Hi Timo,
For my purposes, the FA* 300/2.8 + Pentax F 1.7x AFA is a better choice than the Sigma EX 500 f4.5 because of the much shorter Minimum Focusing Distance (2m vs 4m). I like to shoot small songbirds as close as I can get, and this is usually between 2.4 -3.0m. This combination gives me 510mm at f4.8, and I don't really feel that the optics are compromised significantly. With a crop sensored camera, only the center portion of the image circle is used (which is the best section optically with any lens), and the TC further reduces the FOV by magnifying the very center of the image circle, so with this class of lens, the very few optical flaws in the lens are the only real cause of degradation of IQ you will see.
Most super tele shooters tend to shoot at longer distances, and for this, I'd say that the Sigma might be a better choice, but when considering the alternatives, MFD was a always a top priority for me, and I was already used to using the AFA, so at the time, my choice was between a "bargain" grade FA*300/2.8 from KEH (who are very conservative in their condition grading) for $2500 or a new Sigma for $3200. I chose the FA* and have never regretted the choice.
Here are a few examples. All were shot in jpeg ***, NR and sharpened to taste in PP, cropped as noted and dowsized for posting.
This Green Heron was from between 6-7m. Shot with a K-7, FA* 300/2.8 + 1.7x AF + Sigma EX 1.4x APO TC (714mm f6.7). I was setting up to shoot Pied Billed Grebes from long distance when this guy landed and actually started walking
towards me. I actually had to carefully back off to get the whole bird in the frame. This was from a tripod so SR was turned off and the FL in the exif shows as 0.
This White Capped Sparrow is from less than 3m, shot with a K20, FA* 300/2.8 + 1.7x AFA. This is a vertical 5x7 crop from a landscape frame. IIRC, this crop used about 1/3 of the frame.
Here's a White Throated Sparrow from about 3m. K20, FA* 300/2.8 + 1.7x AFA + Sigma 1.4x APO TC. This is also a 5x7 vertical crop of a landscape frame using the full vertical resolution.
. . .and a 100% crop SOOC from the lower left portion of the image.
Here's one from the K-5, FA* 300/4.5 +1.7x AFA handheld from about 8m, cropped to 8x10 from around 2/3 of the original frame. The lens is wide open (exif shows f8, but the AFA converts by multiplying the lens f stop by 1.7x and rounding up)
Here's another with the K-5, Canon FD 300/4 L + 1.7x AFA, from a tripod at 3+ m (the MFD of the Canon) I have no idea where the f4 in the exif came from
If there's image degradation from the TC(s), it's not enough for me to notice, much less care about. . .The TC doesn't effect the MFD
I'm sure that some will be tempted to pipe in saying that the AFAs are expensive, but when the alternative is spending $4k+ for a 500/4.5, then $350 doesn't sound like that much. . . and I purchased my 3 AFAs for an average of @ $125 each, so they certainly were a bargain for me. . . Even a DA* 300/4 + a 1.7x AFA will cost less than $2K USD new, and give you a handholdable 510mm f6.8 that will give you quick AF in most any daylight conditions.
If I had the 500/4.5, I'd still want the 300/2.8 for its closer focusing and faster aperture since lighting is not always perfect (I don't set up blinds or use camo), and lugging over 13 lbs of lens in addition to a tripod and gimbal is just not a possibility for me. With 2 bodies, 2x 1.7x AFAs, 2x 1.4x TC, an FA* 300/2.8 and an FA* 300/4.5, and a tripod and gimbal, I have a whole lot of versatility in FL/speed with minimal bulk and weight -- 300mm at f2.8 and f4.5, 420mm at f4 and f6.3, 510mm at f4.8 and f7.7, 600mm at f5.6, 714mm at f6.7, and in a pinch, 867mm at f8.1 (which will only AF in pretty bright light, and I have to set up the forward mounted AFA to infinity focus first) -- all with AF -- sometimes with the focus limiting of the AFA.
Personally, I'm pretty invested in the 300mm+TC concept of longer reach. I have an FA*300/2.8, Sigma EX 300/2.8 APO, Tamron SP 300/2.8 Mdl 60B, FA* 300/4.5, Canon FD 300/4 L converted to K mount, Sigma EX 100-300/4 APO, a Tokina 100-300/4 SD (MF), 3 1.7x AFAs, a Sigma 1.4x APO AF TC, a Tamron F 1.4x AF PZ MC4 TC, and a Tamron SP 140F 1.4x TC that only mounts to Adaptall 2 lenses. The onlu lenses that I have that natively go past 300mm are the Tokina 80-400 f4.5-5.6 AT-X, a Tokina 150-500 f5.6 (which is pretty much a monster, though a decent lens), and a Tamron SP 500/8 Cat. I also am using a Pentax Q with adapted K mount lenses to get more reach than is possible with my DSLRs, but that's a different story. . .
I'm not saying that this is optimal for most people, it's the best of alternatives for me, and something that should at least be considered.
Scott