Originally posted by jddwoods I agree with this selection. The DA*300 and the TC is the most cost effective and weight effective way to get the highest quality 420 mm. The DFA 100WR is super sharp fast f2.8. This combo gives you 100mm, 140mm, 300mm and 420mm.
I agree completely.
For the long tele, I've tried consumer zoom (55-300 both screw-driven and PLM versions), long consumer zoom, (Sigma 170-500), longish prime (Sigma 400 f5.6 tele macro) and 300mm prime + TC (FA*300mm f4.5 + Kenko 1.5x TC). The other option is a premium long zoom like the DFA*150-450. Each option has its merits - it's a trade-off between affordability, versatility, weight/bulk, speed, AF performance (55-300 PLM wins hands down here), and image quality. But I give weight a lot of priority - a 2kg+ lens (e.g. Sigma 150-500, Sigma 50-500, Pentax DFA 150-450) is going to be a beast to carry and harder to use handheld. For me the sweet spot is the 300 prime plus tele when required - image quality is excellent, it's fine to use handheld, you have two focal lengths (each with reasonable maximum aperture) and it's a reasonable carry option. YMMV.
Using a TC costs one stop of light, and might slow the AF a bit, but any degradation in image quality, when used with a sharp prime, is negligible IMO. Here are some samples with the DFA 100 + Kenko TC.
And here are some with the FA*300 + TC (effectively 420mm f6.3):
The DA*300 f4 + DA 1.4x TC should offer similar image quality to my combo, with the advantages of WR, 1/3 stop extra aperture, quieter AF, newer coatings, and correct info being passed to the camera for EXIF and SR purposes. Downsides are SDM AF reliability issues and, if you ever go to FF, possible vignetting from the TC. Some people say the screw-drive AF in the F/FA*300 is faster than the SDM in the DA*300, but SDM is said to be more accurate at times - I can't comment because I haven't used the DA*300.