Der Punkt ist ja nicht der Abbildungsmaßstab, sondern der Umstand, dass mir die Tiere reglmäßig näher vor die Kamera laufen/fliegen.
Mit kürzerer Naheinstellgrenze mache ich dann ein Foto und mit längerer eben nicht.

Für mich wäre die ultimative Lösung wohl ein D-FA 400/4.0 DC AW (+TK) - gibt es aber leider nicht...

Noch mal zur Verdeutlichung: Weder das Sigma 500 noch das DA560 eignen sich besonders gut, um mit ihnen "an der Hüfte" durch die Gegend zu ziehen und Spontanfotos zu machen.

Bei mir ist es mehr die Ferngrenze, die mir Sorgen macht.

I have owned the Sigma 500/4.5 for about 18 months and now have had the DA560 for about 9 months.
Both are great lenses, but also very different lenses, in handling as well as image rendering.
Let's list a few of their differences, although other posters have already done so as well.

-The Sigma 500 is a relatively compact, short lens, with the tripod foot pointing backwards (towards you)
The DA 560 is a relatively long lens with the tripod foot pointing forwards (away from you).
This makes handling the lenses very different, but both are fairly easy to shoot handheld with some getting used to and practice. They way almost exactly the same, just over 3kg.
-The sigma has a max. aperture of f4.5 and is 500mm long. It works relatively well with the 1.4tc's either the Sigma or the Pentax (colors/contrast are better with the Pentax version of the 1.4tc), although the AF is a challenge, with no real confidence about getting spot on AF, so you miss a lot of shots if you care about proper AF.
The Pentax has a max. aperture of f5.6 and is 560mm long. The scale tips the other way i.m.o., and it is really better shooting the DA560 without a 1.4tc. On the one hand, 560mm is nearing the point where you won't really want to extend the length any further for handheld shots, on the other hand, the DA560 has a truly remarkable ability to resolve fine detail and this takes a more noticeable hit than the IQ of the Sigma, that is very sharp, but does not really resolve as fine detail. I've completely stopped using the 1.4tc since getting the DA560mm, where it was on the Sigma 500 most of the time.
-The Sigma has screw drive AF, that is accurate as long as you don't use the 1.4tc, but that can make a lot of objectionable noise depending on your use of the lens. Birding with c-af is no joy due to the noise alone..
The DA 560 has almost silent AF that is not really fast, but that is silent and very accurate, and moreover: also very consistent. If you have really gotten into shooting this lens, you will not miss very many shots due to less than perfect focus. Still, for c-af, the AF is really too slow.
-The Sigma has a "vintage style" color rendition, and a very smooth bokeh. Contrast can be low, leading to washed out colors. It is sharp, but does not resolve very fine detail.
The Pentax DA has lively, modern style colors, and tremendous contrast in practically all circumstances. It is very sharp and resolves very highly. It can be very rewarding to not shoot jpeg's with this lens, but use a good raw converter and work a bit more on the images, because there's lot's of detail.
-The Sigma is black, the Pentax DA is white. I don't see a very large white lens as very practical, so I use a black lens cover from lenscoat.
-The Sigma is compact and will fit in many bags, the Pentax is long, few bags will hold it with the camera attached. I use a long lens bag from lenscoat.

Good luck deciding. Both are very special lenses.
Chris

I currently own the Sigma 500 F4.5 and have to say… it is sweeeet but … and it comes with a "BUT", it comes with drawbacks:
1. Not weather sealed – I have been in extreme conditions photographing wildlife in different parts of the world from the rain forests of Rwanda and Borneo to the dry dusty safaris in India and Kenya and needless to say ever so lovely unpredictable British. I have to admit it is a pain to keep the lens covered with the rain and dust coat. However, this is not the biggest minus point. I can live with it
2. Lens movement – That’s the best way I would describe is the lens has a very slight movement (probably 1mm) enough to let the lens start searching (I have attached a pic of what ive seen happen on my 500 and 100-300). I have missed on some of the best action shots because of this. I have had this checked with Sigma and Richo and had the bayonet replaced spending £150. No much luck. The same thing seems to happen just not as much and I know that it will eventually get there. Happens only happens when handheld or on a beanbag. Tripod – no issues.
3. Doesn’t work well with 1.4x TC – well while many might argue that the TC works with the lens, it doesn’t work well enough for me ☺. I rather not use it at all.