Originally posted by OJGoreng Out of curiosity... I noticed you're located a bit more south than I am. How dependent is this reliability on bright light? Reason I ask is that with the K f/4 300mm (yes, a very different lens, but at least with the same maximum aperture) I invariably get horrible results with the AFA, the focus being completely off. I wondered if I just have a bad AFA, but with my faster, shorter lenses there's no problem whatsoever. So maybe the typical low-light cloudy Dutch conditions colour my perception about the viability of this combo.
Hi OJG,
I don't have much experience with light conditions effected by different latitudes, so I really can't say (but it is a little strange to think of my location as a "southern" one
). With the f4.5 max aperture, I'm starting with about a 1/3 stop handicap from the lens. I've shot this combination (FA* 300/4.5 + AFA) for years though, and using 7 Pentax body models ( DS, K100DS, K10, K20, K-7, K-5, K-5 IIs) I've only felt the need to remove the AFA when it gets too close to sunset (about 1 hour) and darker overcast conditions (probably somewhere under EV 12). This is my default walk around birding lens combo, and it's on one of my cameras about 80% of the time during daylight hours, almost regardless of weather conditions.
Another member, Lowell Goudge, in Toronto, Canada, is closer to your latitude, has gotten good results with his K 300/4 and the AFA, so this might be a matter of lens sample variation.
I've also used the AFA with a relatively recently acquired Canon FD 300 f4 L that had been converted to K mount. What I found with this lens is that any deviation from wide open on the aperture causes AF with the AFA to become significantly challenged. It must be noted that the aperture control with this lens was a bit compromised with the conversion -- the ring no longer has detents and there is some roughness right off of wide open -- so there is no easy way for me to tell at what aperture this occurs, but this lens will only AF with the AFA reliably when wide open. If you're trying to shoot with your lens stopped down at all, this might be the problem. I'd also take a look at the aperture blades wide open to see if the blades are visible when the lens is set to f4. I'd assume that they should be retracted completely at this aperture, so if they are visible at all, then that could be a problem.
At these effective apertures (f6.8-7.7), the SAFOX AF sensors are at very close to their limits for what I consider acceptable AF performance. The Pentax AF systems have shown small increments of increased AF sensitivity with each generation, and I've tried pushing each successive body to see how far I can stretch the AFA's ability to supply reasonable AF performance with TCs.
It should be mentioned that the affordable ~500mm alternatives -- the Sigma xx & xxx -500 zooms are f6.3 at 500mm (and I've heard reports that at least some expose at more like f6.8 wide open), so are also in this speed class -- without the focus limiting aspect of the AFA -- so in marginal light, they can hunt, and for me that would be frustrating compared to the very brief lock to lock hunt of the AFA.
Here are some pretty good K-5 examples
With the FA* 300/4.5 + AFA -- 1/1250, lens wide open, ISO 800, +0.3 EvC, Handheld, cropped to 8x10 only from the sides, NR and sharpening applied and downsized for posting.
With the Canon FD 300 /4 L + AFA -- 1/1250, lens wide open, ISO
1600, -0.7 EvC, Tripod mounted, vertical 8x10 crop from a landscape frame, NR and sharpening applied and downsized for posting
K-5 IIs -- one of the first shots with this body:
With the FA* 300/4.5 + AFA --
1/250, lens wide open,
ISO 4500, Handheld, uncropped, NR and sharpening applied and downsized for posting.
Scott