 500/4,5 primes.
I don´t think the Sigma 150-500 can provide the image quality of the DA*300.

I bought a new Sigma 150-500mm OS HSM, returned it after a couple of weeks. I now have a DA*300, and no regrets. I sometimes use the DA*300 with a Kenko 1.5X TC. Auto-focus is poor but works OK for static subjects. I sometimes go with manual focus, especially on birds in trees.

Do you mean poor all the time, or just with the TC on it?

Just got mine in the mail, and the AF doesn't seem much faster than my Takumar F 70-210 (AF is just a lot quieter) I am really hoping it is because today was very overcast and not a lot of contrast for the AF to pick up on. Also, is it just mine, or are the aperture blades funny shaped? Rather than making a perfect circle-shape thing like it is supposed to, one aperture blade seems longer/ flatter than the rest so it throws it from a circle thing to like half circle half pentagon thing. Very hard to explain, I may post photos of it if I can get any. The only other thing other than the AF that bothers me about it is holding it hand held with low light (shooting 100-200 f/4 @ISO 400).

I mean the DA*300 with TC (Kenko PZ-AF 1.5X Teleplus SHQ). It hunts easily, and stops focussing inappropriately. I often have to tap the shutter release a few times until focus is on the right spot. SDM doesn't play well with teleconverters, which may be why Pentax hasn't released the TC shown in the roadmap for the past five years.

SDM is not faster than screw drive, more often the opposite. If you find the DA*300 slow, you won't like the even slower focussing Sigma 150-500.

I don't see anything odd about the shape of the aperture blades. The first DA*300 I bought was having trouble locking focus, possibly because calibration was off. I returned it and requested a replacement. I often shoot in much lower light than you quote above, and have no trouble focussing. Have you checked for focus accuracy?

A Pentax FA 100-300 and a Sigma 170-500.

That I have not done. I didn't really know that was a thing..

My DA*300 had the same issue with the aperture blades when it was brand new. They quickly loosened up, but I can still get some quirky aperture shapes at certain aperture settings. I wouldn't worry about it too much. Give it a bit of a work out and the stickyness should soon reduce.

It is, a critical thing. There's a reason higher end cameras include AF Adjust. I've had to return brand new copies of the DA 15, 18-135 and 55-300mm for calibration. All returned perfectly corrected.

I find this chart easy to use and effective: http://www.dphotojournal.com/focus-test-chart.pdf

I haven't really noticed it to affect IQ any, however it is a used lens (in my opinion it would be classified as 'New without tags') I'll keep using it and see if that solves the problem.

I shoot mostly w/ an original Bigma and I'm reasonably happy with it except for the maximum aperture. Shots can be seen at flickr.com/pictures/avipix. For diversified shots w/ all sorts of cameras by different shooters try the flickr group "The Power of Bigma".

The comparison of a 300mm w/ the Bigma doesn't seem very useful to me; too "apples to oranges". Depends on what subjects you're figuring on, among other things.

Generally, I discourage folks hoping to shoot birds and other animals from trying to do so w/ too short a lens (typically 250-300mm) because I've witnessed too many photographers disrespect and disturb their subjects and the environment trying to get just a bit closer, and that's most commonly when shooting with lenses that were too short for what they wanted to accomplish. (Birders w/o scopes trying to get a better look w/ 8x binoculars are liable to do the same, incidentally.)