This is quoted from a previous Pentax forum answer I provided to another individual who was having difficulty getting sharp pix from their Sigma 150-500.
BTW, you're not alone in not getting sharp pix from the 150-500. Before I bought mine, I checked out both the Canon forum (I also have Canon equipment) and the Pentax forum. I noted that ...in both forums...some got out of focus pics, some got tack sharp pics.
I believe the issue is not with this lens, nor is it with either Canon or Pentax DSLR's. I believe it is a question of technique. It's a big, heavy, super telephoto.
A lens this size, needs careful technique to get the most out of it.
These settings and procedures work well for me, when using my Sigma 150-500.
I don't use a tripod, generally. Normally I use the Sigma 150-500 when the light is good, plenty of sun.
I use it handheld.
I find I get sharp pictures from it...yes...even at 500mm. I use it with my Pentax K-5 (with grip and extra battery) and use the Pentax shake reduction, rather than the Sigma shake reduction (OS)...although the Sigma OS works fine. Remember to shut off the OS system if you have the Pentax SR switched on. Also if using a tripod...don't use any shake reduction system...either Pentax or Sigma.
I support the lens by either holding the lens handle or holding the lens body with one hand, gripping the handle with my hand under the handle and bracing my arm against my body.
I use the other hand, to hold the K-5.
I'm an older man, but find I can handhold the body and lens ok, this way. I am 6' 3" and around 240 lbs...so maybe my size is a factor, when it comes to hand holding and supporting the K-5, Sigma combo. I don't know.
I find settings are very important for a large, heavy lens.
On my Sigma I use 800-1600 ISO, F 10, F 11 and shutter speeds over 1/1000th of a second if possible. I've used as low as around 1/350th, 1/500th...and although I've been happy with the picture...I find I'm pushing my luck if I use shutter speeds this low, on a large, heavy lens.
I use F 10, F 11, due to the fact that I find a super telephoto has minimal depth of field at F 5.6, 6.3. F 10, etc...gives me enough depth of field to get a bird the size of a flying pelican ...all in focus...wing tip from wing tip.
I use it to take pics of wildlife, wild birds...I always spot focus on the eye. If you get the eye sharp and have reasonable F stop/shutter speeds...the pix will be sharp....well so far, this works for me.
I also use it to take pix of racing cars at the drag strip. It is tack sharp here too.
What I'm saying is technique is important with any large lens....even when using a tripod. I'm sure you have both the Pentax and Sigma shake reduction switched off using a tripod. Again, with a long lens at the 500mm setting....F stop is critical for adequate depth of field.
Hopefully some of my experience is helpful.
Good luck .
Les
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